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WIRELESS TELEGRAPH
BY ELMER E.BUCHE
Practical \Vireless Telegraphy
A COMPLETE TEXT BOOK
for STUDENTS of
RADIO COMMUNICATION
BY '
ELMER E. BUCHER
Instructing Engineer
Marconi Wireless Telegraph Co. of America Member Institute of Radio Engineers
Fully Illustrated
WIRELESS PRESS, INC
42 BROAD STREET, NEW YORK
Ubrary
COPYRIGHT, 1917
BY WIRELESS PRESS. INC.
S. L. PARSONS & CO., INC., Printers, 45 Rose St., New York.
AUTHOR'S NOTE
In preparing this volume, the author has endeavored to give the non-technical student and the practical telegraphist an understanding of the functioning of present day commercial wireless telegraph apparatus, and he has varied the usual procedure followed in text books by covering first in a general way the fundamentals of electricity, electromagnetic induction, the dynamo, the motor, the motor generator, storage batteries, charging panels, etc., a knowledge of which is quite as essential to the practical wireless worker as the more com- plicated phenomena of radio-frequency circuits.
It was not possible in the space available to treat the elements of electricity and magnetism in detail, but an effort was made to cover some of the more important prin- ciples to prepare the student to understand the functioning of radio telegraph apparatus.
As in the case of the ordinary electrician working in the more common branches of electrical practice, one of the first essentials in training a wireless telegraphist in the practical operation of a radio set is to instil in his mind a thorough understanding of electrical circuits, i. e., the wiring of electrical apparatus, for only as this all important knowledge is assimilated is the learner qualified to take charge of a commercial wireless station ; hence, this book is largely devoted to describing the circuits of practical radio sets together with a simple explanation of the basic principles of the apparatus associated therewith.
No attempt therefore has been made to treat the subject with rigid scientific accuracy or completeness. The idea has been rather to show the student what the apparatus consists of and how it is manipulated. Only general notions of how and why it operates are presented and neither completeness of treatment nor rigidly scientific as distinguished from popular and non-technical use of terms have been attempted.
In selecting the apparatus to be described the author has chosen that which is most widely in commercial use and such other apparatus as is of general interest. In line with this policy, systems using radio frequency alternative and direct current arc transmitters have been treat- ed and chapters on undamped oscillation receivers and Marconi transoceanic wireless tele- graph apparatus have been included.
The attention of prospective wireless operators is directed to the series of questions in the Appendix (Section F), which bear particularly on the salient points of a practical operator's course and which were prepared as a guide for the beginner to qualify him to take the examination for a Government license certificate.
The student who has knowledge of electrical circuits and requires instruction only in the details of commercial wireless apparatus is advised to read Chapters Four to Twelve inclusive, but those who only require a working knowledge of the ship apparatus used in the American Marconi Company's service are directed to Chapters Nine and Twelve.
One of the first questions often asked by a beginner who has had no previous elec- trical training or experience is "What is the object of the study of the elements of elec- tricity and magnetism" or "Why is such instruction required previous to taking up the subject of wireless apparatus proper?"
To this it may be answered that the functioning of wireless telegraph apparatus is based upon fundamental electrical and magnetic principles and consequently, when the simple laws of the magnet and electrical currents are. thoroughly understood the ground is at least two-thirds gone over. The primary object of the elementary work is to prepare the student step by step to understand the apparatus for the production of radio-frequent currents — the currents of extremely high frequency by which the electric waves of wire- less telegraphy are set into motion. The second object is to explain and describe the apparatus by which the energy of these currents can be radiated in the form of electric waves and detected at a distant receiving station, and to explain the apparatus by which such currents are finally made audible in a telephone receiver or some sort of telegraphic recording apparatus.
The author desires to acknowledge his indebtedness to the Marconi Wireless Telegraph Co. of America, the Crocker-Wheeler Manufacturing Co. and the Electric Storage Battery Com- pany for the loan of photographs, cuts, blue prints, wiring diagrams and literature which have greatly assisted in the preparation of this work. He has also freely consulted the columns of the Wireless Age and the Proceedings of the Institute of Radio Engineers.
E. E. B.
New Yor4c, June, 1917. '
oooUlo
CONTENTS
PART I.
MAGNETISM.
THE MAGNETIC CIRCUIT.
J>-jO-£
1. Natural Magnet. 2. Flux. 3. Polarity. 4. Magnetic Induction. 5. Permanent and Temporary
Magnets. 6. Laws of Magnetic Poles. 7. Magnetic Circuit 1
PART II. THE PRODUCTION OF ELECTROMOTIVE FORCE.
ELECTRIC CURRENT AND CIRCUITS.
8. Electrical Current. 9. Classification of Currents. 10. Electromotive Force. 11. Conductors and Insulators. 12. Production of Electromotive Force. 13. Electricity by Friction (Static Elec- tricity). 14. Electricity by Chemical Action (Primary or Secondary Batteries). 15. Secondary Cell. 16. Current Strength and Quantity. 17. Electrical Resistance. 18. Grouping of Electrical Cells. 19. Ohm's Law and Practical Application. 20. Divided Circuits. 21. Electrical Work. 22. Electrical Horsepower. 23. Definition of Electrical Units. 24. Current Output and Voltage of Various Current Sources 4
PART III. ELECTROMAGNETIC INDUCTION.
THE DYNAMO— THE FLOW OF ALTERNATING CURRENT.
25. Electromagnetism. 26. Magnetic Field About Two Parallel Conductors. 27. The Solenoid. 28. Induced Currents. 29. Mutual Induction. 30. Self-induction. 31. Value of Induced E. M. F. 32. The Dynamo. 33. Determination of Frequency. 34. Strength of Magnetic Field. 35. Diagram of an Alternating Current Dynamo. 36. Direct Current Dynamo. 37. Shunt, Series and Compound Wound Dynamos. 38. The Electric Motor. 39. The Effect of Counter Electromotor Force. 40. Motor with Differential Field Winding. 41. Dynamo and Motor Armatures. 42. Development of Armature Windings. 43. The Alternating Current Transformer. 44. Electrostatic Capacity. 45. Reactance and Impedance. 46. Capacity Reactance. 47. Lag and Lead of Alternating Current. 48. Effective Value of Alternating E. M. F. and Current. 49. Measuring Instruments or Electric Meters. 50. Induction Coil. 51. Practical Electric Circuits 16
PART IV. MOTOR GENERATORS.
HAND AND AUTOMATIC MOTOR STARTERS.
52. The Motor Generator. 53. Field Rheostats. 54. Dynamotor and Rotary Converter. 55. The Motor Starter. 56. Automatic Motor Starters. 57. Protective Condensers. 58. Care of the Motor Generator. 59. How to Remove Motor Generator Armature » 51
/
PART V. STORAGE BATTERIES AND CHARGING CIRCUITS.
60. The Necessity for a Storage Battery in a Radio Installation. 61. General Construction and Action. 62. The Charging Process. 63. The Fundamental Actions of a Storage Cell. 64. The Electrolyte. 65. The Hydrometer. 66. How the Capacity of a Storage Cell is Rated. 67. Funda- mental Facts Concerning the Storage Cell. 68. How to Charge a Storage Cell. 69. How to Determine the Value of the Charging Resistance. 70. Lamp Bank Resistance. 71. The Use of the Ammeter and the Underload Circuit Breaker. 72. The Ampere Hour Meter. 73. Over- charge. 74. How to Charge a Battery When the Voltage Exceeds That of the Charging Gen- erator. 75. How to Determine the Polarity of the Charging Generator. 76. Determination of the State of Charge and Discharge of a Battery. 77. Keeping the Level of the Electrolyte. 78. Pro- tecting the Cells from Acid Spray. 79. General Instructions for the Portable Chloride Type of Accumulators. 80. General Operating Instructions for the Exide Cell. 81. The Edison Storage Battery. 82. The Charge and Discharge of the Edison Cell 67
vi PRACTICAL WIRELESS TELEGRAPHY.
PART VI. THE RADIO TRANSMITTER.
CONDENSERS— OSCILLATION GENERATORS— RADIATION OF ELECTRICAL WAVES- DAMPING OF OSCILLATIONS.
Page
83. Methods of Generating Radio Frequency Current. 84. The Condenser. 85. Connections for Con- densers. 86. How to Place a Charge in a Condenser. 87. Analysis of a Spark Discharge. 88. Effect of Resistance on Oscillations. 89. Electrical Resonance. 90. The Open Circuit Oscillator. 91. The Length of the Electric Wave. 92. The Determination of Wave Length From the Inductance and Capacity. 93. Logarithmic Decrement of the Oscillations. 94. Methods of Exciting Oscillations in an Aerial. 95. The Reaction of Coupled Circuits. 96. The Standard Waves of Commercial Wireless Telegraphy. 97. Fundamental Circuit of a Complete Radio Transmitter. 98. Simple Explanation of the Circuits. 99. Numerical Values for a Standard Radio Set 80
PART VII. APPLIANCES FOR A RADIO TRANSMITTER.
SPARK DISCHARGERS— OSCILLATION TRANSFORMERS— CONDENSERS— TRANSFORMERS.
100. In General. 101. Spark Dischargers for Radio Telegraphy. 102. Adjustment of the Spark Note. 103. Oscillation Transformers. 104. Aerial Tuning Inductance. 105. The Short Wave Condenser. 106. High Potential Condensers. 107. High Frequency "Choking" Coils. 108. High Voltage Transformers. 109. Reactance Regulators. 110. Aerial Changeover Switch. 111. Transmitting Keys 101
PART VIII. AERIALS OR ANTENNAE.
112. Function of the Aerial. 113. Determination of the Wave Length From the Dimensions of an Aerial. 114. Fundamental Considerations. 115. Various Types of Aerials. 116. Directional Aerials. 117. Standard Marconi Aerial. 118. The Deck Insulator. 119. Installation of the Aerial. 120. Earth Connection. 121. Radiation. 122. Antenna Decrement. 123. Transmis- sion Range • 116
PART IX. RECEIVING CIRCUITS, DETECTORS AND TUNING APPARATUS.
STANDARD MARCONI RECEIVING SETS.
,24. In General. 125. The Problem. 126. Simple Receiver. 127. The Inductively Coupled Re- ceiver. 128. Other Methods of Coupling. 129. The Carborundum Detector and Tuning Cir- cuits. 130. Adjustment of the Inductively Coupled Tuner. 131. The Action of the Car- borundum Crystal. 132. Adjustment of Crystal Detectors. 133. Detector Holders. 134. Classi- fication of the Receiving Detectors. 135. Fleming Valve Detector and Tuning Circuits. 136. Marconi Type 107-A Tuner. 137. Marconi Magnetic Detector and the Multiple Tuner Circuits (English Marconi Company). 138. The Marconi Type 106 Receiving Tuner. 139. Marconi Receiving Tuner Type 101 (American Marconi Company). 140. The Marconi Universal Re- ceiving Set (English Marconi Company). 141. Electrolytic Detector. 142. The Three Element Valve Detector. 143. A Repeater Vacuum Valve Circuit. 144. The Vacuum Valve Amplifier. 145. Amplification of Radio Frequencies. 146. The Effects of Distributed Capacity. 147. The "End Turns" of a Receiving Tuner and End Turn Switches. 148. The Variation of a Radio Frequency Inductance. 149. Buzzer Excitation Systems. 150. Receiving Telephones. 151. Mi- crophonic Relays or Sound Intensifiers. 152. Brown Amplifying Relay. 153. Atmospheric Electricity. 154. The Marconi Balanced Crystal Receiver (English Marconi Company). 155. Type I Aerial Changeover Switch. 156. Marconi Type 112 Receiving Tuner. 156A. General Advice for the Manipulation of a Receiving Tuner 129
PART X. AUXILIARY APPARATUS OR EMERGENCY TRANSMITTERS.
!57. Statute Requirements. 158. Tuned Coil Set. 159. The Electric Storage Battery Company's
Accumulators and Charging Panel 179
CONTENTS. vii
PART XI. PRACTICAL RADIO MEASUREMENTS.
MEASUREMENT OF WAVE LENGTH— DECREMENT - CALIBRATION— TRANSMITTING AND
RECEIVING APPARATUS.
Page
160. The Importance of Electrical Resonance. 161. Indicators of Resonance. 162. Uses of the Wavemeter. 163. Simple Use of the Wavemeter. 164. General Instructions for Tuning a Radio Transmitter. 165. Tuning by the Hot Wire Ammeter. 166. Tuning the 2 K. W. 500 Cycle Panel Transmitter. 167. Determination of Coupling. 168. Plotting of Resonance Curves. 169. Measurement of the Logarithmic Decrement of Damping. 170. Calculation of the Decre- ment of the Wavemeter (or Decremeter). 171. Wavemeter as a Source of High Frequency Oscillations. _ 172. Calibration of the Secondary and Primary Circuits of a Receiving Tuner. 173. Calibration of the Open and Closed Circuits Simultaneously. 174. Measurement of the Natural Oscillating Period of a Coil. 175. Measurement of Electrostatic Capacity. 176. Meas- urement of the Effective Inductance of a Coil at Radio Frequencies. 177. Calculation of In- ductance from the Constants of the Coil. 178. Measurement of the Effective Inductance and Capacity of an Aerial. 179. Calibration of a Wavemeter from a Standard. 180. Measurement of Mutual Inductance at Radio Frequencies. 181. Comparative Measurement of the Strength of Incoming Signals. 182. "Tight" and "Loose" Coupling. 183. Measurement of High Voltages. 184. Tuning and Adjustment Record 188
PART XII. STANDARD MARINE SETS OF THE AMERICAN MARCONI COMPANY.
PANEL TRANSMITTERS— COMPOSITE TRANSMITTERS.
185. Panel Transmitters. 186. Details of Type P-4 Panel. 187. Description of Apparatus. 188. Complete Adjustment of Type P-4 Set. 189. Type P-5 Panel Transmitter. 190. Description of Apparatus. 191. Complete Adjustment of the Type P-5. Set. 192. How to Remove the Arma- ture of the % K. W. Motor Generator. 193. The 1 K. W. Non-Synchronous Discharger Transmitter. 194. Description of the Set. 195. Installation. 196. Adjustment of the 1 K. W. Set. 197. Type "E-2" One-half Kilowatt, 120 Cycle Panel Transmitter. 198. Details of the Circuits and Apparatus. 199. General Instructions for Tuning and Adjusting. 200. Marconi 2 K. W. 240 Cycle Transmitter. 201. Type P-9 Y* K. W. Cargo Transmitting Set. 202. Aerial Current and Reduction of Pover. 203. General Instructions for the Panel 223
PART XIII.
MARCONI DIRECTION FINDER OR WIRELESS COMPASS AND ITS
APPLICATION.
204. In General. 205. Description of Equipment. 206. The Direction Finder Aerials. 207. The Circuit Complete. 208. The Tuned Buzzer Tester. 209. How Current is Induced in the Looped Aerials. 210. Direction of Magnetic Forces Within the Goniometer. 211. General Instructions for Operation of the Direction Finder. 212. To find the Direction of a Radio Station 255
PART XIV. TRANSMITTERS OF UNDAMPED OSCILLATIONS.
ARC GENERATORS— RADIO-FREQUENCY ALTERNATORS— PLIOTRON OSCILLATOR.
213. In General. 214. The Arc Generator. 215. Signalling with the Arc Transmitter. 216. The Alexanderson High Frequency Alternator. 217. Goldschmidt Radioi-'Frequency Alternator. 218. The Joly System for the Protection of Undamped Oscillations. 219. Marconi's System for the Production of Continuous Waves. 220. The Pliotron Oscillator 264
PART XV. RECEIVERS FOR UNDAMPED OSCILLATIONS OR CONTINUOUS WAVES.
221. The Problem. 222. The Tikker. 223. The Heterodyne System. 224. The Vacuum Valve as a Source of Radio-Frequency Oscillations. 225. Vacuum Valve as a Combined Detector, Amplifier and Beat Receiver. 226. Oscillating Vacuum Valve Detector Circuits of the U. S. Navy. 227. The Goldschmidt Tone Wheel. 228. Marconi System for Reception of Undamped Oscillations , 277
PART XVI. MARCONI TRANSOCEANIC RADIO TELEGRAPHY.
229. Marconi Development and General Considerations. 230. Marconi's Duplex System. 231. The Balancing Out Aerial. 232. Glace Bay-Clifden Stations. 233. Marconi Directional Aerial. 234. Marconi Transoceanic Stations. 235. Marconi Tubular Masts. 236. Radio-Frequency Circuits of the Damped Wave Transmitters. 237. Other U. S. High Power Stations. 238. Long Distance Receiving Sets. 239. Condensed List of High Power Stations 288
Practical Wireless Telegraphy
PART I. MAGNETISM.
THE MAGNETIC CIRCUIT.
1. NATURAL MAGNET. 2. FLUX. 3. POLARITY. 4. MAGNETIC INDUCTION. 5. PERMANENT AND TEMPORARY MAGNETS. 6. LAWS OF MAGNETIC POLES. 7. MAGNETIC CIRCUIT.
Because the flow of an electrical current is invariably accompanied by a mag- ictic field, a brief explanation of the phenomena surrounding the simple bar nagnet will be given. This is to be followed in a successive chapter by a descrip- ion of the electromagnet.
1. Natural Magnet. — A substance which has the property of attracting >its of iron or steel is called a magnet. Natural magnets found in various parts of he earth are known as lodestone and a piece of lodestone dipped into a pile of ron or steel filings exhibits this property of attraction to a considerable degree.
If a bar of hard steel be rubbed with a piece of lodestone the steel is found to )e magnetized and is then known as an artificial magnet. If the same bar is lipped into a pile of iron filings, the majority of the filings cling to the tips of the >ar, there being no tendency towards attraction at the center. Since the strongest nagnetism exists at the ends of the bar, these ends are known as the poles of the nagnet.
2. Flux. — If a piece of paper, over which iron filings have been sprinkled, s placed above and parallel to a bar magnet, the filings will arrange themselves
^ _^ into a series of well defined lines as in
^"" ~^^N Fig. 1. These may be said to show the
/ ,--—•• -^ NN general direction of the magnetic force.
\ I /' ^NN \ / These lines indicate that the space about
\ * / ^ -•*--»„ \ f. *' the poles of a magnet is in a state of \ \ \ ' / / / ' . stress or strain, and therefore, they are
called the magnetic lines of force or simply lines of force. The space sub-
' \ \ "\ N jected to this strain is called the mag-
^__^ -' i \ \ netic field and the total lines of force
1 \ crossing a given space or field are
/
^-~^ — - / termed the magnetic flux.
3. Polarity. — A magnetic needle Fig. i-Fieidf a Simple Bar Magnet. suspended or pivoted as in a compass
and left to swing freely will, as is well
known, point in the direction of the north magnetic pole. The end which points in that direction is known as the north pole of the magnet and the opposite end, the south pole.
2 PRACTICAL ; WIRELESS TELEGRAPHY
4. Magnetic Induction.— A' piqce of soft iron placed in the magnetic field of another niftgixct, ". fcrecoja'Jes .temporarily magnetized and will have two unlike poles. Magnetism H'htis ; indtice(i in a piece of soft iron is said to be due to magnetic induction. If, for example, the north pole of a steel bar magnet be placed near to a bar of soft iron, the end of the iron bar nearest to the magnet will exhibit south magnetism and the opposite end north magnetism. It should be understood that magnetism can be induced in the iron bar whether in direct contact with the inducing magnet or slightly separated from it but when the exciting magnet is removed, the induced magnetism will practically disappear.
5. Permanent and Temporary Magnets. — Because a bar of soft iron re- tains its magnetism only while under the influence of a given magnetizing force it is called a temporary magnet. On the other hand a piece of steel, when once magnetized, retains its magnetism permanently, and thereafter is known as a permanent magnet.
LINES IN OPPOSITE DIRECTION REPULSION
LINES IN SAME D1RECT10H ATTRACTION
m
.-•>)} rcc
Fig. 2 — Diagram Showing the Attraction and Repulsion of Magnetic Fields.
The power of steel to resist magnetization and once in this condition to resist demagnetization is termed its retentivity. Steel possesses greater reten- tivity than iron because, as previously mentioned, soft iron becomes saturated with magnetism very quickly and loses it almost immediately when the inducing magnetic field is removed.
The capability of any substance for conducting magnetic lines of force is termed its permeability. Iron, for instance, possesses much greater permeability
MAGNETISM. 3
•
than steel and steel possesses greater permeability than air. This means that if the circuit for the magnetic lines of force from pole to pole of a magnet is completed through an iron core, a greater number of lines of force will pass than if the circuit were completed through a piece of steel or through air.
6. Laws of Magnetic Poles. — If two bar magnets are suspended by a cord as in Fig. 2, and the north pole of one brought near to the north pole of the other, they will be found to repel. On the other hand, if the south pole of a bar magnet is brought near to the north pole of another magnet, they are found to attract one another. The foregoing actions may be summed up by the funda- mental law : Like magnetic poles repel, unlike magnetic poles attract.
A variation of this law is encountered when a very strong south pole, let us say, is placed near a weak south pole. The stronger magnet will attract the weaker one because of its over-powering field. Similar effects are observed between two north poles of dissimilar strength.
7. Magnetic Circuit. — Each line of force of a magnet (as in Fig. 1) is assumed to pass from the south pole to the north pole, through the bar and from the north to the south pole outside the bar. This is said to be the direction of the lines of force and the path they take is called the magnetic circuit. Such a circuit is usually made up of magnetic material like iron or steel but in com- mercial apparatus very often one or more air gaps are included in the path of the flux.
If a magnetic substance such as a bar of iron is suspended free to move in a magnetic field, it ivill tend to turn and lie parallel with the Held, or, as is more often said, ^vill take such a position as to accommodate through itself the greatest number of lines of force. On the other hand, if a permanent magnet is suspended free to move in a magnetic field of definite direction (such as suspending a bar magnet above a stationary magnet} it will tend to take a parallel position with the field but in a particular direction, that is, its internal lines of force will be in the same direction as those of the field.
Advantage of this fundamental principle is taken in the design of many electromagnetic devices and in electrical measuring instruments to be described later on.
Powerful magnetic fields may be created by an electric current. Mag- netism so produced is known as electromagnetism. The great advantage of the electromagnet is the fact that the strength of the magnetic field can always be controlled, whereas the field of the permanent magnet is more or less of fixed strength. Electromagnetism will be taken up in its proper order in a following chapter.
PART II.
THE PRODUCTION OF ELECTROMOTIVE
FORCE.
ELECTRIC CURRENT AND CIRCUITS.
8. ELECTRICAL CURRENT. 9. CLASSIFICATION OF CURRENTS. 10. ELECTROMOTIVE FORCE. 11. CONDUCTORS AND INSULA- TORS. 12. PRODUCTION OF ELECTROMOTIVE FORCE. 13. ELEC- TRICITY BY FRICTION (STATIC ELECTRICITY). 14. ELECTRICITY BY CHEMICAL ACTION (PRIMARY OR SECONDARY BATTERIES). 15. SECONDARY CELL. 16. CURRENT STRENGTH AND QUANTITY. 17. ELECTRICAL RESISTANCE. 18. GROUPING OF ELECTRICAL CELLS. 19. OHM'S LAW AND PRACTICAL APPLICATION. 20. DIVIDED CIRCUITS. 21. ELECTRICAL WORK. 22. ELECTRICAL HORSEPOWER. 23. DEFINITION OF ELECTRICAL UNITS. 24. CUR- RENT OUTPUT AND VOLTAGE OF VARIOUS CURRENT SOURCES.
8. Electric Current. — When we speak of a current of electricity as flowing through a wire or circuit we simply express in a convenient way certain phenomena associated therewith. We do not, in fact, know what actually trans- pires in the transfer of electricity from point to point in a conductor. Electricians generally agree that a so-called "current" of electricity flows in a definite direc- tion throughout a given circuit, but there is no direct evidence at hand to prove the actual existence of a "current", in the commonly accepted meaning of the word. The term, however, is universally adopted to designate the flow of elec- tricity from point to point in an electrical circuit.
9. Classification of Currents. — Electrical currents are called direct if they flow in one direction throughout a given circuit, and alternating if they con- tinually reverse, flowing first in one direction and then in the other.
A primary current is said to be one which flows directly from a generating source. A secondary current is one induced by a primary current acting in- ductively on a circuit having no direct connection with the primary circuit. A current is said to be of low tension when its pressure or voltage is rela- tively low, and conversely, it is said to be of high tension when its pressure or voltage is relatively high.
10. Electromotive Force. — In order to produce a steady electrical current, two conditions are necessary. There must be a steadily maintained electric pressure known as electromotive force and a suitable conducting path to pass the current.
11. Conductors and Insulators. — A metallic circuit in which a current of electricity flows with little opposition is said to be a conductor; one which offers considerable resistance is known as a partial conductor, but a substance which completely impedes the flow of current is termed an insulator. It should be understood at the beginning that these terms are purely relative for an abso- lute insulator or a perfect conductor does not exist.
The best conductors of an electric current among the common metals, in order of their increasing resistance, are silver, copper, gold, aluminum, zinc, iron, platinum and nickel.
Examples of insulators given in order of their increasing value are dry air, shellac,
THE PRODUCTION OF ELECTROMOTIVE FORCE.
paraffine, amber, resin, sulphur, wax, glass, mica, ebonite, india rubber, silk, paper and oils.
12. Production of Electromotive Force. — To produce an electromotive force, it is necessary first to create a difference in potential or difference in electric pressure between two bodies or two points in the same body.
An electromotive force can be produced by various methods, for example :
(1) By friction (static machine);
(2) By chemical action (primary and secondary batteries) ;
(3) By mechanical motion (dynamos or generators) ;
(4) By thermal action (thermo junction).
In the following chapters these four methods will be considered consecutively and in detail.
Electromotive force is denoted by the unit termed the volt. The term pressure and voltage are used to express difference of potential or electromotive force (abbreviated E. M. E.) as well.
13. Electricity by Friction (Static Electricity).— When a piece of amber is rubbed with silk the amber is said to be electrified, and the presence of this electrification can be detected by holding the amber near to small bits of paper. The paper will be attracted to the amber. The silk is also in a state of electri- fication and if it is held near to another piece, similarly electrified, it will be repelled. Likewise two pieces of electrified amber will repel one another, and if the amber is held near the silk, the silk will be attracted to it.
This action of attraction and repulsion is said to be due to electric charges residing on these elements. The amber is said to possess positive ( + ) electri- fication and the silk negative ( — ) electrification. The electric charges are said to be caused by friction and are known as static electricity, meaning electric charges at rest or stationary.
It is to be noted that if a body containing a positive charge is brought in contact with one containing a negative charge, both charges being of equal intensity, they will neutralize and disappear ; the bodies are then to be discharged. Again if two charged bodies are joined by an electric conductor, all signs of electrification will disappear and there will pass through the conductor a momen- tary electric current.
There are other elements which when rubbed together will produce static charges of electricity, but the foregoing example is sufficient to illustrate the method. Machines for the production of electro- motive force by friction are known as static or frictional machines but since they bear no particular relation to the principles involved in the functioning of wireless tele- graph apparatus, a description will not be given.
14. Electricity by Chemical Action (Primary or Secondary Batteries). — A convenient and practical appar- atus for setting up a steady electro- motive force is the electrochemical cell which consists of two dissimilar elements, in other words, two un- like metals immersed into a dilute acid or alkali solution.
A simple cell, for example, consists of strips of zinc and carbon immersed in a conducting solution of sal ammoniac (am- monium chloride) as in Fig. 3. If the Fig. 3— Simple Electric Cell.
6 PRACTICAL WIRELESS TELEGRAPHY
exposed terminals of these plates are joined by a metallic conductor, the cell is capable of supplying a continuous flow of electricity through the wire. It is observed as the current flows that the zinc strip wastes away, in fact, the consumption of the zinc furnishes the electro- motive force necessary to drive the current through the cell and through the external circuit. The chemical changes within the cell, consisting of copper and zinc strips immersed in a dilute solution of sulphuric acid may be briefly described as follows: When the copper and zinc strips are connected together by a metallic circuit and the current begins to flow, the sulphuric acid attacks the surface of the zinc plate and forms a compound known as sulphate of zinc. During the formation of this sulphate some of the hydrogen contained in the sulphuric acid is liberated in the form of bubbles which immediately appear on the copper plate. Some of these bubbles rise to the surface of the liquid and escape into sur- rounding air, but others cling to the copper plate which gradually becomes covered with a film of hydrogen. Since hydrogen is a non-conductor of electricity, the amount of surface of the copper plate in contact with the battery solution gradually decreases as the accumula- tion of hydrogen gas increases, and accordingly the current output of the cell diminishes. In electrician's parlance the cell is now said to be "run down." Part of. this reduction of current is due to the fact that hydrogen tends to set up a current within the. cell in the opposite direction to the normal flow as well as cover the copper plate. A cell in this condition is said to be polarized, and various chemical and mechanical means have been devised to prevent the hydrogen bubbles clinging to the copper plate.
An electroscope (a device for determining the presence and nature of electric charges) indicates a strongly negative charge at the exposed end of the zinc element. The zinc plate is therefore known as the negative ( — ) pole of the cell, and the carbon or copper terminal, the positive ( + ) pole of the cell.
We learn from this that the action of the battery solution upon one plate more than on the other tends to keep the plates in a continuous state of electrification, the stronger manifestation being exhibited at the exposed end of the zinc plate and it is this difference in pressure which causes the current to flow round the external circuit.
The direction of the current inside the cell will be from the zinc plate through the solution to the carbon plate and outside the cell from the carbon plate through a metallic conductor to the zinc plate.
The conducting fluid in which the elements of the electric cell are immersed is known as the electrolyte or the exciting fluid. The plates and the metallic terminals attached thereto are termed the poles or electrodes of the cell. A number of cells connected together are known as a battery.
The type of cell just described is called a primary cell to distinguish it from a storage or secondary cell which will be described in detail further on.
It has been mentioned that the electromotive force or corresponding flow of current pro- duced by the electrochemical cell is caused by two dissimilar elements. The list of metals given below are arranged in such order that any single element will be the negative pole of the battery when used with the metal next below it on the list, and the positive pole when used with the element next above it.
( — ) Sodium Iron *
Magnesium Copper
Zinc Silver
Tin Gold
Cadmium Platinum
Lead Carbon (+)
Referring to this list, although there will be a difference of potential, and consequently a flow of current between carbon and copper if joined together by a wire and immersed in a battery solution, there will be a very much greater electromotive force if carbon and zinc are employed. 15. Secondary Cell. — A simple secondary cell popularly known as a "storage battery" consists of two or more plates of lead placed in a dilute solution of sulphuric acid as in Fig. 4. One of the plates in this diagram is connected to the positive terminal of two primary cells (connected in series) and the other plate to the negative terminal. When current flows from the primary battery for some time through the solution from plate to plate as in Fig. 5 and afterwards the wires from the primary battery are disconnected and
FOOTNOTE:— Cells of various types are described in books on elementary electricity to which the reader is referred for a more detailed description.
THE PRODUCTION OF ELECTROMOTIVE FORCE.
ANODE
LEAD mp^ m
SECONDARY CELLS
..CATHODE
the plates are joined by a conductor, current will flow from the lead plate which was connected to the positive terminal of the primary battery to the opposite lead plate, and within the cell, in the opposite direction.
j , f ; When the current flows from plate to
plate through the electrolyte, the plate connected to the positive pole of the battery receives a brown coating of peroxide of lead but the opposite plate becomes spongy or porous. Since the passage of the cur- rent through the cell has left one plate unchanged while it has coated the surface of the other plate with lead peroxide, it is reasonable to expect that if the charging battery is disconnected and the two dis- similar lead plates connected by a wire, a current of electricity will flow through the external circuit. In fact, we now have the essentials of an ordinary chemical cell. This cell will continue to supply current until the lead peroxide is partly used up and the plates will gradually return to the state they were in before the charging process took place. In order that the plates may be put in condition to deliver
PRIMARY CELLS
Fig. 4 — Simple Diagram for Cell.
"Charging" a Storage
current again, they must be reconnected to the charging source and a new coating of peroxide of lead deposited upon the positive plate. It will be seen, therefore, that it is not really electricity which is "stored up" in the storage cell but that the current supplied to the cell during the charging process produces an electrochemical change which gives the plates dissimilar properties, and so long as this change is evident, there will be a difference of potential at the terminals and therefore an electromotive force. In commercial practice storage cells are "charged" by electric dynamos or generators rather than by primary cells.
The electromotive force of primary cells varies from .06 to 1.5 volts according to the nature of the battery elements and the grade of electrolyte. The electromotive force of the lead plate secondary cell lies between 2.1 and 2.6 volts.
In electrical equations, potential or E. M. F. is represented by the letter E. Instruments for measur- ing potential difference are known as "voltmeters.
16. Current Strength and Quan- tity.— Up to this point we have not made mention of the strength of the current or the quantity of elec- tricity flowing through a given cir- cuit. We have simply referred to the potential difference and the conse- quent electromotive force generated by chemical cells. Just as we might use in the system of hydraulics the gallon per second as a unit to express the quantity of water flowing from a given source, so in electrical cir- cuits, we express the quantity of
electricity flowing by the Unit Fig. S-Elemental Storage Cell.
8 PRACTICAL WIRELESS TELEGRAPHY
termed the coulomb. We must not confound the measure of the total quantity of electricity in a given circuit with its strength or rate of flow. The strength of an electrical current should be described as the rate of flow of electricity through a circuit per second of time. When one practical unit of quantity of electricity (one coulomb) flows every second continuously, the rate of flow or the strength of the current is said to be one ampere; if three unit quantities flow continuously every second, the strength of the current is three amperes and so on. Hence iJc may define the ampere as the quantity of electricity flowing past any point in a circuit per second of time.
The strength of the current in amperes will be seen to be independent of the length of time the current flows in a given circuit whether it flows for a fraction of a second, a minute, or an hour ; if the quantity of electricity that would flow in one second is the same in any two or more cases the current in amperes is the same.
We may now define the coulomb as the amount of electricity that would pass in one second through a given circuit in which the strength of the current is one ampere.
If a current of one ampere flows every three seconds, the quantity of electricity delivered is three coulombs, or if three amperes of current flow for one second, the quantity is also three coulombs. From this we see that the quantity of electricity in coulombs is equal to the current strength in amperes multiplied by the time it flows in seconds or,
0= I X t,
Where Q = Quantity of current "in coulombs,
I = Current in amperes, and t = Time in seconds.
Hence to find out the quantity of electricity that flows around a circuit in ten minutes when the strength of the current is ten amperes, we substitute the value of I and t in this equation and multiply or, Q = 10 X 600 — 6,000 coulombs.
It is more convenient in electrical practice to measure the strength of the current in amperes than to compute the total quantity of electricity flowing ; hence, when we speak of the current available from a given electrical source, we employ the unit, the ampere, which indicates the rate at which it flows.
In electrical equations the ampere is represented by the letter I. Instruments for measuring the strength of current are called ampere-meters or ammeters.
17. Electrical Resistance. — If the terminals of a primary or secondary cell, or a battery of cells, are connected to a length of copper wire and a current measuring instrument such as the ammeter, connected in series with the circuit, a much greater reading or deflection of the ammeter will be obtained with a given length of copper wire than with an iron wire of the same length and diameter. This experiment indicates that a cell producing a constant E. M. F. (abbreviation for electromotive force) can force a very much stronger current through a copper wire than through an iron wire of the same proportions. We may conclude from this that iron offers a higher resistance to the passage of electricity than copper.
Resistance in electrical circuits may be defined as that property of bodies which opposes the flow of electric current. Just as water passes with difficulty through a small pipe of great length, but very freely through a large pipe of short length so, also, a small wire of considerable length and poor conducting qualities opposes the flow of electricity considerably, but a good conductor of short length and large diameter offers but very little resistance.
All substances are found to resist the passage of electricity but the resistance of metals is by far the least. Of all metals, silver is found to be the best con- ductor, and it therefore possesses less resistance than copper, for example. In fact, the ability of silver to conduct electricity is taken as unity or the base from which the specific resistance of other metals is computed.
The specific resistance of any material is the resistance of a piece of unit length and unit
THE PRODUCTION OF ELECTROMOTIVE FORCE. 9
cross section at an arbitrarily adopted degree of temperature. It is, in fact, the resistance of an inch cube of any substance at the temperature of melting ice.
The following table shows the relative resistance of chemically pure metals at the tem- perature of 32 degrees Fahrenheit. Resistance in Microhms Metal Relative Resistance per cubic inch
Silver annealed 1.000 .5904
Copper annealed 1.063 .6274
Silver hard drawn 1.086 .6415
Copper hard drawn 1.086 .6415
Gold annealed 1.369 .8079
Aluminum annealed 1.935 1.144
Zinc pressed 3.741 2.209
Platinum 6.022 3.555
Iron annealed 6.460 3.814
Lead 13.05 7.706
German Silver 13.92 8.217
We learn from this table that a cubic inch of German silver, for instance, has a little more than 13 times the specific resistance of a cubic inch of annealed silver.
We find by experiment that the total resistance of a conductor varies directly as the specific resistance and length, and inversely as the cross sectional area. These quantities are related in the following way :
L
R_ c — «J >
A where R = the resistance in ohms ;
L = the length of the conductor ; A = its cross sectional area; S = the specific resistance of the material.
Hence if we know the length and cross sectional area of a conductor, take the value of S from the foregoing table and substitute all three values in this formula, the total re- sistance is readily determined.
The resistance of metals is also affected by temperature; usually it increases with in- crease of temperature but certain substances decrease their resistance under rise of tem- perature, an example being carbon lamp filaments and certain electrolytic conductors such as battery solutions. The hot resistance of the carbon filament of an incandescent lamp is approximately one-half the value when cold. The resistance of a conductor, however, is always constant, if the temperature remains constant, irrespective of the strength of current flowing through it. If a conductor offers unit resistance to a current of one ampere, it offers the same resistance to a current of twenty amperes provided the tempera- ture does not change appreciably. In most circuits encountered in practice, the rise of temperature is not appreciable, but in case a conductor does heat considerably, the actual resistance can only be obtained by taking the temperature into account as well as the specific resistance.
The unit of resistance is called the ohm. The international ohm is the re- sistance offered to the flow of an unvarying electric current by a column of mercury 106.3 centimeters long, weighing 14.4521 grams, at a temperature of 32 degrees Fahrenheit.
Examples of conductors in ordinary electrical practice having approximately this value of resistance follow :
1 ohm = 250 ft. of No. 16 B. and S. copper wire, which is l-20th of an inch in diameter.
1 ohm =r 1,000 ft. of No. 10 B. and S. copper wire, which is l-10th of an inch in diameter.
One thousand feet of No. 32 B. and S. bare copper wire has resistance of 170.7 ohms. In electrical equations resistance is expressed by the letter R.
18. Grouping of Electrical Cells. — Battery cells may he grouped in three ways :
(1) In series;
(2) In parallel;
(3) In series multiple or series parallel.
Keeping in mind the units for expressing the strength and pressure of an electric current, we shall now see how the grouping of cells in various ways affects the current and pressure available for a given external circuit.
10
PRACTICAL WIRELESS TELEGRAPHY
rL5Vi
A series connection is made by joining the positive pole (the carbon plate) of one cell to the negative pole (zinc plate) of the next cell, as shown in the diagram,
Fig. 6. The upper part of this dia- gram shows two chemical cells con- nected in series and the lower figure shows ten cells connected in series as they would be represented in or- dinary electrical diagrams.
In the upper diagram of Fig. 6, current flows from the carbon plate of the cell through the switch S, through a concen- trated resistance coil R (which may be of German silver or other metal of high specific resistance) to the zinc plate of the right hand cell. The circuit continues from the zinc plate through the electrolyte to the carbon plate, from the carbon plate to the zinc plate of the left hand cell, and finally through the electrolyte to the carbon plate from which the flow originally began. When the switch S in this diagram is opened, the circuit from the battery cells is said to be broken or interrupted; but
|
S U- |
1.5V |
vvvvvvv R 1 r |
|||
|
c |
t ii |
||||
|
§& |
I |
1 |
|
c |
1 |
1 |
1 |
1 |
1 i«; |
I i |
I |
I |
I |
i! |
|
* |
r^ |
Fig. 6 — Electric Cells Joined in Series.
when switch S is closed, the circuit it called
a closed circuit, current flowing freely from the positive to the negative pole of the battery. Now the total resistance of this circuit is made up of the resistance coil R, the re- sistance of the connecting leads and the internal resistance of the battery cells, that is the resistance of the electrolyte from plate to plate. The strength of the current flowing through this circuit, as will be explained in detail further on, is governed by the total E. M. F. of the cells and the total resistance of the circuit and since the internal resistance of ordinary primary cells is rather high, it cannot be ignored in the grouping of cells.
When cells are connected in series, the total electromotive force is that of one cell multiplied by the number of cells in the group (provided all cells have identical potential) ; but the strength of the current will not exceed that of a single cell, and more likely will be less, due to the fact that the total resistance of the circuit increases as more cells are added (o the battery. Grouping of the cells in either series or parallel affects the total internal resistance as follows : When a number of cells in a battery are connected in series, the total internal resistance is equal to the sum of the internal resistances of all the cells. When a number of like cells are connected in parallel, the total internal resistance is
equal to the resistance of one cell divided 1-5 V
by the number of cells in the battery.
A parallel connection is made by connecting the positive terminal of one cell with the positive terminal of an- other cell and the negative terminal of the first cell with the negative terminal of the second cell as in the diagram Fig. 7. The left hand figure shows the direction of the flow of cur- rent of two cells connected in parallel and by this connection, the total electromotive force of the cells is no more than that of a single cell but the current available (in amperes) is that of one cell multiplied by the number of cells in the group.
Applying this to the right hand dia- gram of Fig. 7 where four cells are con-
v VWWWV — — J
|
+ |
• — |
|
1. |
5V |
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5V |
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fc |
5V |
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R |
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9 OHMS
Fig. 7 — Electric Cells Joined in Parallel.
THE PRODUCTION OF ELECTROMOTIVE FORCE.
11
UNIT A 3- 1.5V -4.5V
nected in parallel, the potential difference across the terminals is 1.5 volts and the current available in amperes will be 4 X 15 or 60 amperes. If the resistance of R is rather high, of the order of a few hundred ohms, this potential difference can be measured by connect- ing a voltmeter across the terminals. It should indicate E. M. F. of \y2 volts. We see from this that the final effect of a parallel connection in the case cited is the same as if 4
zinc plates joined together and 4 carbon plates joined together were immersed in a single battery tank.
A series parallel connection of electrical cells is shown in the dia- gram, Fig-. 8, where groups A and B each * consist of three battery cells joined in series and shunted by the resistance R.
Since groups A and B consist of three cells connected in series, then (in accord- ance with the statements made concerning
III
UNIT "B 3»I.5V«4.5V
III
R = 45 OHVAS
-vvwww
4.5 VOLTS
Fig. S; — Cells Grouped in Series Parallel.
series connection of cells in previous para- graphs) the voltage of each group will be 3 X 1.5 or 4.5 volts, but the current avail- able from either groups A or B will not be greater than that of a single cell in either unit. Assuming that each group is
capable of delivering current of 15 amperes, there will be available for the external circuit 15 -f- 15 or 30 amperes, but the total electromotive force of the two groups is that only of one group or 4.5 volts.
When the resistance of the external circuit is small in comparison with the internal resistance of the cells, there is no advantage in the series connection be- cause the flow of current will be governed by the resistance of the battery rather than by the external circuit. In this case parallel grouping of the cells is most desirable. When the external resistance is large in comparison with the internal resistance of a single cell, the cells are most advantageously connected in series. It is thus plain that the connection to be adopted will depend upon the resistance of the external circuit as compared to that of the internal resistance of the battery. In the majority of cases, the most efficient grouping is the one where the internal resistance of the cells about equals the resistance of the external circuit.
19. Ohm's Law and Practical Application. — The relation between electro- motive force, current strength and the resistance of an electrical circuit is dis- closed by Ohm's law which states that the strength of the current in amperes in any given circuit is directly proportional to the E. M. F. and inversely proportional to the resistance, or using the symbols of the previous paragraph,
E
I = - R
which may be written
Volts
Amperes =
Ohms
The student cannot overestimate the importance of this law because only as it is thoroughly understood can electrical circuits be handled and cared for intelli- gently. Applying the law practically, if an E. M. F. of 6 volts is applied to a circuit having a total resistance of 3 ohms, the current strength in amperes is obtained as follows :
6
I = - =z 2 amperes 3
By transposing this equation we may write
12 PRACTICAL WIRELESS TELEGRAPHY
E = I X R E
or R = —
I
It is plainly evident that if we know any two of the quantities involved in this expression, the third may be readily determined.
To illustrate : If the flow of current in a given circuit is 2 amperes and its total re- sistance 220 ohms, the E. M. F. applied to set up this value of current must have been. 220 X 2 = 440 volts (E = I X R).
As a second illustration we may take the case of an ordinary carbon filament carbon lamp which takes 0.5 amperes under pressure of 110 volts. According to this law, the
110 E
resistance of the filament must be or 220 ohms (R = — ).
0.5 I
We learn from Ohm's equation that to increase the flow of current through a circuit of fixed resistance we must increase the voltage. If the voltage be doubled, the flow of current is doubled and so on. By the same law if the flow of current through a given device and the pressure across its terminals can be measured, the resistance in ohms is obtained by simply dividing the pressure in volts by the current in amperes. •
Ohm's law applied to the circuit of Fig. 6 yields the following results : If the coil R has a resistance of 9 ohms and the E. M. F. of the cells is 3 volts, the strength of the cur-
3 rent through R = — = 0.33 amperes (assuming the internal resistance of the cells and
9
connecting wires to be negligible). If R had 18 ohms resistance, 0.166 amperes would flow through the circuit. If, in the left hand drawing, Fig. 7, R has 9 ohms resistance and the
1.5
E. M. F. is V/2 volts, the current = — = 0.16 amperes. Also if R in Fig. 8 had resistance
9
4.5 of 9 ohms, the flow of current would be — or 0.5 amperes.
9
If a number of electrical devices are connected in series as in diagram, Fig. 9, the cur- rent through each element is the same, irrespective of its resistance. In this diagram a source sf direct current potential B, of 100 volts is applied to the circuit comprising an electric lamp L, of 180 ohms, a resistance coil R, of 50 ohms, and a telegraph sounder S, of 4 ohms. We may calculate the current flowing at any point through the circuit such as at A, by first determining the total resistance. This, exclusive of the cells and connecting
100 wires leading therefrom, is 180 + 50 + 10 = 240 ohms; the current in amperes — -
240 0.41 amperes.
It is to be especially noted that the strength of the current through all the elements of this circuit is the same, irrespective of the resistance of the individual elements but the current is governed principally by the greater resistance, that of the lamp L.
If a voltmeter be connected to the terminals of any of the various resistance
elements of the circuit (see V in Fig. ^-;fv 9),. a difference of potential or elec-
(TD r~VV~~~! tromotive force will be found to exist
across the terminals that varies as the resistance and the strength of the cur- rent. The electromotive force may Ttenpl be calculated directly by Ohm's law si -^- anil if the resistance and the current are
R 50 OHMi
I — j~— known jf the current flowing
L 10 OHMS , , . .
^^ through each resistance element is
— -""^^ 0.41 amperes, the pressure in volts is
Fig. 9— Electrical Devices Connected in Series. obtained by multiplying the current
THE PRODUCTION OF ELECTROMOTIVE FORCE.
13
strength in amperes by the resistance in ohms. Thus the potential difference across R = 0.41 X 50 = 20.5 volts; similarly across L =- 180 X 0.41 - 73.8 volts (the calculation being made oruthe assumption that the internal resistance of the cells is zero).
20. Divided Circuits. — A divided or shunt circuit is an additional circuit provided at any part of a circuit through which the flow of current sub-divides. One branch of such a circuit is said to be in multiple or in parallel with the other branch or branches.
Fig. 10 represents a divided circuit of 3 branches, R-l, R-2 and R-3. If resistances R-l, R-2 and R-3 are equal, the current flowing from A to B will divide equally between
the 3 branches. If a current of 9 amperes
+ is flowing in the main circuit as indicated
" by the ammeter A, 3 amperes will flow through each branch. If the resistances are unequal, the current divides inversely as their relative resistance.
The current in the branches of the divided circuit, Fig. 10, can be determined by finding the voltage across the terminals of each branch, and dividing the result by the resistance of each branch.
Thus the current in branch R-l = E E E
Fig. 10 — Diagram Showing Branch Electrical Circuits. — — ? R-2 — and in R-3 '=. - — .
R-l R-2 R-3
100 100 100
R-l passes - - or 3.33 amperes; R-2 passes - - or 5 amperes and R-3, - - or 10
30 20 10
amperes. An ammeter connected in series with the circuit as at point C should indicate 3.33 -f 5 -f- 10 or 18.33 amperes. (Resistance of the connecting leads being ignored).
When several resistances are connected in parallel their joint resistance is computed as follows :
R-l 30 OHMS
R-2
20 OHMS
R-3
'0 OHMS
R =
1
where R = the joint resistance. the 3 elements is equal to:
R-l R-2 R-3 Hence, in the circuit of Fig. 10, the joint resistance of
1
= — =r 5.4 ohms. 11
30 20 10 60
It is now clear that two or -more resistances in parallel will conduct an electric current more freely than one, and the joint resistance of several resistances in parallel is less than the resistance of the smaller one.
When a nnmber of resistances are connected in series their joint resistance is the sum of several resistances taken separately.
21. Electrical Work. — When a current of electricity flows through a con- ductor, it encounters frictional resistance and a certain amount of the energy is transformed into heat. The heat of a conductor under certain conditions may be so great that unless due precaution is taken, the wire will melt. We find that when an electric current has passed through a substance, the development of heat is proportional,
(1) To the time during which the current flows;
(2) To the square of the current;
(3) To the resistance of the conductor.
14 PRACTICAL WIRELESS TELEGRAPHY
This may be expressed :
J == I2 X R X T, where J — the electrical energy expended in the form of heat in joules.
The joule is defined as that amount of energy which is expended during one second, by current of one ampere flowing through a resistance of 1 ohm and the joule per second is the practical unit of electrical power which has been named the ivatt.
joules Since power is the rate of doing work per unit of time, watts = - — .
time
Hence if 2,000 joules of electrical work are done in twenty seconds, the power exerted 2000 is - - = 100 joules per second — 100 watts. Power may also be expressed in the units
20
of pressure and current strength. The power in watts in a given circuit in which direct current is flowing is equal to the product obtained by multiplying the current in amperes hy the electromotive force in volts or
W = I X E.
Hence, if in any given direct current circuit we measure the pressure by a voltmeter, and the current strength by an ammeter, the power in watts is obtained by multiplying together the readings of each instrument.
22. Electrical Horsepower. — The unit of mechanical work is a foot pound. It is the work done in raising a mass of 1 Ib. against the force of gravity through a distance of 1 ft. Work done at a rate equal to 33,000 ft. Ibs. per minute is called the horsepower (abbreviated H. P.).
One Mechanical H. P. = 33,000 ft. Ibs. per minute = 550 ft. Ibs. per second. Also it can be shown that 1 joule = 0.7373 ft. Ibs., hence, 1 joule per second or 1 watt = 0.7373
1 watt
Ibs. per second. Therefore 1 ft. Ib. per second = - — .
0.7373
1 H. P. 1 watt 550
Since 1 ft. Ib. per second = — — or — — , therefore 1 mechanical H. P. =
550 0.7373 0.7373
746 watts.
Now 746 watts = 1 mechanical H. P., therefore
W
H. P. = or,
746
IXE
H. P. =
746
Where I = the current in amperes, E — pressure in volts. For example, an electric motor requires 30 amperes current at pressure of 110 volts; its
110X30 3300
rating in H. P. = - - = 4.4 horsepower.
746 746
1 kilowatt = 1000 watts = 1.34 H. P. 1 H. P. = 746 watts = .746 K. W.
23. Definition of Electrical Units. — The practical units of electricity may be defined as follows:
The practical unit of electromotive force is the volt, and by definition the volt is that E. M. F. required to maintain the flow of current of one ampere through a resistance of one ohm.
The practical unit of current strength is the ampere, and it is that strength of current maintained by an E. M. F. of one volt through a resistance of one ohm.
The ohm is the unit of resistance and is such resistance of conductor or circuit that permits the passage of a current of one ampere under an E. M. F. of one volt.
THE PRODUCTION OF ELECTROMOTIVE FORCE.
15
The unit of current quantity is the coulomb which is the quantity of electricity flowing in a circuit when one ampere passes a given point during one second of time.
The watt is the unit of electrical power and is equal to one joule per second. It is the power of a current of one ampere flowing under electric pressure of one volt.
In connection with these units, the prefixes of kilo, micro and milli are employed, mean-
1 1
ing respectively, 1,000 times, - - of and - - of. Thus a kilo-volt = 1,000 volts; a
1,000,000 1,000
1 1
micro-ampere — - - ampere; and a milli-volt = - - of a volt.
1,000,000 1,000
24. Current Output and Voltage of Various Devices. — For students' information, we may review here the values of voltage and current to be expected from various current sources and circuits in daily use. For example, primary cells of various types generate an E. M. F. varying between 0.6 to 1.75 volts. The current output varies with the size and nature of the elements, lying between 5 and 30 amperes for common sizes. Storage cells generate an E. M. F. between 2.08 and 2.6 volts. The rated current output may vary from 5 to 200 amperes, depending upon the size of the cell.
Generators or dynamos are constructed to supply potentials from 4 to 6,000 volts, the latter value being rarely exceeded. Certain types of generators, for instance those used in electroplating establishments, may have a current output of 10,000 amperes, with an E. M. F. of 4 to 8 volts. The electric lighting wires of homes generally carry current at pressure of 110 volts, either direct or alternating current. Transmission lines for carrying large amounts of power over great distances may have voltages as high as 200,000 volts, but the strength of the current is comparatively small. The potential of trolley wires is generally about 550 volts. Voltages in excess of 110 volts are considered dangerous to human life, particularly those in excess of 500 volts.
Fig. lOa — Portable Wireless Transmitting and Receiving Set for Junior Military Organizations.
Note : — The physical standard for the ohm has been noted. The standard for the strength of current is an arbitrary one. It is found that if a silver and a platinum electrode are dipped in a neutral solution of silver nitrate (consisting of 15 parts by weight of silver nitrate and 18 parts of water) a steady current of one ampere flowing from the silver to the platinum will deposit .001118 grams of silver on the platinum per second.
The standard for the volt is the Weston Cadmium cell which has an electromotive force of 1.018 volts at a temperature of 20 degrees Centigrade.
PART III. ELECTROMAGNETIC INDUCTION.
THE DYNAMO— THE FLOW OF ALTERNATING CURRENT.
25. ELECTROMAGNETISM. 26. MAGNETIC FIELD ABOUT Two PARALLEL CONDUCTORS. 27. THE SOLENOID. 28. INDUCED CUR- RENTS. 29. MUTUAL INDUCTION. 30. SELF-INDUCTION. 31. VALUE OF INDUCED E. M. F. 32. THE DYNAMO. 33. DE- TERMINATION OP FREQUENCY. 34. STRENGTH OF MAGNETIC FIELD. 35. DIAGRAM OF AN ALTERNATING CURRENT DYNAMO. 36. DIRECT CURRENT DYNAMO. 37. SHUNT, SERIES AND COM- POUND WOUND DYNAMOS. 38. THE ELECTRIC MOTOR. 39. THE EFFECT OF COUNTER ELECTROMOTOR FORCE. 40. MOTOR WITH DIFFERENTIAL FIELD WINDING. 41. DYNAMO AND MOTOR ARMA- TURES. 42. DEVELOPMENT OF ARMATURE WINDINGS. 43. THE ALTERNATING CURRENT TRANSFORMER. 44. ELECTROSTATIC CA- PACITY. 45. REACTANCE AND IMPEDANCE. 46. CAPACITY RE- ACTANCE. 47. LAG AND LEAD OF ALTERNATING CURRENT. 48. EF- FECTIVE VALUE OF ALTERNATING E. M. F. AND CURRENT. 49. MEASURING INSTRUMENTS OR ELECTRIC METERS. 50. INDUC- TION COIL. 51. PRACTICAL ELECTRIC CIRCUITS.
25. Electromagnetism. An explanation of some of the more important phenomena surrounding a current carrying conductor follows : If a conductor
through which a current of electricity _ is passing is laid parallel to and above 1 a compass needle as in Fig. 11, the needle will tend to turn at a right angle to the conductor, but if the cur- rent is turned off, the needle will re- turn to its original position. As we have previously mentioned a magnet suspended freely will tend to lie parallel to a given magnetic field, hence, it follows from this experi-
Fig. 11 — Deflection of Compass Needle by Electric , n r ,1
Current. meiit that the flow of current through
the wire of (Fig. 11) must have set
up a magnetic field and the direction of which is evidently at right angles to the conductor. j
// the current in a horizontal conductor is flowing towards the north, and a compass is placed under the wire, the north pole of the needle will be deflected towards the west; if the compass is placed over the zvire, the north pole of the needle will be deflected towards the east. Or, if the current is reversed in the conductor, the needle will point in the opposite direction in each case respectively. From this and other experiments we deduce that if the current in a conductor is flowing away from the reader, as in Fig. 12a, the direction of the lines of force will be around the conductor in the direction of the hands of a clock. If, on the other hand, the current flows towards the reader as in Fig. 12b, the direction of the lines of force will be around the conductor in the direction opposite to the movement of the hands of a clock or counter clockwise.
ELECTROMAGNETIC INDUCTION.
17
26. Magnetic Field About Two Parallel Conductors. — The magnetic fields of two parallel conductors are either mutually attractive or repellent, according
to the direction of the current in each.
In the diagram, Fig. 13, the current in the left hand wire is flowing away from the reader, but in the right hand wire towards the reader. Since the gen- eral direction of the lines of force is op- posite in either wire, their magnetic fields are in opposition or in repulsion. In the diagram, Fig. 14, current is as- sumed to be flowing in both wires in the same direction and since the lines of force have the same general direction, they combine and coalesce as shown by the outer lines.
27. The Solenoid. — If a number of turns of wire be wound in a spiral, as in Fig. 15, the lines of force generated by each turn of wire will unite with those set up by adjacent turns. The lines of force inside each turn will have the same general direction, forming several long lines of force that may be said to pass through the entire helix. These lines pass out of the coil at one end and enter at the other end, just as in the case of the bar magnet described in Part 1.
If the general direction of the lines of force inside this coil is from right to left, the left hand end will be a north pole, the opposite end, a south pole. The polarity of the coil may always be determined if the di- rection of the current is known. The rule is that if in looking at the end of the coil, the current flows around its turns clockwise, the end nearest to the observer will be a south pole, but if the current flows in the opposite direction, it will be a north pole.
A helix consisting of a number of turns through which current flows is known as a solenoid. We see from the foregoing that a solenoid has north and south poles and, in fact, possesses all the properties of a permanent steel magnet with the advantage that the magnetism in the case of the solenoid is entirely under control.
The strength of the magnetic field of a solenoid is proportional to the strength of the electric current passing through it and the number of turns of wire com- posing the coil, but the magnetizing power may be increased from 200 to 2,000 times by merely inserting an iron core or bar of soft iron within it.
In order that the phenomena of electromagnetic induction to be explained later may be better understood, the expansion and contraction of the magnetic field around a current carrying coil should be considered. // direct current of unvary- ing strength Hows through the solenoid, the lines of force stand still when the now of current is fully established. If the rate of How of current is increased or decreased the lines of force increase or decrease accordingly, -or stated in another
Fig. 12a — Showing Lines of Force Around a Conductor with the Current Flowing Away from Reader.
18
PRACTICAL WIRELESS TELEGRAPHY.
way, when the current rises, the lines of force move away from the wire but when the current falls, the lines of force collapse back upon the wire.
The general direction of the magnetic field around a horse shoe magnet is shown in Fig. 16. If the direction of the flow of current from the battery around the convolu- tions of the two coils is as indicated, the left hand pole has north magnetism and the right hand pole has south magnetism. A piece of soft iron A placed near to the tips of the poles will be forcibly drawn to them and will only be released when the current is turned off. If a coil of resistance wire R, such as a German silver resistance regu- lator, is connected in series with the wind- ings, the strength of the magnetic field can be closely regulated. If high values of re- sistance are inserted, the current may be re- duced to a degree that the magnet will barely attract the piece A. Variation of the cur- rent flow would affect the field of a straight solenoid winding in the same manner. The point to be taken from this is that whenever electromagnets are employed for mechanical work such as lifting masses of iron or for exciting the magnets of a dynamo or motor, the strength of the field can be regulated over certain limits by a simple variable re- sistance.
If a horse shoe of hard tempered steel be inserted in the magnetic windings in place of the soft iron core and allowed to remain for a few seconds, upon removal it will be found to be permanently magnetized.
We have explained that the direction of the magnetic field around a conductor de- pends upon the direction of the flow of cur- rent. It is clear that if the current of the magnet in Fig. 16 is reversed the polarity will be reversed as shown in Fig. 17.
The strength of the magnetic field about a solenoid can be varied by fluxes of opposite directions as shown in Fig. 18. The solenoid windings A and B are wound in opposite directions connected in series and finally to the terminals of the battery. Since current flows through the two coils in opposite directions their magnetic fields are repellent and if the coils are telescoped together (one within the other) the magnetic field will be nearly destroyed. If the two coils are partially telescoped, the resultant magnetic field varies accordingly. Advantage is taken of this principle in construct- ing an instrument known as the variometer, which is particularly useful for tuning wireless telegraph circuits.
The electromagnet in some form is employed in nearly all electrical ma- chinery, and, therefore, the laws r
J .' r i « til Fig. 13— Lines of Force About Two Conductors Carrying
governing magnetic fields Should Current in Opposite Directions,
Fig. 12h — Showing Lines of Force Around a Con ductor with Current Flowing Towards Reader.
ELECTROMAGNETIC INDUCTION.
19
have careful attention. Study of the phenomenon of magnetic induction is par- ticularly important as it is encountered at many points in a wireless telegraph set.
28. Induced Currents. — We have already seen that a magnetic field in-
variably accompanies the flow of electricity through a conductor and conversely we find that whenever a conductor is moved through a magnetic field, an electromotive force will be induced therein, and a flow of current will take place if the conductor forms a closed or continuous circuit. This is the fundamental principle upon which the operation of the dynamo or generator is based.
Experiment reveals that the production Fig. 14 — Lines of Force Around Two Conductors of the E. M. F. is conditioned by the fol-
lowing rule: The motion of the coil must
take place in such a way as to change the total number of magnetic lines of force which are enclosed by the coil.
Eor instance, simply moving a coil in a uniform field from one position to another, so that the lines of force enclosed by the coil remain of constant number, will not induce a flow of current, but if the coil is rotated, for example, so that the lines of force enclosed by it either increase or diminish, an E. M. E. will be induced which varies according to the rate at which the lines of force change.
The induction of current by a magnetic field threading in and out of a coil can be shown by a simple experiment. If the terminals of a solenoid wound with fine wire are connected to a current indicating device, such as a galvanometer, and a permanent bar magnet plunged into the interior of the winding, a momentary deflection of the galvanometer is observed. If the bar remains within the coil there is no further movement of the current indicator. If the bar be suddenly withdrawn, the galvanometer gives a second deflection in the direction opposite to that cited in the first instance. This experiment proves that the cutting of the flux through a coil of wire induces a current therein and that the direction of the current reverses with the flux. Currents will be induced in the coil if it remains stationary and the magnetic flux passes in and out of the coil, or if the field is stationary and the coil is moved through it. In either case, an E. M. F. is generated proportional to the rate at which the conductor cuts through the field.
We may substitute for the bar magnet just mentioned a solenoid winding P and cause its magnetic field to act upon a second winding S as in Fig. 19. When the circuit of wind- ing P is opened at key K, no lines of force are in evidence, but at the moment the key is closed, the lines of force expand from the core P and intersect or cut through the winding S. The galvanometer then gives a momentary deflection. If the current is left to flow through P, there is no further effect in S until the circuit of P is opened by the key; the galvanometer now gives a second momentary deflection but the needle moves in the opposite direction just as in the case of the bar magnet. Thus for each "make" and "break" of the first circuit, two pulses of current flow through the winding S, the first in one direction around the circuit, and the second in the op- posite direction. This current is said to be induced in S by electromagnetic induction.
29. Mutual Induction. — It is of
great importance to note that the effect in S takes place only when the circuit of P is made and broken. When current is flowing in P continuously, the magnetic lines of force are stationary, and conse- / i-
t ,'"'
I I
quently current is not induced in S. But when the lines of force about S rise and \ fall, then there will be a movement of cur- rent through S. If the winding S is placed at a right angle to P instead of lying paral- lel to it, a change of flux in P will have little or no effect upon S.
V
Fig. 15 — Magnetic Field of Solenoid Winding.
20
PRACTICAL WIRELESS TELEGRAPHY.
Fig. 16 — Magnetic Field of Horse-shoe Magnet with Current Flowing in Definite Directions.
HIM
Fig. 17 — Showing How Magnetic Field Reverses with Reversal of Current.
The direction of the current induced in S should be observed. When the lines of force increase through S, the induced pres- sure is counter to that which originally flowed in winding P, but when the lines of force decrease through S, the induced current has the same direction as the orig- inal current from the battery through winding P.
It is clear that the lines of force in S are in the opposite direction to those which set up the current in S. The field of force created around S therefore reacts upon winding P tending to build up a current in opposition to that already flowing in P. That is, the change in strength of the primary current in P induces a secondary current in S which in turn induces a back pressure in P. The induction due to the two circuits reacting upon each other is called their mutual induction which is a measurable quantity.
30. Self-induction. — We have seen that the expanding field of wind- ing P induces an electromotive force in winding S. Similarly the field produced by each turn in winding P will cut neighboring turns, thereby in- ducing in them electromotive forces that tend to oppose the E. M. F. of the original current. On the other hand, when the current in winding P diminishes, the lines of force contract and thereby induce electromotive forces in adjacent turns, that tend to set up currents in the same direction as the original current.
This inductive action of a coil or conductor upon itself is called self- induction.
Self-induction may be defined as the property of a circuit that tends to prevent any change in the strength of currQnt passing through it. This is clear from the fact that self-induced currents either tend to prevent the rise or the fall of current through a circuit.
The effects of self-induction are noticeable only in direct current cir- cuits when the current is turned on and off, but in alternating current cir- cuits they are ever present. All con- ductors have self-induction. the amount depending upon their size and shape. Coiled wires have greater self-induction than a long straight wire. The self-induction of a coil
ELECTROMAGNETIC INDUCTION.
21
without an iron core is practically constant. If a given coil has an iron core, the self-induction is greater in proportion to the permeability* of the iron.
The coefficient of self-induction or inductance is also defined as the prop- erty of a conductor by which energy may be stored up in magnetic form.
The unit of inductance is the henry and represents the cutting of 100,000,000 lines of force when one ampere of current is turned on and off per second ; that is, if one ampere is turned on and off, in a given conductor, the electromotive force induced by the collapse of the magnetic field, equals one volt.
Fig.
k
18 — Variation of Magnetic Field by Opposing Coils.
Fig. 19 — Diagram Illustrating the Principle of Electromagnetic Induction.
This means that a conductor or coil to have self-induction of one henry must be of such length and shape that when one ampere is flowing it is surrounded by 100,000,000 lines of force, and when the current is turned on and off, 100,000,000 lines of force cut through the conductor setting up a pressure of one volt.
This can be expressed :
M X T
L-
I X 100,000,000 where T = the total number of turns in a given coil;
M = the total lines of force threading through the coil ; and I = the current in amperes.
If M = the lines of force threading through the coil when the current = 1 ampere, then
M X T
L =
100,000,000
The unit, the henry, is applicable to coils of a great numlDer of turns having iron cores, but for coils encountered in wireless telegraph transmitters, some sub-multiple of the henry is desirable, such as the micro-henry, the milli-henry and the centimeter.
* Permeability is a measure cf the ability of a magnetic substance to conduct magnetic lines of force.
22
PRACTICAL WIRELESS TELEGRAPHY.
1,000 centimeters = 1 micro-henry;
1 1 micro-henry =.— —henry;
1,000,000 1
1 rmlli-henry —
henry;
1,000
The inductance of a given circuit is generally calculated by one of several formulae. 31. Value of Induced E. M. F. — Referring- to Fig. 19: The electromotive force induced in winding S is conditioned on the ratio of the turns in the two windings and the rate of the change of flux threading through S. For instance, if P has 100 turns of comparatively coarse wire such as No. 14 or No. 16 B. & S. wound over an iron core and S has many thousand turns of fine wire such as No. 36 B. & S., an electromotive force of several hundred thousand volts may be induced in S. Should winding S have less turns than winding P, the E. M. F. induced in S will be lower than that of winding P. Ad- vantage of this principle is taken in the design of the apparatus known as the induction coil, in which the circuit of P is interrupted from thirty to one hundred times per second. Then if winding S is given a large number of turns and its terminals are separated by a space of several inches, the voltage may be so great as to jump the gap in the form of an electric spark.
To properly distinguish the various circuits, winding P is called the primary; winding S the secondary winding. The current in P is termed the primary current and in S the secondary current.
Fig. 20 — Fundamental Diagram of Simple Alternator.
32. The Dynamo. — We may briefly define the dynamo as a machine for converting mechanical energy into electrical energy by the principle of electro- magnetic induction. But unlike the simple battery or storage cell the dynamo may generate either direct or alternating current. Alternating current dynamos are frequently called alternators. The student can nearly always distinguish between the two machines by observing the part of the dynamo at, which the current is collected. If the brushes rest on a commutator made up of a number of copper segments separated by insulating material, it will be a direct current dynamo, but if the brushes simply rest on two brass rings, it will be an alternating current dynamo.
The fundamental principle of the dynamo follows : Whenever a coil of wire rotates through a magnetic field of uniform strength in such a way that the
ELECTROMAGNETIC INDUCTION.
23
number of lines of force enclosed by the coil increase or diminish uniformly, a current of electricity will be induced in the coil, the strength of which at any instant is proportional to the rate of the change of flux. Hence the essentials of a dynamo are :
(1) A magnetic field of constant strength;
(2) A number of coils mounted on a shaft and rotated in such a way as to cut through the magnetic field;
(3) Means for conducting the current induced in the rotating coils to an outside circuit.
A diagram of an elementary alternator appears in Fig. 20. A uniform magnetic field is set up between the magnetic poles N and S by the current from battery B-l which flows through the magnet windings M, M. The rectangle of wire A, B is mounted on a shaft which rotates clockwise. Two brass rings, C, D, are mounted on the shaft but insulated from it. The copper brushes H and L make contact with these rings, and the circuit is completed through F (any current absorbing apparatus).
According to the principle just explained, if A, B rotates around its axis, an E. M. F. will be induced in the loop, the magnitude of which depends on the rate of charge of the number of lines of force threading through the loop. When in the vertical position of Fig. 20, the loop encloses the maximum number of lines of force, but when side A goes under- neath the S pole and side B goes under- neath the end pole, as in Fig. 21, the rect- angle will enclose the minimum number of lines of force when it has moved 90 de- grees or in a horizontal position. As A moves out of the field of the south pole and B out of the field of the north pole, the rectangle reaches another vertical posi- tion (but with the two sides of the rect- angle reversed) and again encloses the maximum number of lines of force. As the rotation of A, B continues, side A -goes into the field of the N pole and side B goes into the field of the S pole, where for a second time the minimum number of lines of force are enclosed after which the loop returns to the position mentioned at the beginning.
Now, according to the rule which gov- erns the direction of the flow of current in a conductor cutting through a magnetic field, when A, B, is in the position of Fig.
Fig. 21 — Showing Position of Armature Conductors for Maximum Cutting.
21, a current will flow towards the rear of the rectangle in the left hand side, and towards the front of the rectangle on the right hand side. Then if A, B, continues y2 revolution, so that side A is cutting through the N field and side B through the S* field, current will flow in A, B, in the opposite direction. It is clear that in a complete revolution, A, B, undergoes two changes of current which flows first in one direction around the rectangle and then in the opposite direction. The current is said to have gone through a complete cycle.
We see that during the first quarter revolution of loop A, B, or from 0° to 90°, the E. M. F. increases from zero to maximum; from 90° to 180° the E. M. F. decreases from maximum to zero; from 180° to 270° the current reverses and the E. M. F. increases from zero to maximum, and from 270° to 360° the E. M. F. again decreases from maximum to zero.
The changes in the strength of the current induced in A, B, can be shown by a wave-like curve as in Fig. 22, in which the successive positions of the rectangle are shown by the positions, 1, 2, 3, 4, 5, 6, 7, 8, etc. From position 8-16 the E. M. F. gradually rises, maximum E. M. F. being attained in position 4-12. This increase of E. M. F. is indicated by the ascending slope of the curve B to C. From position 4-12 on, the E. M. F. decreases (as indicated by the descending slope of the curve B to C), the minimum cutting of the lines of force taking place at point 8-16. This corresponds to the point X1 on the horizontal line. As the rectangle continues the revolution, the lines of force are cut on an increasing angle, another maximum of E. M. F. being attained at point 12-4, but of the opposite sign as
24
PRACTICAL WIRELESS TELEGRAPHY.
shown at point D. From, this point the E. M. F. decreases to zero or when the rectangle A, B, is in the position of 16-8. This curve depicts the gradual rise and fall of the E. M. F. in a dynamo coil and is known as a sine curve, the plotting of which is explained in the principles of co-ordinate geometry. The curve shows the relation between time (fractions of a second) and the strength or amplitude of the current at any given point during the complete revolution of a dynamo coil. The curve represents a complete cycle of alternating current. Vertical lines drawn from the horizontal A, B, represent time in fractions of a second. The horizontal lines drawn from the successive positions of the coil, 1, 2, 3, 4, etc., correspond to the position of the dynamo coil at any particular instant. At points where the horizontal and vertical lines intersect, a common line is drawn connecting them, which results in the wave-like curve.
33. Determination of Frequency. — The frequency of an alternating cur- rent dynamo is expressed in cycles per second. We see from the previous para- graph that one complete cycle of current is generated when A, B, makes a single revolution. Hence if A, B, rotates 60 complete revolutions per second, there will be 120 reversals or alternations of current per second. Since two alternations of
IN CASE OF 500 CYCLE ALTERNATOR Fig. 22 — Sine Curve Showing Rise and Fall of Current during One Complete Cycle.
current constitute a complete cycle, the frequency of this generator is said to be 60 cycles.
The frequency of any alternator may be determined by first counting the number of field poles and by measuring the speed of the armature per second of time, or
N X S
Frequency =
v 2
Where N = the number of field poles ;
S = the speed of the armature in revolutions per second.
Direct reading frequency meters are in daily use. They are connected in shunt to the power circuit like the voltmeter. (Frequency meter is described in Paragraph 49, section C.)
34. Strength of Magnetic Field. — The strength of the magnetic field about the poles N and S, Fig. 20, is proportional to the strength of the current in amperes and the number of turns of the coil. The strength of the magnetic field Is the same whether a current of a large number of amperes is flowing through a few turns 6f wire or a relatively weak current flows through a greater number of turns. The turns of the field winding of any dynamo are of a fixed number ; hence the strength of the magnetic field is regulated by increase or decrease in the strength of the current flowing through tthe field winding.
ELECTROMAGNETIC INDUCTION.
25
The field current is regulated by a device known as a field rheostat which is simply a variable resistance connected in series with the circuit.
The voltage developed in any given dynamo coil is proportional to the rate of cutting of the magnetic field. In the case of the rectangle A, B, of Fig. 20 or 21, the total flux passes in the coil twice and out twice during one revolution or during one cycle. If the coil enclosed 100,000,000 lines of force and made one complete revolution per second, 100,000,000 lines of force would be thrust into the coil twice and thrust out twice. This would be the equivalent of cutting 400,000,'OOQ lines of force per second and in this particular case the induced E. M. F. would be 4 volts. If the number of turns on the armature winding were doubled, all other conditions remaining equal, the voltage would
1 10 VOLT D.C.
FIELD POLE
IIOVOLTS
60-500
CYCLES
-O- -O-
LQAD
Fig. 23 — Fundamental Diagram of 4 Pole A. C. Generator.
be double, or we might state that if there were N turns, the E. M. F. developed would be N X that of 1 turn.
The fundamental equation for the dynamo is :
4XNXnXS
E —
100,000,000 X P Where E = the average voltage of the dynamo ;
n = the revolutions of the armature per second ; N — the number of conductors on the surface of the armature ; P = the number of pairs of poles ; S = the total number of lines of force. In a commercial dynamo, the only factors in this equation which are variable,
26
PRACTICAL WIRELESS TELEGRAPHY.
are (1) the density of the magnetic field and (2) the speed of the dynamo arma- ture per second. We see from this that the voltage of any generator may be increased by increase of the speed of the armature or by increase of the strength of the magnetic field surrounding the armature coils. Commercial generators are built usually for constant speed. Regulation of the voltage is obtained by means of the field rheostat.
35. Diagram of an Alternating Current Dynamo. — The essential parts of an alterating current dynamo are :
(1) Field Magnets.
(2) Armature.
(3) Collector Rings.
The diagram of Fig. 23 is merely intended to show the general details of the construction and connections of an alternating current dynamo. The field poles which are firmly bolted to the circular iron frame are represented by N, S, N, S, the armature at M and the collector rings at E, F. The field poles are wound alternately in opposite directions so that the current circulates about the turns in opposite directions, giving the poles alternatively north and south polarity. The armature M is built up of a number of slotted sheets of soft iron which are pinned together and mounted on a common shaft, the copper conductors lying lengthwise of the core in such a way that the coils will be filled and emptied with magnetic flux (coils not shown). If these coils are properly connected together, the currents induced therein (by the change of flux) will flow in the same gen- eral direction, the voltage of one coil being added on to that of the next coil. It is to be especially noted that the source of continuous or direct current for exciting the field poles of an alternator is generally supplied from an external source which may be either a small direct current dynamo known as an exciter or a battery of storage cells. In most cases encountered in wireless working, the pressure of the direct current source is 110 volts and the number of the turns of the field winding are such that the correct amount of current flows with small amounts of resistance in series at the rheostat R. As already explained, this resistance is known as the field rheostat or field regulator.
When the armature M revolves at a uniform rate, an alternating current is induced in the coils which is collected by the brushes E, F in contact with 2 collector rings, the volt- tage varying with the design of the machine. For purposes of wireless telegraphy the voltage of the generator may vary from 110 to 500 volts and the frequency of the current may vary from 60 to 500 cycles standard frequencies being 60, 120, 240 and 500 cycles.
If the armature of Fig. 23 were revolved 1800 revolutions per minute, current at a frequency of 60 cycles per second would be obtained from its armature. Remembering the formula given for determining the frequency we see that in a complete revolution of the armature any point passes through four fields which would set up four reversals of current.
If the armature revolves at the rate of 1800 revolutions per minute, corresponding to 30 revolutions per second, there would be a 4 X 30 or 120 reversals of current, or a frequency of 60 cycles. If the genera- tor had 32 field poles, the frequency would be 32 X 30 or 960 alternations correspond- ing to 480 cycles.
The student should understand that the foregoing description and drawing simply shows in an elementary way the construc- tion and functioning of a generator. The diagram is merely intended to indicate the connections of the machine, the direction of the magnetic lines of force and the method by which the voltage generated by Fig. 24.— Showing the Function of a Simple Commutator, the armature is regulated.
ELECTROMAGNETIC INDUCTION.
27
36. Direct Current Dynamo. — Direct current is obtained from dynamo coils by a commutator, which is placed on one end of the armature driving shaft. In simple form it consists of a split brass or copper ring of two parts, C, D, which is thoroughly insulated from the armature shaft (shown in Fig. 24). The circuit from the loop A, B is completed through the contact brushes E and F through an external load as at R.
The function of the commutator should be clear from the following explanation : As- sume the coil A, B to be in rotation in the direction of the arrow ; then in the particular position shown in Fig. 24, the segment D will be a (-f ) pole and segment E a ( — ) pole. The current will therefore flow in the external circuit from brush F to brush E. When A, B turns completely over so that side B goes under the south pole and side E under the north pole, the current will flow in B as it did formerly in side A, that is, towards the brush F. Similarly, when A is in the north field, the current will flow away from brush E. Therefore, the current will flow in the external circuit in the same direction as in the first case.
We see also that in the second position mentioned, the current in B is flowing oppositely to that when B was cutting through the north field, but we must keep in mind that com- mutator segment B now makes contact with brush F instead of brush E. Thus the cur- rent will flow in one direction in the external circuit irrespective of the rate at which A, B revolves.
A steady flow of current like that obtained from a battery of chemical cells cannot be obtained from the dynamo; the latter in reality generates a pulsating current. If the dynamo armature is composed of a great number of coils, the pulsations are so minute and follow each other so rapidly that the current is practically continuous. That is, these pulsations are made to overlap one another by mounting a number of loops of the armature and connecting them in series so that immediately one set of coils passes the position of maximum cutting of the lines of force, another set will take their place. The greater the number of the armature coils the greater will be the number of commutator segments required. In fact, commutators in commercial dynamos may have from 50 to 150 segments depending upon the design of the dynamo.
37. Shunt, Series, and Compound Wound Dynamos. — We have already explained that continuous or direct current must flow through the field windings of an alternating current dynamo and that this current is obtained from an external source. In the direct current dynamo, the current for excitation of the field is obtained from its own armature.
When the terminals of the field winding are tapped across the brushes of a direct cur- rent dynamo, it is called a shunt wound dynamo. The circuit for this machine is shown in the diagram, Fig. 25, where the terminals of the field winding are tapped across the arma- ture circuit at points C and D. A regulating rheostat connected in series with the field circuit at R permits an increase or decrease of the strength of the current flowing. The field winding of the shunt dynamo is composed of a large number of turns of com- paratively fine insulated wire, the actual number of turns being governed by the flux required, whereas the armature coils have comparatively coarse wire. Two paths are presented to the current as it flows from the armature of this machine, one being the field circuit and the other, the external circuit.
In well designed shunt dynamos, the re- sistance of the shunt circuit is always greater than the resistance of the armature and external circuit, but the strength of the current flowing in the shunt coil is in fact comparatively small even in the larger types of generators.
The student may question how current is set up in a machine of this type when it is first put into motion. The fact is that the initial building of the current is due to residual magnetism in the field cores. When Fl>. 25-Circuit of Shunt Wound D. C. Generator.
28
PRACTICAL WIRELESS TELEGRAPHY.
FIELD
ARMATURE
a piece of soft iron has been magnetized, no matter how soft the iron may be, a certain num- ber of magnetic lines of force are retained when the magnetizing current has been turned off. These lines are known as the residual lines of force and the cores of the field winding are said to possess residual magnetism.
When the dynamo armature is first set into rotation, the residual lines of force pass in and out of the armature conductors through the core, generating therein a feeble current which flows to the field winding and increases the number of lines of force threading through the armature coils. This induces a stronger current in the armature conductors which con- tinually adds to the strength of field until the normal voltage of the dynamo is established. The complete process usually requires from 10 to 50 seconds. After the generator armature attains its normal speed, the voltage across its terminals may be raised or lowered by the rheostat R. If the resistance of R is increased, the voltage diminishes, or if the resistance of R is decreased, the voltage increases.
A diagram of a series wound generator appears in Fig. 26. The field magnets of this type are wound with a few turns of thick wire joined in series with the
armature brushes and all of the cur- rent generated by the armature passes through the coils of the field magnet to the external circuit. The current in passing through the windings of the field magnet, energizes them and strengthens the weak field due to the residual magnetism of the cores which results in a gradual building up of the magnetic field. The important char- acteristic of this machine is its ability to furnish current at increased voltage as the load increases for it is clear from previous explanations that the greater the strength of the field current, the greater the strength of the magnetic field from pole to pple. The strength of the field current flowing through a series wound generator, and therefore the voltage across its armature is reg- ulated by cutting out turns of the field lerator. through the medium of a multi-point
switch or as may be done in the case of any type of generator, the voltage can be regulated by variation of the speed of the armature.
The compound wound dynamo combines the desirable characteristics of both the series and shunt wound machine, and it gives a better regulation of voltage on circuits of varying load than is possible with a dynamo of either type. A suitable diagram of connections appears in Fig. 27. The field magnets of the compound dynamo are wound with two sets of coils, one set being connected in series with the armature as shown at R, and another set in shunt to the armature and external circuit as shown at V. The function of the series winding is to strengthen the magnetic field by the current taken through the external circuit, and thus automatically sustain the voltage under variation of a load.
In the case of the shunt wound dynamo, as the external load is increased, the potential difference at the armature terminals will fall, but in the case of the compound wound gen- erator, this fall of pressure is counteracted by the series winding, the current which flows in it increasing with the load and causing the pressure to rise. The number of turns of each winding and the relative strength of current is proportioned so that a practically constant pressure is maintained under varying load. Initial adjustments of the voltage can of course be secured by means of a field rheostat such as shown at R-l.
The student should note carefully that current must circulate in both the
ELECTROMAGNETIC INDUCTION.
29
series and shunt windings in the same general direction in order that the re- sultant magnetic fields may have the same general direction.
38. The Electric Motor.— A motor is a machine for converting electrical energy into mechanical energy. There is essentially no difference between a motor and a dynamo. Any dynamo connected to a source of electric power will run as a motor and any motor driven by mechanical power such as a steam engine, etc., will generate a current of electricity. The differences between the two machines are mainly mechanical. The fundamental operating principle of the motor is as follows: A wire carrying a current placed in a magnetic field will tend to move in a direction at right angles both to the direction of the field and to the direction of the current. For example, if the plane of a given coil of wire lying be- tween the poles of a magnet is parallel to a magnetic field, and a current is passed through the coil, it will tend to turn or to take up a position at a right angle to the magnetic field. If the current is reversed when it has reached this position, the coil will continue to revolve.
The action of the motor can be simply explained by the diagram of Fig. 28 where a motor armature, commutator and brushes as well as the field poles, are represented in a conventional manner. If the terminals G, H, be connected to a source of direct current, part of the current will circulate through the field windings and part through the coils of the armature between the two
brushes. If the current flowing through Fig. 27— Circuit of Compound Wound D. C. Generator, the armature coils bears the correct direction to that flowing through the field winding, a state of magnetism such as shown in Fig. 28 may be produced. The upper half of the arma- ture core above the imaginary line X, will be a south pole and the lower half a north pole. The lower half of the armature will then be attracted by the south field pole and repelled by the north field pole and the upper half will be repelled by the south field pole and attracted by the north field pole. This may be stated in another way by stating that the coils of the armature tend to turn until they enclose the greatest number of lines of force from the field poles. The general strain of this attraction and repulsion is seen to be clockwise.
The movement of the armature will be continuous because the commutator acts to main- tain in the same direction the flow of current through the two sides of the armature always. Consequently, the upper half of the armature will always be a south pole while the lower half will be a north pole, irrespective of the speed at which the armature revolves.
Now it would have no effect on the general direction of rotation if the connections from
the source of current to the motor were reversed because the polarity of the flux in both the armature and the field poles would be reversed accordingly and be- cause the strain of the two magnetic fields would have the same general direction the motor would revolve in the same di- rection as before. Careful consideration of this fact reveals that in order to change the direction of rotation of the armature, the flow of current must be reversed independently in either the armature or field poles.
Like generators, motors may have series, shunt, or compound wind-
110 VOLT O.C.
Fi6'
d
30
PRACTICAL WIRELESS TELEGRAPHY.
ings. The type known as the differential wound motor appears in Fig. 29 and will be described further on.
39. The Effect of Counter Electromotive Force. — When a motor armature is set into motion by an external current, the loops of wire composing its coils cut through the magnetic field and induce a reverse electromotive force counter to that which originally caused the motion. This back pressure is known as counter electromotive force which gov- erns directly the speed of a motor. The difference between the impressed and the counter voltage determines the actual flow of current in the armature and the counter voltage is proportional to the speed of the armature, the number of armature wires and the strength of the magnetic field which is enclosed.
The speed of a motor supplied with current at constant pressure varies directly with the counter electromotive force and in any given machine the stronger the field, the slower will
be the speed of the armature. If the field of a motor be weakened by inserting re- sistance in the excitation circuit, the armature will increase its speed up to a certain point, or until the increased speed of the armature increases the counter E. M. F. to such an extent as to cut down the armature current. Up to this point, however, the speed of any given motor can be varied by simply increasing or de- creasing the field strength.
The pull* of the motor armature is di- rectly proportional to the strength of the armature current and to the strength of the magnetic field. In the case of the shunt wound motor where the field is of constant strength, the pull of the arma- ture depends upon the- amount of current through its winding. Hence if we weaken the field, the reduced counter E. M. F. will permit increased flow of current in the armature, and therefore, will increase its speed.
The speed of a shunt wound motor is self-adjusted in the following manner: If a load is thrown on suddenly, the armature will have a tendency to slow down, but this decreases the reverse electromotive force and there- fore increases the current flowing through the armature winding. This causes the motor to return to its normal speed of rotation.
We see from the foregoing that the speed of a motor can be regulated in two ways : ( 1 ) by connecting a variable resistance coil known as a field rheostat in series with the field winding; (2) by connecting a variable resistance of large current-carrying capacity in series with the external circuit or in series with the circuit to the armature itself.
The motors of the motor generators used in wireless telegraphy are designed to permit variation of the speed 20% above and below the normal speed.
40. Motor with Differential Field Winding. — As we have explained, the speed of a motor is increased or decreased by regulation of the strength of the magnetic field and any reduction of field flux of a given machine will increase the speed of the armature. By the use of the differential field winding shown in Fig. 29, the flux of the shunt field is automatically weakened in accordance with the external load and the speed therefore self-regulated. Confining our vision strictly to the windings of the field poles, two distinct set of coils will be seen, one a series winding (SR) in series with the armature and the other a shunt winding (SH) connected across the main power line. If the current in these two windings circulates in opposite directions, a differential field is produced and the resultant field will be of greater or less intensity according to the current
*The term "torque" is applied to the twisting force produced in the armature when the current is turned on. "Torque" is the result of "pull" and "leverage."
110 VOLT D.C Fig. 29— Motor with Differential Field Winding.
ELECTROMAGNETIC INDUCTION.
31
taken by the armature. A suddenly applied load will tend to slow the armature down, and this will reduce the counter E. M. F. of the armature coils ; accord- ingly increased current will flow through the series winding, which will reduce the counter E. M. F. to a still lower figure, permitting such increase of armature current as will restore the motor to normal speed.
Through use of the differential winding motors may be designed to give very close speed regulation and are therefore distinctly suitable to drive the A. C. generators for wireless telegraphy.
If we keep before us the fact that the counter electromotive force developed in a motor armature acts effectively as a resistance to the flow of current, and that this reverse electromotive force increases with the speed, it is easily seen that a considerable difference must exist between the armature resistance when standing still and its effective resistance when in rotation. If such a motor armature were started by connecting its terminals directly to the power mains an excessive current would flow which would do injury to the windings or the commutator. A device known as a motor starter is, therefore required to reduce the starting current to a safe value.
Motor starters will be treated in detail in Part IV7, paragraphs 55, 56 and 57.
41. Dynamo and Motor Armatures. — Armatures may be classified with
A particular reference to their shapes, the
r \ t \ two principal types being known as the
I El_UIIIIIII!lllllllllllllllinilllllllllllllllllll^ Tr'd,?gram,aFidg.1of
outline of the drum wound armature, the
A »• V I || I/ core of which is made up of a number of
Nlllllllllllllllllllllllllllllllllllllllllllllllllllllly thin sheets of soft iron mounted on the
* ' shaft B to form the support for armature
Fig. 30-Gencral Outline of Drum Wound Armature. coils The coUs f()r ^ armature arc
placed lengthwise in slots, one coil being shown as at A, B. One terminal, A, B, is con- nected to a segment of the commutator and the winding continues through a slot to the rear of the armature core underneath a south pole, and returns in the case of a four-pole dynamo or motor about 90° away or underneath the north pole where the second terminal is attached to the next adjacent commutator segment. A number of these coils are con- nected in series, taps being brought from the terminals of each coil to the successive seg- ments of the commutator. The iron punchings of the core 0 are insulated from one an- other by shellac or varnish to prevent the induction of current in the core as well as in the armature coils. A solid core would occasion great energy losses in this way.
An armature coil constructed of thin discs or punchings is said to be laminated. The field poles and armatures of both dynamos and motors are laminated to prevent induction losses.
In the ring wound armature shown in Fig. 31, the armature conductors are wound about a ring-shaped iron core, separated from one another and equally spaced, the term- inals of each coil being connected to ad- jacent segments of the commutator. Be- cause the conductors on the outside surface of the core only are active in cut- ting the lines of force, the ring wound armature is more or less wasteful and is seldom encountered in wireless telegraph installations.
42. Development of Armature Windings. — The subject of arma- ture windings is too comprehensive to be treated in detail here. These windings are exhaustively covered in many textbooks on dynamo engi- neering which should be referred to
for additional details. The drum Fi 31_Gen,rai Outlin? of Ring Woun<J Armatu
32
PRACTICAL WIRELESS TELEGRAPHY.
armature windings may be classed into two principal types, the lap winding and the wave winding.
The development of the lap winding is shown in Fig. 32, where a number of armature coils, numbered 1, 2, 3, 4, 5, etc., successively, are assumed to be mounted on the armature of a dynamo and to cut through the magnetic fields of the poles N. S, N. >S. The arrow indicates the direction of rotation, and the letters correspond to the segments of the com- mutator. The position of the positive (-f ) brush of the armature is shown.
In this diagram, an armature having 18 conductors revolves in a four pole field and the flow of current will be observed to have the following direction. If we start from com- mutator segment I, the point where the current enters the armature through the negative brush, then the current flows through conductor 17, through commutator segment A, through conductors I and 6, and out at segment B. The current is thus seen to take two paths from the negative brush through the armature coils to the positive brush. And it will be clear also that one side of a given armature coil lies underneath a north pole and the opposite side underneath a south pole 90° distant. The student should note carefully the direction of the flow of current in all coils of the armature winding, taking particular note of the fact that in parts of the armature, the current is flowing towards the positive brushes and in other parts, away from the negative brushes. The coils composing the
Fig. 32— Development of Lap Winding on Armature.
armature winding are connected so that the voltage induced in one adds on to that of the next coil, hence, current flows in the same general direction through various groups of coils although the sides of a given coil are under magnetic fields of opposite polarity.
It is to be noted that the brush shown in Fig. 32 short circuits a particular coil of the armature, which lies in the neutral magnetic field. It is self-evident that if a given armature coil were short circuited by a brush when the coil occu- pies a position other than the neutral position, it would be surrounded by a mag- netic field and current of great strength -would be induced therein. This would overheat or melt the conductors or at least would cause destructive sparking at the commutator.
If the armature wiring in Fig. 32 is carefully traced out, it will be observed that the winding, so to speak, laps back upon itself. It is therefore termed the lap winding. In a four-pole generator, four brushes would be required for this winding and in a six-pole generator, six brushes.
In the diagram of Fig. 32 the coils of the armature are shown as consisting of a single turn of wire but they may have several turns between segments as
ELECTROMAGNETIC INDUCTION.
33
shown in Fig. 33 where a single coil is connected to commutator segments
B and C.
Fig. 34 shows the development of the so-called wave winding. The path of the current
is as follows : Current enters commutator segment I, continuing through conductor 17,
or to segment E, continuing through conductors 9, 14, finally coming out at segment A. The current having passed through a conductor under each field pole, it returns to the commutator segment A, the one adjacent to segment I, at which it originally started. There are but two paths for the current through the armature, hence but two contact brushes are required. The majority of motors encountered in wire- less work have lap wound armatures.
The general construction of a drum-wound armature is shown in Figs. 35 and 36. Fig. 35 shows a complete Crocker- Wheeler motor generator armature. It should be observed that the D. C. armature coils lie lengthwise in slots on the iron core and their terminals are soldered in slots at the end of each commutator segment. Fig. 36 shows the terminals of the armature coils placed in the slots ready for soldering. The construction of the commutator should be noted. It is
_ made up of a number of copper bars separated by fiber
J insulating material.
I B I C I
COMMUTATOR
43. The Alternating Current Transformer.— We
011 have shown in paragraphs 28 and 29 how a varying
magnetic field threading in and out of a coil of wire wound over an iron core can induce a flow of current into another coil wound about it. Mention was made of the fact that direct current flowing through the first coil must be interrupted or its strength changed periodically to induce a current in the second coil.
It is clear from Fig. 19, that the lines of force produced by winding P cut each turn in S just once, and, therefore, the pressure or electromotive force induced
Fig. 34 — Development of Wave Winding.
in winding S increases or decreases accordingly as the number of turns in S are greater or less than in P (see paragraph 31).
The apparatus built upon this principle is known as a transformer and the different types are called step-up or step-down with respect to the ratio of the primary and secondary turns.
34
PRACTICAL WIRELESS TELEGRAPHY.
The essentials of a transformer are:
(1) A primary winding;
(2) A secondary winding;
(3) An iron core.
In order that the current may be induced in the secondary of a transformer, the primary winding must be traversed by either a pulsating or interrupted direct current or an alternating current.
Fig. 35 — Construction of Crocker Wheeler Motor Generator Armature.
A. CARICATURE
W.L!CTOR RINGS
Fig. 36 — Showing Crocker Wheeler Motor Armature with Coil Terminals Unsoldered.
Alternating current transformers for the production of high voltages may be broadly classified under two general types :
(1) The constant current transformer;
(2) The constant voltage transformer. In terms of the ratio of transformation, they may be classified as :
(1) Step-up transformer;
(2) Step-down transformer.
According to the design of the coils and the core or magnetic circuit, they may be classified as :
(1) Open core transformer;
(2) Closed core transformer;
(3) Auto transformer;
(4) Air core transformer.
Fig. 37 is an elementary diagram of a closed core, step-up, constant voltage transformer. The primary and secondary windings P and S respectively, are supported by a rectangular iron core built up of strips of sheet iron. The primary winding, for example, may consist of one or two layers of comparatively coarse wire such as No. 10 or No. 12 B. & S. gauge. The secondary winding S may have several thousand turns of fine wire such as No. 30 or No. 32.
The process of transformation is as follows : The alternating current flowing from a dynamo through the primary winding P magnetizes the iron core periodically, causing a
ELECTROMAGNETIC INDUCTION.
35
|
* |
1 |
||||
|
OLT5 C YCUS P £ C |
-_ — — • « — T~ — - |
) |
;i • i .•: :T |
- • • |
I |
|
"- |
radio work.
varying flux to flow through the iron core in accordance with the alternations of current. This varying flux induces an E. M. F. in the secondary which will cause a current to flow if the secondary circuit is closed. The current in the secondary circuit flows in the opposite direction from that in the primary circuit and as it increases, it sets up a flux in opposition to that already in the core, reducing its strength. This reduces the self-induction of the primary, permitting more current to flow in the primary and in this way the transformer becomes self-regulating — a rise of the secondary current causing an increase in the primary current.
If, for example, current at 110 volts, 500 cycles, flows through winding P, the flux will alternate through the core 1,000 times per
SeCOnd, Setting Up 1,000 alteriia- Fig. 37-Mnguetie Circuit of Step-up Closed Core
tions of current in winding S. Transformer.
Since S consists of a great number of turns, the voltage of the current induced in S will be very much greater than the voltage of the current in winding P. In fact, it is found that the voltage in the secondary winding is almost a direct ratio of the primary and secondary turns, e. g., E-s T-s
E-p ~T-p
where T-p = the current in the primary ; where T-s = the current in the secondary ;
E-p and E-s = the voltage in the primary and secondary circuits respectively. Hg. 38 shows the open core, step-up voltage, constant current transformer employed C, the core constructed of a bundle of fine iron wires or of sheet
iron is covered with several layers of insulating cloth followed by one or two layers of coarse copper wire. An insu- lating tube (not shown) is placed over the primary. It is made of some mate- rial which will withstand the heat and possess the requisite insulating qualities. Over this tube is placed a secondary winding which consists of several thousand turns of fine wire wound up in the form of pancakes as at S-l, S-2, S-3. There is little magnetic reaction of the secondary upon the primary in this type of transformer owing to the lack of a continuous iron path for the flux and the self-induction of the primary therefore remains nearly constant. The transform- er will draw practically the same current when the secondary is on short circuit as when it is open.
The closed core transformer can be designed to have this operating characteristic when fitted with a magnetic leakage gap shown by the dotted line, Fig. 37. The reactive lines of force from the secondary pass through this gap and the self-inductance of the primary winding, therefore, remains nearly constant under all variations of the secondary load.
Both the open and closed core transformers are employed in wireless telegraphy to generate current at voltages between teii thousand and fifty thousand volts at power input varying from Y4 K. W. to 500 K. W.
The ratio of transformation in the open core transformer is not exactly in proportion to the turns, due to magnetic leakage. The design is, therefore, altered to meet these conditions. Generally the secondary is given more turns than the usual transformer equation would require.
The so-called auto transformer with a step-up ratio of turns is shown in Fig. 39. In this type the primary and secondary windings have turns in common, a single coil being
Fig. 38 — Open Core Transformer.
36
PRACTICAL WIRELESS TELEGRAPHY.
39 — Auto Transformer, copper strips, the tubing being from l/> to
used for both circuits. A portion of the current flowing in the secondary winding is in- duced by the passing of the flux through the core from the primary turns, but another portion flows into the secondary circuit by direction conduction.
Although transformers of this type are not employed for the production of high voltages (with low frequency currents), they are frequently used as step-down transformers to
obtain 10 to 30 volts of alternating cur- rent from a 110- volt source. Without an iron core, auto transformers are used in the circuits of radio frequency in both the transmitting and receiving apparatus of wireless telegraphy.
The air core transformer in Fig. 40 is used principally in radio-frequency cir- cuits for transferring oscillations at ex- tremely high frequencies from one circuit to another, and when used in this man- ner it might properly be called a radio- frequency transformer. For such a trans- former if used in the transmitting ap- paratus of a radio set, winding P is made of a few turns of coarse copper tubing or inch in diameter for the small size sets. Wind- ing S may have several turns, a dozen or more of insulated wire or small copper tubing. On the other hand, if the auto transformer is used in receiving sets, winding P may consist of several hundred turns of No. 24 B. & S. wire, and winding S may have several hundred turns of No. 32 B. & S. wire.
Whether or not the voltage of the sec- ondary circuit will be greater or less than in the primary, in radio-frequency transforma- tion, depends upon the values of capacity included in either circuit as well as the ratio of the turns. Owing to the phenomenon of resonance and the effects of capacity, a step- up ratio of turns may be the equivalent of a step-down voltage or vice versa.
When the primary circuit of an open core transformer is supplied with interrupted di- rect current, it is called an induction coil. This coil will be described in detail in Para- graph 50.
44. Electrostatic Capacity.— In or- der that certain phenomena involved in the flow of alternating current may be understood, it will be necessary to con- sider another quality of an electric cir-
30,000 TO 1,000.000 CYCLES
SECONDARY
PRIMARY
Fig. 40 — Radio Frequency Transformer.
cuit known as electrostatic capacity. We have mentioned two qualities of an electric circuit, i. e., resistance and inductance. The third quality, capacity, is of particular importance in wire telegraph apparatus and will now be defined.
Further on, we shall show how
c these three qualities govern the flow
of an alternating current.
Capacity may be defined as that property of a conductor or circuit by which energy can be stored up in electrostatic form. The electrostatic capacity of a conductor is measured by the quantity of electricity in coulombs with which it must be charged to raise its potential to one volt/
EFFECTIVE VALUE 10 5 AMP
Fig. 41— Rise and Fall of Alternating Current.
ELECTROMAGNETIC INDUCTION.
STATIC FIELD
Fig. 42 — Simple Condenser.
A device for storing up energy in the form of an ^electrostatic field is known as a condenser. When two copper sheets or other conducting material are separated by a small air space as in Fig. 42, and a source of direct or alternating current connected to
the two plates, the intervening space fills up with electrostatic lines of force. If the charging source be disconnected, and the terminals of the condenser be recon- nected to a galvanometer, the latter will give a momentary deflection indicating the passage of an electric current. This experiment proves that the electrostatic field within a condenser will, when re- leased, set up a flow of an electric current. The unit for expressing the ca- pacity of a condenser is the farad which is a condenser of such dimen- sions that one volt of electricity will store up in it a charge of one coulomb. The farad is too large for practical measurements, hence the micro-
1 farad is in general use. One (1) microfarad = - farad.
1,000,000
The quantity of electricity that can be placed in a condenser is directly proportional to its capacity and the difference of potential between its plates, or Q — C X E- Hence, a condenser of .000002 farad capacity, charged to a potential of 10,000 volts would have stored up in it 10,000 X .000002 — .02 coulomb.
When current flows into a condenser, its potential difference rises uniformly until the E. M. E. of the condenser and that of the charging source are equal. At any instant,
Q
the E. M. F. of the condenser is proportional to — but since the charging process is uniform
C E
the average E. M. F. = — . 2
The work done in joules in placing a quantity of electricity into a condenser — the quantity of electricity multiplied by the average E. M. F., hence
E
the work in joules = Q X —
2 Since Q = E X C
C E2
therefore W — EXCX — —
2 2
Where W = the work in joules. Now if a condenser is charged N times per second, the power is expressed:
C E2 P=i- - X N
2 . Where N = the number of charges per second ;
C =i the capacity of the condenser in farads ; E — the potential difference in volts.
Hence if a condenser of .002 microfarad capacity were charged to a potential of 30,000 volts by a 500 cycle alternator, the power expended in watts would be : .000000002 X 30,0002
• X 1,000 = 900 watts. 2 A condenser of concentrated capacity always consists of :
(1) Two or more opposing surfaces;
(2) An insulating medium between the plates which may be air or any of the well-
known insulating materials, such as glass, micanite, hard rubber, waxed paper, etc.
38 PRACTICAL WIRELESS TELEGRAPHY.
This medium is known as the dielectric. The capacity of a condenser is found to vary:
(1) Directly as the area of the opposed surface and the ability of the dielectric to
conduct electrostatic lines pf force;
(2) Inversely as the separation of the plates.
This may he written :
K X A X 2248
T X 1010 Where C = the capacity of the condenser in microfarads ;
A = the area of the opposed surfaces in squares inches ;
K = a certain constant ;
T = the separation of the opposed surfaces, or the thickness of the di-electric.
It can be proven that different di-electric mediums conduct static lines of force with more or less ease depending upon their nature. Air is taken as unity and all other insulating mediums are compared to it. Certain grades of glass are said to have a dielectric constant of 9, meaning that a condenser with a plate of glass between conducting surfaces will permit 9 times the quantity of electricity to be stored up as with air at ordinary pressure. In the same way, the dielectric constant of micanite is said to be 5, paraffin paper 2, etc. (Note complete table in the Appendix).
Condensers of large capacity are made by taking a number of sheets of tin or brass foil and separating them with thin sheets of waxed paper or other insulating material, alternate sheets of foil being connected together on either side, so there is no direct con- nection between them. This constitutes a condenser of concentrated capacity which may store up temporarily considerable amounts of energy in electrostatic form.
Condensers may be classified with respect to their dielectric strength which may be defined as the ability of the dielectric to resist puncture when subjected to electric pressure. Condensers which will withstand high voltages without rupture of the dielectric are termed high potential condensers and conversely those which will withstand low voltages only are called low potential condensers. High voltage condensers are used in circuits of several thousand volts pressure. Low voltage condensers are employed in circuits of less than 500 volt pressure.
We have mentioned that a condenser when first connected to a charging source, has zero potential, and as the current flows, the potential difference rises until the voltage of the condenser is equal to voltage of the charging circuit ; the flow of current then stops. If the applied potential is decreased, the condenser will start to discharge and current will flow out in the opposite direction to which it was charged. The voltage of the condenser is thus seen to set up a back pressure which tends to drive the charging current back.
We have already seen how inductance tends to prolong the flow of current in a circuit and we now see that the condenser tends to extinguish it or drive it back. Thus the back pressure of the condenser opposes that set up by an in- ductance coil. We shall now see how these counter E. M. F.'s govern the flow of alternating current.
The effects of self-inductance will first be noted.
45. Reactance and Impedance. — When a coil of wire is connected to a source of direct and then to a source of alternating current of the same voltage, the flow of current (in amperes) will be considerably greater with the former connection than with the latter. This is due to the fact that the counter E. M. F. of self-induction in a direct current circuit is only momentary, the effects being observed when the current is turned on and off, whereas in a circuit carrying alternating current, the effects of self-induction are continuous and the back pressure resulting therefrom must always be considered to determine the strength of current.
The flow of a direct current through a given circuit is opposed only by the ohmic resistance, but the flow of alternating current is impeded by the counter electromotive force of self-induction as well as by the ordinary resistance. The
ELECTROMAGNETIC INDUCTION.
39
extra resistance of self-induction is termed reactance, and is expressed in equiva- lent ohms. The combined opposition of reactance and resistance in any circuit is termed impedance, and accordingly the flow of current through a circuit carrying alternating current is governed by the impedance and not alone by the ohmic resistance. It should be understood that the counter E. M. F. of self-induction entails no loss of energy in an electric circuit as does resistance (where the energy is lost in the form of heat), but a higher voltage is required in that circuit to force a given value of current through it.
The flow of alternating current is nearly always controlled by coils of high self-induction which are termed reactance coils or "choking" coils.
The reactive pressure occasioned by a circuit loaded with inductance is termed in- ductance reactance. It is expressed :
Reactance = 6.28 X N X L Where N — the frequency in cycles per second. L =.the inductance in henries.
If the coil L of Fig. 43 has inductance of .055 henry, and it is connected to a 60- cyqje alternator, the inductive reactance = 6.28 X 60 X -055 = 20 ohms. If the frequency be increased, the reactance (in ohms) increases in the direct ratio; thus if N == 100,000 cycles, the frequency of a radio-frequency alternator, then the reactance of coil L = 34,540 ohms.
The flow of current through L is governed both by the reactance and the resistance, and the impedance of such a circuit is expressed as follows :
60 CYCLES A C
43 — -Alternating Current Circuit with Concentrated Inductance.
Impedance = V
Where R = the resistance of the coil in ohms;
X = the reactance of the coil in ohms.
Then if the coil L of Fig. 43 had inductance of .055 henry, resistance of 10 ohms and reactance of 20 ohms, then
Impedance = V 202Xl02 = 22.3 ohms approximately.
E For direct or continuous current, Ohm's law is expressed I — — , but for alternating
R E current the formula is modified to, I =r — . If the pressure of the alternator is 110 volts,
Z
110 then there will flow through L, - - — 4.9 amperes nearly.
22.3
46. Capacity Reactance. — We have shown how a condenser connected in series with an alternating current circuit acts as an effective resistance and exerts a back pressure on the charging E. M. F., and also that this back pressure opposes that set up by inductance. To distinguish these counter E. M.'F/s, the reactance occasioned by inductance is expressed as positive reactance and that by a con- denser as negative reactance,
The capacity reactance of a condenser is determined as follows :
1 Capacity reactance = —
6.28 X N X C Where N = the frequency of the current in cycles per second ;
C = the capacity of the condenser in farads.
The important point to be noted from this formula is that a large condenser will have a small value of reactance and conversely a small condenser will have a large value of reactance.
40
PRACTICAL WIRELESS TELEGRAPHY.
If the condenser C connected in series with the 60 cycle alternator of Fig. 44 has capacity of .00013 farads, and the frequency of the alternator is 60 cycles, then
1
Capacity Reactance = = 20 ohms approximately.
6.28 X 60 X .00013
If the frequency of the alternator is 100,000 cycles, then,
1
Capacity Reactance =
— = .012 ohms.
6.28 X 100,000 X -00013
It is clear that by proper selection of capacity and inductance values in the alternator circuit of Fig. 45, the counter electromotive forces can be made to balance and the reactance there-
1 fore reduces to zero, or 2ir N, L = —
27T N,C
The circuit then acts as if neither induct- ance or capacity were present and the flow of current is governed solely by the ohmic resistance of the circuit.
If capacity reactance overbalances in- ductance reactance, then the resultant value takes the notation of the predom- inating figure, e. g., if the capacity react-
60 CYCLES A C
Fig. 44 — Alternating Current Circuit with Condenser Series.
>" ance exceeds the inductance reactance, the difference between the two will be ex- pressed in ohms, and the circuit said to have so many ohms capacity reactance. In case inductance reactance predominates, the opposite statement applies. We see from all this that a much greater current can be made to flow through the circuit from the alternator by the use of a condenser and a coil than if but one of these were used.
Reviewing the foregoing, it is clear that the reactance of a given coil for frequencies in excess of 100,000 cycles per second (as compared to lower frequencies) may attain a rather large value. It is therefore nec- essary in such circuits to insert a cer- tain amount of concentrated capacity to build up the current. In the radio- frequency circuits of wireless telegraph apparatus, current flows at frequencies between 20,000 and 1,000,000 cycles per second and if this current is to be trans- ferred by magnetic induction from one circuit to another, the second circuit must contain a certain amount of in- ductance and capacity of such values that inductance reactance and capacity reactance neutralize one another. The
60 CYCLES A.C
Fig. 45 — Showing How Resonance Is Obtained in Alter- nating Current Circuits.
second circuit is then said to be resonant to the impressed frequency and the flow of current is governed solely by its resistance.
Straight wires possess both capacity and inductance, which are said to be dis- tributed rather than concentrated as in the case of a condenser or a coil of wire The laws of electrical resonance apply to such circuits as well as those having concentrated capacity and inductance.
47. Lag and Lead of Alternating Current. — A certain phenomenon, in- volved in the flow of alternating current throughout a given circuit, is termed phase displacement. Given a circuit in which inductance reactance predominates, it is found that when a given alternating electromotive force is applied thereto, the pressure and current do not reach their maximum values simultaneously. The current lags behind the impressed voltage by a certain degree dependent upon the self-induction of the circuit and such a circuit is said to haye a lagging phase.
As it is convenient to express a complete cycle of current in terms of the degrees of a circle, l/4 cycle being equivalent to 90°, l/i cycle to 180°, and so on, we express the lag of the current in terms of the degrees of the circle. Hence, a certain circuit is said to have
ELECTROMAGNETIC INDUCTION. 41
an angle of lag of 35° or some other degree dependent upon the constants of the circuit. (All this is explained in any strictly theoretical text book on alternating current).
In a circuit wherein capacity reactance predominates, the opposite condition is obtained, e. g., the current leads the voltage reaching its maximuhi value before the impressed E. M. F. A circuit of this type is said to have a leading phase.
The point to be brought out here is that in circuits having either lead or lag, the actual power consumption in watts cannot be determined from the reading of a voltmeter or ammeter. To illustrate : When a voltmeter or ammeter are connected in the primary wind- ing of a high voltage transformer of the type used in wireless telegraph transmitters, the voltmeter may indicate 110 volts and the ammeter current strength of 14 amperes. Apply- ing the power formula in simple form the apparent reading in watts would be 110 X 14 = 1,540 watts, but a wattmeter connected in this circuit may indicate a reading of 1,000 watts which is the true reading because the wattmeter is constructed to read correctly inde- pendently of the degree of phase displacement. The result obtained by multiplying the pressure by the current is only an apparent reading of watts ; but the true reading is always obtained by the meter. The ratio of the true watts to the apparent watts is ex- pressed by the term, the power factor. Tn the circuit taken as an example, the power
1000
factor = — , or approximately 65%. 1540
R The power factor can also be obtained from the ratio of — , that is if the impedance
Z
and resistance are known, and the value of the former is divided by the latter, the power factor of the circuit is obtained.
lie formula for power in direct current circuits, W = I X E is changed in the case of an alternating current circuit wherein the current lag's behind the voltage to read : W i= I X E X Cos. 0.
The cosine <i> is the power factor expressed as a function of an angle of a circle and
R as mentioned, is equal to — . Hence if the total resistance of a given circuit is known, also
Z
the total impedance and the reading of current and E. M. F. is obtained by an ammeter and voltmeter respectively, we can determine the true power in watts in any circuit without the use of a wattmeter.
48. Effective Value of Alternating E. M. F. and Current.— It is self- evident from the alternating current curve of Fig. 41 that the current constantly changes in value' as well as reversing its direction. Hence, to express the effectiveness of a given electromotive force in such circuits, we must employ some value other than the maximum E. M. F. or maximum current per alternation. Take, for example, any given circuit in which the maximum current for each alternation amounts to 15 amperes (as in Fig. 41), it is evident that at all points off maximum during the complete cycle, the strength of the current is less than 15 amperes. It is clear that we must take some sort of an average value in order to determine the effectiveness of an alternating current. Since the heating effects of direct current in a given circuit are uniform, the effectiveness of an alternating cur- rent is expressed in terms of the strength of a given amount of direct current which would produce the same power or heating effect. To illustrate: If 15 amperes of direct current pass through a resistance of 2 ohms, the power of the current converted to heat will be \- X R = 152 X 2 = 450 watts. Now if we pass an alternating current through the same wire and adjust its strength until 450 watts are consumed in the form of heat, we would then have 15 amperes of alternating current flowing.
This is the so-called effective value of the alternating current which in the case of a sine wave curve is found to be .707 of the maximum value per alternation. Suppose, the maximum value of current per alternation in the curve of Fig. 41 is 15 amperes, then the effective value will be 15 X -707 •=. 10.5 amperes. That is, the current rises and falls uniformly between a value of -(-15 amperes and — 15 amperes producing the same heating effect as a direct current of 10.5 amperes. Now an ammeter connected in such a circuit would indicate 10.5 amperes because these instruments are constructed to indicate the effective value of current and not the maximum value per alternation. Similarly, voltmeters indicate the effective voltage in a given circuit. All this means that the maximum voltage per cycle of an alternating current supplied from power mains at
42
PRACTICAL WIRELESS TELEGRAPHY.
pressure of 500 volts, is somewhat greater, in fact, is 500 X 1.41 = 705 volts. Similarly the maximum voltage per alternation in 110 volt alternating current circuits is 155 volts. (This is only true when the wave form of the current follows the curve of sines.)
When speaking of the pressure of the high voltage transformers used in wireless telegraphy, the secondary voltage is generally given as the maximum voltage per cycle and not the effective value.
The student will see from the foregoing that the problems of alternating current circuits are largely different than those of direct current circuits and that the flow of current is governed by conditions other than the ohmic resistance. Also the actual power consumption in watts depends upon whether or not the pressure and current in a given circuit are in exact phase.
49. Measuring Instruments or Electric Meters. — The principle measuring instruments employed in connection with a wireless telegraph transmitter are :
(1) The voltmeter;
(2) The ammeter;
(3) The wattmeter;
(4) The frequency meter;
(5) The hot wire ammeter.
In the circuits of a radio-transmitter, these instruments occupy the positions fol- lowing: The voltmeter is joined across the terminals of the alternator; the ammeter is connected in series with the primary winding of the transformer; the wattmeter is connected in the circuit from the alternator to the transformer; the frequency meter is shunted across the terminals of the alternator; the hot wire ammeter is used prin- cipally in circuits of radio-frequency, and to some extent, in circuits of lower frequency. Before entering into a description of these meters, we shall explain the workings
of the current-detecting instrument knovi the galvanometer. This instrument may take one of several forms, but the type shown in Fig. 46 is the least difficult to understand. A rectangular coil of sev- eral turns of copper wire D is suspended between the poles of a horse shoe permanent magnet, P, P. Between the poles is the stationary iron core C* The coil is suspended from the screw F and the current to be measured enters at the wire A and leaves by the wire B. When current is passed through the coil B, it tends to turn so as to include the greatest number of lines of force but is resisted by the torsion of the suspending wires. If a pointer and suitable scale are attached to this coil, comparative readings of the strength of current may be made. In- struments of this construction are sen- sitive and will easily measure a current of .000001 of an ampere. It is an impor- tant instrument to demonstrate the ele- mentary principles of electromagnetic induction, and should be a part of all students' equipment.
Now if the coil D had several
Fig. 46 — Simple Galvanometer.
thousand ohms resistance, the galvanometer might be calibrated in volts and em- ployed as a voltmeter. If, on the other hand, coil D were wound with a few turns of relatively coarse wire, it might be calibrated in amperes and would, therefore, be known as an ammeter. As an ammeter it would be connected in series with the circuit under measurement.
*The Core C intensifies the field across the air gap P to P.
ELECTROMAGNETIC INDUCTION.
— vWW
(a) The voltmeter may be constructed along the lines of galvanometer. A simple drawing of the Weston voltmeter appears in Fig. 47.
A rectangular coil of fine wire, A, B, is mounted on the metal bobbin G. It is supported by jewelled bearings and held in the zero position of the scale by the spiral springs, S-l and S-2, through which the circuit of the coil is com- pleted. When the pointer is in zero po- sition, the coil rests at a slight angle to the pole pieces of a permanent magnet. N, S. When current is flowing through the coil, the normal field from N to S is "lengthened" out and in trying to "shorten" themselves, the lines of force actually "twist" or turn the coil. When the tension of the spring is equal to the pull of the magnetic field, the pointer comes to rest and the reading of the instrument may then he observed. An external resistance coil, R, is connected in series with the winding of the bobbin to reduce the flow of cur- rent to a minimum value. This coil may have resistance of 100,000 ohms and may be provided with two taps making the meter a double scale instrument.
There is essentially no difference in the construction of the ammeter and
the voltmeter except the resistance of the windings and the calibration. The coils of the ammeter have relatively low resistance whereas the voltmeter as already mentioned, has high resistance.
The windings of an ammeter may be proportioned to carry a small amount of current, but the meter can be used to measure very large values by connecting its terminals across an external shunt as in Fig. 48. This shunt consists of a number of metal strips of comparatively low resistance stretched between two large copper lugs. A potential difference exists across the terminals of the shunt which causes a certain amount of the current to sub- divide and flow through the meter. An increase of current through the shunt will increase the flow of current through the instrument, and the meter, therefore,
may be calibrated to read very large Flg" 48-Show'«* the Use of a Shunt Wlth an Ammeter" values of current, although but small values pass through the instrument itself. Such instruments are generally supplied with a certain length of connecting leads between the shunt and the instrument. The length of these leads must not be altered or the calibration of the instrument will be interfered with. In electrical diagrams, both the voltmeter and the ammeter may be designated by the symbol of Fig. 49.
One type of meter for use in alternating current circuits is shown in Fig. 50. Coil VV is a spool of wire with several layers like the winding of an electromagnet. The shaft carrying the pointer P has the semi-cylinder of iron A surrounded by the brass semi-cylinder B. Outside B is another semi-cylinder of iron C. When current is flow- ing through the coil W, the semi-cylinder A tends to move into the unfilled space of C, but is resisted by the spiral springs S.
If coil W is wound with fine wire and has a coil of high resistance connected in series, it is a voltmeter, but if W has a coarse wire winding, the instrument becomes an ammeter.
(b) The wattmeter is a positive necessity for determining the power flowing
AMMETER
LOAD
44
PRACTICAL WIRELESS TELEGRAPHY.
Fig. 49— Symbol for Measuring Instru- ment.
in an alternating current circuit because, as already explained, the product of the
volts multiplied by the amperes does not give the true reading of watts. Due to
the self-induction of the circuit, the E. M. F. and current do not reach their maxi- mum values simultaneously. The current in fact, lags behind the impressed E. M. F., and there- fore the product of volts multiplied by amperes gives what is known as an apparent reading of watts. This lagging of the current behind the E. M. F. is known as phase displacement.
Wattmeters are constructed to be independent of phase displacement and they will give true readings of power consumption over the range for which they are designed. The student will always recognize the wattmeter by it having four binding posts; tzvo very large binding posts which are connected in series with the apparatus under measurement and two small binding posts which are connected in shunt to the terminals of this apparatus.
The general design for this instrument is shown in Fig. 51. Coil A, called the current coil, is connected
in series with the load; coil B, called the voltage coil, is connected in sliunt to the load
but a large resistance R is connected in series. The position of the coil A is fixed, but
coil B is mounted on bearings and fitted with a pointer P which is held in the zero
position by a spiral spring S. When current is passed through these two coils, two
magnetic fields are set up, which act mutually to pull the movable coil B parallel to the coil A. The current in the pressure coil will vary as the potential difference between its
terminals and the current through the series coil will vary as the current in the circuit
in which it is inserted. The force acting
upon the movable coil will be proportional
to the product of the current and potential
difference. That is, the deflection of the
coil is proportional to the power of the
current flowing in the circuit, and the
scale of the instrument may be calibrated
directly in watts. In the diagram binding
posts C, C, are for the current coil, and
posts V, V, are for the pressure coil.
(c) Frequency meters are not ex- tensively supplied to commercial wire- less telegraph sets, but one of these in- struments is always a- part of the radio inspector's testing equipment. The Hartmann & Braun meter is shown in Fig. 52. It has much the appearance Fig-
of a voltmeter, and, like that instrument, it is connected in shunt to the terminals of the alternator.
In this diagram the single elongated magnet winding M has joined in series with it the coil R ; the two terminals E, E are shunted across the circuit under measurement.
The soft iron piece, P, P, completes the magnetic circuit for the poles of the horse- shoe magnet, N, S, N, S, etc. A number of small vibrating reeds are placed directly in the path of the flux between the soft iron piece and the poles of the magnet. Each of these reeds have a different period of mechanical vibration and, consequently, are only set into vibration by the flux of the magnet when it alternates at such rates as to correspond to the natural mechanical period of the reed.
Four permanent horse-shoe magnets, N, S, N, S, etc., keep the core in a constant
ELECTROMAGNETIC INDUCTION.
45
LOAD
state of magnetization, but wh'en alternating current flows through winding M, the reed having a natural period corresponding to the particular frequency of the current flowing will be set into violent oscillation. The scale reading corresponding to this
particular reed is the frequency of the cir- cuit under measurement. The instrument will perhaps be better understood from the side elevation, Fig. 52. Frequency meters of this type are very accurate.
(d) The mechanism of one typt of hot wire ammeter is shown in Fig. 53. Meters of this construction are particularly suitable for measurement of the current at radio-frequencies. It should be self-evident that measure- ment instruments having bobbins or coils of wires are totally unsuited to this work, first, because the current of high voltage and high frequency would burn out the coil, and, second, the length of the windings would seriously affect the oscillating prop- erties of the circuit. The self-induc-
, • ^ 5 ' *c ^> tion of the hot wire in a hot wire
l/n p meter is practically zero, and there-
fore there is no danger of burn-out or short circuit.
In the diagram, Fig. 53, a steel plate P is made to pull against the wire C, D, by the spring S-l. One end of the wire C, D is attached to the plate P, passed about the pulley K, and again attached to P at point R where it is insulated. The pulley K carries the arm S with two prongs between which is stretched a silk thread T wound around the shaft X. X carries the pointer P1, which moves over the scale.
The current to be measured enters the wire at point A and leaves at the shaft K; as the current flows, the temperature of the wire rises, causing it to expand, but owing to the tension of S-l, the slack is taken up at side B and equilibrium can be re- stored only when the pulley K rotates sufficiently to equalize the pull on the spring. The rotation of K carries S with it, and S in moving causes the silk fiber to rotate the shaft which carries the indicating needle.
When the hot wire ammeter is used to measure large values of current, a shunt must be supplied to sub-divide the current flow, but an inductive shunt, even jvith one-half a turn of wire can- not be employed to measure current at radio-frequencies because the inductance of the shunt would vary with each change of frequency. Consequently, hot wire meters are constructed after the de-
Fig. 51 — Mechanism of Wattmeter.
sign of Fig. 54, where several resistance
Fig. 52 — Mechanism of Frequency Meter.
wires are stretched in parallel between two large copper blocks, B, B1. All of these wires are of small diameter, such as No. 36 or No. 40 B. & S. gauge, hence they offer practically the same resistance to current of radio-frequency as to a direct current; that is, irrespective of the frequency of the current, the reading in amperes will be accurate. One of the wires, C, D, is selected to work the indicating mechanism in the
46
PRACTICAL WIRELESS TELEGRAPHY.
DAMPING DISC
Fig. 53 — Mechanism of Hot Wire Ammeter.
following manner: A wire, E, F, is attached to the center of C, D, and a silk thread attached to it at K, which is wound about the shaft in such a way as to work against the tension of the spring S which, normally, would cause the pointer to move to the full scale position. However, by means of the thread, it is drawn to the zero position of the scale. When current is flowing through C, D, the expansion of the wire releases
the pull of the thread which allows the pointer to move across the scale ac- cording to the degree of expansion and the tension of the spring.
The fundamental principle of another instrument for measuring radio-frequency currents, used by the Marconi Company, is shown in Fig. 55. The complete details for construction are not opened for pub- lication, but briefly, the operation is as follows : The current of radio-frequency Hows through several wires stretched be- tween blocks B and B1. A thermo- couple mounted on one of these wires as indicated at C, D, is heated by the current of radio-frequency flowing be- tween the copper blocks. As is well known, this junction sets up a direct current which flows through the meter A. The latter is a sensitive di- rect current 'instrument with magnetic windings, and is calibrated directly in amperes.
The production of E. M. F. by a thermo-couple may be better understood from Fig. 56. If a piece of bismuth B and antimony A be soldered or welded together and their ends connected to a galvanometer, then if the temperature of the junction is raised higher than the remainder of this circuit, a current flows in the external circuit from antimony to bismuth. If the junction is cooled below temperature of the rest of the circuit, current will flow in the opposite direction. There are a number of other metals which when joined and heated will produce a direct E. M. F. under change of temperature, notably copper and iron, which are frequently used as a thermo- junction.
50. Induction Coil. — \Ye have shown how an alternating current can be raised to a pressure of several thousand volts by means of the alter- nating current transformer. In a somewhat .similar manner,, a direct current of low voltage can be changed to an alternating current of many thousand volts by an apparatus known as an induction coil.
Fig. 57 is a diagrammatic sketch of an induction coil. P is the primary winding and S the secondary winding. B is a piece of spring brass fitted with a soft iron button that may be attracted by the core C. A is an adjustable thumb screw, platinum tipped, which makes contact at C1, closing the circuit of the battery through winding P. In practice sole- noid S is wound about C.
When the battery circuit is closed at K, the core becomes saturated with magnetism and attracts the armature B. B being drawn to the end of the iron core, the flow of current is broken at C1. Since the current is now cut off from P the magnetic field disappears and the tension of the spring causes the circuit to be closed again at C1. This process is repeated continuously, resulting in from 30 to 100 breaks per second.
Hot Wire Ammeter with Internal Shunt.
ELECTROMAGNETIC INDUCTION.
47
9
The coil P generally has 1 or 2 layers of coarse insulated copper wire of different sizes (according to design) between No. 12 and No. 16, which are thoroughly insulated from the core C. Winding P is covered with an UILLIAMMFTFR
insulating tube which supports winding S. The secondary winding may have many JUNCTION
thousand turns of very fine wire which are wound in the form of pancakes and connected in series. Thus the electromo- tive force at the terminals of winding S
Fig. 56 — Production of Thermo-Electric Current.
may be as great as 150,000 volts when the pressure of the current through P is 20 to 30 volts.
The student should note that, al- though the interrupted direct current in winding P induces an alternating current in the winding S, the induced pressure (in winding S) is considerably more in- tense at the "break" of the primary cur- rent than at the "make." This is due to the more rapid change of flux thread- ing through the winding S when the lines of force collapse than when they rise; in
Fig. 55-Aerial Ammeter with Thermo-Couple. othef WQrds? it requires a longer period
to saturate the iron core with lines of force than to empty it. The wave form of the induced current is shown in Fig. 58.
MAGNETIC AMMETER
B-i
MINI
pig< 57 — Fundamental Diagram of Induction Coil.
48
PRACTICAL WIRELESS TELEGRAPHY.
Fig. 58 — Wave Form of Secondary Induced Currents.
(a) Interrupters. In addition to the magnetic interrupter shown in Fig. 59 there are several types of interrupters for induction coils, but since they are seldom used in modern wireless systems, they will not be described. The electrolytic
interrupter is frequently employed but not extensively. A diagram of connections and a sketch is shown in Fig. 59.
A lead plate of convenient size is immersed in a dilute solution of sul- phuric acid together with a platinum electrode covered with a porcelain sleeve. The amount of platinum ex- posed to the action of the acid is closely regulated according to the conditions of the circuit. When the interrupter is connected in series with the primary winding of the induction coil the action is as follows : The current flowing through the solution from platinum point to the lead plate sets up an electrochemical action which form a gas bubble on the tip of the platinum electrode. This gas bubble insulates the platinum exposed, thereby opening the primary 'circuit. The cur- rent flow having discontinued, there is nothing to sustain the gas bubble, which accordingly collapses again, allowing the current to flow through the winding, when the above action is repeated. A rather high rate of in- terruption is thus secured which in- duces a rapidly pulsating current in the secondary winding. Interrupters of this type frequently give 1,000 breaks of the primary current per second of time. They will not function well on potentials less than 80 volts direct cur- rent. Fair results are obtained with alternating current.
51. Practical Electric Circuits. — With' the sole purpose of conveying to elementary students an idea of the wiring and certain fundamental facts surrounding practical electrical circuits, a few examples are herewith appended. In the circuit of Fig. 60, direct current at pressure of 110 volts enters at the terminals A, B, flows through the fuses F, F, through the switch blades D, D, and thence on to the bank of lamps assumed to consist of 8 lamps connected in parallel or in shunt to the terminals of the power line. If the lamps have resistance each of 220 ohms, one-half ampere will pass through the individual lamps; hence, 8 lamps will pass 4 amperes.
Now the voltage of this circuit may be measured by the voltmeter V which is an instrument of high resistance, taking a very small amount of current, and the current can be measured by the ammeter A which conversely is an instrument of low resistance. In the problem cited, ammeter A should indicate current of 4 amperes, and meter V pressure of 110 volts. The power in watts = 4 X HO = 440 watts = 0.44 K. W.
(a) Fuses. Fuses are required at points F, F, to protect the generator connected to terminals A, B, from accidental short circuit or overload. These consist of lead or com- position alloy wire which melts when more than a predetermined number of amperes pass through them. In order to protect the circuit of Fig. 60, the fuses should have currem
o o o o o
o o o o o
r£j O O O O O
;3 O O O O O
o o o o o
110 V. D.C LEAD' PLAflNUM
Fig. 59 — Electrolytic Interrupter Connected to an Induction Coil.
ELECTROMAGNETIC INDUCTION.
49
\
LAMPS
carrying capacity of 5 or 6 amperes. Then if current in excess of this value is drawn, the alloy wire melts, completely cutting off the current.
In the power circuits of the Marconi wireless system, enclosed fuses are employed ex- clusively. These consist of a strip of fusible material of the requisite current capacity,
stretched between two brass lugs and en- closed within a fiber cylinder.
A second example of a practical circuit is shown in Fig. 61 where the terminals A, B, connect to a 500 volt direct current generator. Assume that only 110- volt incandescent lamps were available for light- ing purposes; then to prevent the lamps being burned out by excessive voltage they are connected in series. The complete cir- cuit will pass y?, ampere of current at a IF JF pressure of 110 volts. A voltmeter con-
I I nected around any of the lamps would in-
I I dicate approximately 110 volts. Should any
of the lamps burn out, the circuit will be Fig 60 — Simple Power Circuit. . . , i u i i_i« i_ j i_
broken and can only be established when a
new lamp is substituted for the burnt one. Since the potential across the entire bank of lamps is 500 volts, and the current ^2 ampere, the bank of lamps would require l/2 X 500 = 250 watts or t/4 K. W.
A third example of a practical circuit is shown in Fig. 62 wherein alternating current is transmitted, let us say, from a central power station at 2200 volts pressure, passed
through the primary winding of the trans-
former P which is of relatively high re- sistance. By electromagnetic induction, a current is induced in the secondary wind- ing at a pressure of 110 volts with a corre- sponding increase of current as compared with that flowing in the primary winding. This current may be employed convenient- ly for lighting a bank of lamps such as indicated at L.
Assume the bank of lamps in this draw- ing to consist of 8 lamps connected in
rig. 61 — Lamps in Series on 500 Volt Circuits.
parallel; they will require 4 amperes of current and the fuses at F-l should have capacity of 5 or 6 amperes. On the other hand, the strength of the current in the primary winding P is relatively low and much smaller fuses will be employed to protect this circuit. When electrical energy is transferred from the primary winding to the secondary wind- ing, certain losses due to resistance, heating of the core and magjietic leakage take place, hence, the total power flowing through P exceeds that taken from S. If winding S
F
-o~~ o
ZZOO VOLTS A.C.
Fig. 62 — Showing Use of Step-Down Transformer.
and the circuits associated therewith take 440 watts we may assume a value of 500 watts for the circuit P. Since the number of watts in a given circuit = I X E, then (assuming
W 500 no phase displacement) I = — or for winding P, I = = 0.22 amperes, the approxi-
E 2200
mate value of the current flowing through winding P. A 1 ampere fuse would therefore protect the primary circuit from overload.
50
PRACTICAL WIRELESS TELEGRAPHY.
The installation of all types of a power or radio equipment are made in accordance with certain definite rules or regulations which have been adopted by the National Association of Electrical Inspectors. These rules vary slightly in different cities of the United States, but are generally in accordance with the national code. The installation of power or wireless apparatus should not be undertaken until these rules have been carefully gone over. The foregoing explanations and examples explain but partially the problems of ordinary electric circuits, but they serve to show certain fundamental facts which should be of some value to the elementary student of radio-telegraphy.
Fig. 62a.— A High Power Radio Station in the Tropics.
PART IV. MOTOR GENERATORS.
HAND AND AUTOMATIC MOTOR STARTERS.
52. THE MOTOR GENERATOR. 53. FIELD RHEOSTATS. 54. DYNAMOTOR AND ROTARY CONVERTER. 55. THE MOTOR STARTER. 56. AUTOMATIC MOTOR STARTERS. 57. PROTECTIVE CONDENSERS. 58. CARE OF THE MOTOR GENERATOR. 59. How TO REMOVE MOTOR GENERATOR ARMATURE.
52. The Motor Generator. — The required high voltage current for the operation of a radio-transmitter is obtained from ( 1 ) a source of alternating cur* rent, (2) an alternating current step-up transformer; but practically all vessels that have been fitted with radio sets to date have a direct current dynamo making it necessary to install a motor generator to obtain an A. C. source of supply.
A motor generator is simply a motor and a dynamo coupled together on a com- mon cast iron base, the motor being set into rotation by direct current, the dynamo in turn generating an alternating current of the required voltage and frequency. Such machines may have two or four bearings, two for the motor armature and two for the generator armature or the shaft may be strengthened at the center and have both armatures mounted on it. In this case the shaft has two bearings.
MOTOR FIELD MOTOR /
GEN FIELD
A.C.
Fig. 63 — General Construction of Motor Generator.
A general outline of the construction of a motor generator is shown in Fig. 63 where a direct current motor is mounted on the left of the cast iron base, the alternating current generator on the right. The motor receives direct current at pressure of 110 volts (gen- erally) and the dynamo generates alternating current at frequencies from 60 to 500 cycles and at voltages varying from 110 to 500 volts according to the design. The student should note from Fig. 63 that (1) the generator field windings receive current from the same source as the motor, e. g., in the case of a ship installation, from the ship's dynamo ; (2) both the motor and generator field windings are connected across the D. C. power line.
52
PRACTICAL WIRELESS TELEGRAPHY.
For the operation of radio-transmitters, a motor generator is required that will give :
(1) Constant speed under variable load;
(2) Constant alternating current voltage under variable load.
For the quenched spark transmitter, a constant current generator having a falling voltage characteristic under a load is preferred.
When a motor generator is connected to a wireless transmitter, it is subjected to an intermittent load following the closing of the transmitting key, and there-
MofcrFJe/d
Generator F/'e/c/
Mofor/fteo,
6O-5OO Cyc/es
Fig. 64 — Simple Shunt Wound Motor Generator.
fore, some means must be provided whereby either a uniform alternating current frequency or voltage for both can be maintained. In practice the necessary regu- lation is obtained by special motor field and generator field windings. Hence, motor generators may be classified with respect to their windings.
Three different types are in commercial use :
(1) A shunt wound motor coupled to a simple alternating current generator:
(2) A shunt wound motor coupled to an alternating current generator, having a compound wound field:
(3) A motor with differentially compounded fields coupled to a simple alternating current generator.
An example of type (1) is the 2 K. W. 500 cycle Crocker Wheeler motor generator used with the Marconi panel transmitters ; of type (2) the 1 K. W. 60
MOTOR GENERATORS.
53
cycle Robbins & Meyers motor generator; of type (3) the 2 K. W. 240 cycle motor generator. All three types are in use in the radio sets of the American Marconi Company.
The fundamental circuit of type (1) is shown in Fig. 64; of type (2) in Fig. 65 and of type (3) in Fig. 66. The student should take careful note of the position of the generator and motor rheostats in all three diagrams because in addition to
Gen l?heo.
60-5OO Cycles
Fig. 65 — Motor Generator with Compound Generator Field Winding
the automatic regulation which these machines are designed to give, initial adjust- ments of either voltage or frequency can be made by the rheostats. For example, if resistance be added at the motor field rheostat, the motor speeds up and, there- fore, increases the frequency of the generator. If resistance be added at the gen- erator rheostat, the generator field current reduces and the voltage across the arma- ture terminals drops. If more current is admitted to the field coils, the voltage of the armature increases.
Explanations given in Part III will cover the shunt wound motor generator of Fig. 64. By proper design, fair regulation of frequency and voltage is secured with this type under the conditions imposed by a wireless transmitter.
54
PRACTICAL WIRELESS TELEGRAPHY.
It will be noted from the diagram of Fig. 65, the generator has two field windings, a series winding connected in series with the motor armature and a shunt winding connected directly across the D. C. line. The windings are disposed on each field pole of the gen- erator so that the lines of force generated by the scries winding and those of the shunt winding Hozv in the same general direction. When a motor generator of this type is subjected to load, there will be a tendency towards a decrease in speed, but there will then be an in- crease of current through the series winding (because it is connected in series with the
O.C
Fig. 66 — Circuits of Motor Generator with Differential Motor Field Winding.
armature) which has the effect of increasing the strength of the generator field, thereby maintaining the voltage of the alternator fairly constant under variable load. The motor of this machine has a simple shunt winding with a speed regulating rheostat, connected in series. The voltage of the generator may be increased or decreased by means of the generator field rheostat.
The differential motor of Fig. 66 has been explained in Paragraph 40. The principal advantage of the motor generator in Fig. 66 is that it maintains a uniform speed under variable load which results in a uniform frequency of current at the terminals of the alternator.
A photograph of the 2 K. W. 500 cycle Crocker Wheeler motor generator ap- pears in Fig. 67a wherein the motor generator armatures are clearly shown. The
MOTOR GENERATORS.
55
MOTOR FIELD
GENERATOR FIELD
Fig. 67a— Details of Crocker Wheeler 2 K. W. 500 Cycle Motor Generator.
generator has 30 field poles and the motor 2 field poles. The armature revolves at 2,000 R. P. M., hence there are 33 1-3 revolutions per second which multiplied by 30 (the number of field poles) gives 1,000 alternations of current per second. And since two alternations of current constitute a cycle, the frequency of this generator is 500 cycles per second. The motor of this machine takes 29 amperes at pressure of 110 volts D. C, but the generator armature delivers alternating
Fig. 67b — 2 K. W. Crocker-Wheeler Motor Generator Assembled.
56
PRACTICAL WIRELESS TELEGRAPHY.
ROTOR OF GENERATOR
GENERATOR - ARMATURE MOTOR ARMATURE
Fig. 67c— Details of % K. W. 500 Cycle Crocker Wheeler Motor Generator.
current at pressures varying between 120 and 380 volts with current output of 20.8 amperes. At normal saturation the generator and the motor fields require about
29 X HO
2l/2 amperes each. The complete motor, therefore, is rated at = 4.2
746 horsepower.
Fig. 67a shows the gen- erator and motor field wind- ings, the end plates and the oil gauge of the 2 K. W. 500 cycle motor generator. The machine completely assem- bled but with the rotary disc discharger removed is; shown in Fig. 67b.
Details of the y2 K. W.. 500 cycle Crocker Wheeler motor generator are shown! in Fig. 67c and the assem- bled machine in Fig. 67d. It is to be noted that the rotor of the generator is composed of two toothed' discs without windings and
Fig. 67d—y2 K. W. Motor Generator Assembled.
MOTOR GENERATORS.
57
that both the field and armature windings are stationary. The rotor closes and opens the magnetic circuit between the armature and field windings and thereby induces current in the armature.
53. Field Rheostats.— A type of field rheostat supplied with motor gen- erators is shown in Fig. 68. A coil of composition resistance wire baked in a
heatproof insulating cement is mounted in a metal case. The wire is tapped at intervals and con- nected to brass studs over which a sliding contact arm moves. An- other type used by the Marconi Company has a number of turns of bare resistance wire wound on a slate base. A sliding contact mov- ing over the turns permits very close regulation of the flow of cur- rent in the field winding. A rheo- stat of this type is essential for motor generators used with quenched spark transmitters which require extremely close regulation of the generator voltage.
54. Dynamotor and Rotary Converter. — The dynamotor and the rotary converter are employed occasionally to generate alternat- ing current from a D. C. source of supply. The distinguishing feature of these machines is the use of a single arma- ture for both A. C. and D. C., hence, but two bearings are required and the con- struction of the machine as a whole is simplified. These machines also require less space to erect, but they possess the marked disadvantage of not allowing full control over the voltage. Also they are not as efficient as the motor generator when connected to a radio transmitter.
Fig. 68 — Cutler Hammer Type Field Rheostat.
110 VOLT D.C
A/WW
Fig. 69 — Fundamental Circuit of Rotary Converter.
The rotary converter shown in Fig. 69 has a single winding on one armature for both alternating and direct current, but the dynamotor of Fig. 70 has two distinct windings (on the same armature) one to rotate it as a motor and the other for the production of alternating current.
Explanation of the circuits of the rotary converter of Fig. 69 follows : Direct current from an external source enters the armature coil C, D through brushes A, B, and also flows to the shunt field windings (wiring not shown) causing the armature to revolve in the usual way. Taps taken from this winding at the commutator segments directly under-
58
PRACTICAL WIRELESS TELEGRAPHY.
neath the brushes are connected to the collector rings on the opposite ends of the shaft, the circuit continuing through the A. C. external circuit E. The voltage of the alternating current will be maximum when the taps to the collector rings are underneath the brushes and minimum when midway between the brushes. It is easily seen that as C, D revolves and attains the position opposite to that in Fig. 69, the current taken from the collector
1 10 VOLT D.C
WWV
Fig. 70 — Fundamental Circuit of Dynamotor.
rings will flow in the opposite direction and therefore as the armature revolves an alter- nating current can be taken from the armature, the frequency of which varies with the speed. An important point in connection with this machine is that if the D. C. supply is 110 volts, the A. C. voltage cannot exceed 70.7 volts. If 110 volts is desired a small step-up transformer must be used.
The A. C. voltage of the converter may be increased by increasing the speed of the motor, but the frequency of the current is likewise increased. The converter does not permit the nicety of control of the voltage and the frequency as does the motor generator and, therefore, it operates at a disadvantage.
The circuit of the dynamotor is shown in Fig. 70. Here the armature coils for the production of alternating current have no connection with the coils for direct current. The two windings are mounted on the same core but in separate slots. The A. C. winding can be given the correct number of turns so that 110 volts A. C. may be obtained when the armature is connected to 110 volts D. C.
F-i
00000
no VOLT D.C
Fig. 71 — Cutler Hammer Starting Box Connected to a Simple Shunt Wound Motor.
A small number of either of the foregoing types of machines are in use by the Marconi Wireless Telegraph Company of America but the rotary converter is very popular abroad.
55. The Motor Starter. — It has been mentioned in Paragraph 40 that the counter E. M. F. of a motor armature is very low upon starting, and if the terminals of a motor were connected directly to the source of current, an exces- sive current would flow which might injure the commutator or burn out the armature windings (unless the circuit is properly fused). A motor starter is, therefore, required to reduce the starting current to a safe value.
A diagram of the Cutler Hammer hand starter is shown in Fig. 71. The principal
MOTOR GENERATORS.
59
elements of the starter are the resistance coils R-l, the small holding magnet M and the handle H. The coils of R-l are of German silver wire or composition wire tapped at certain intervals and connected to the studs 1, 2, 3, 4, 5. The circuit from the negative side of the D. C. power line may be traced to the L post of the starter through the handle H which when placed on the first point of contact permits the current to flow through the coils of R-l to the terminal A, thence to the brush F of the motor armature. The circuit continues through the armature coils back to brush E and to the positive side of the line. One terminal of the field windings F-l and F-2 receives current at the positive side of the line at brush E, but the other terminal has the field rheostat R-2 connected in series, also the holding magnet M ; also this circuit continues to the first tap on the resistance coils R-l. Now as the handle is moved slowly across the contact studs on the starter current is admitted to the motor arm- ature by small increments which sets it into rotation, the speed gradually increasing as the handle moves toward the final or full running po- sition. When this position is at- tained, the magnet M grips the handle and holds it in position until it is released by opening the main D. C. line switch. The diagram shows the Cutler-Hammer starter connected to a shunt wound motor.
It is important that a motor be started neither too rapidly nor too slowly. If the former condi- tion obtains the fuses in series
Fig. 71a — Cutler-Hammer Hand Motor Starter.
with the line to the motor armature will melt, but if the starting handle is moved too slowly across the contact studs, the internal resistance coils will overheat and perhaps burn out. The speed of acceleration of the starting handle can usually be gauged by observing the speed of the motor armature. It should require no more than 15 seconds to start the motors used in connection with wireless tele- graph apparatus.
ARMATURE
1 10 VOLT D.C
Fig. 72 — General Electric Company's Hand Starter Connected to Shunt Wound Motor.
The release magnet M, Fig. 71, serves to protect the motor in case the main line circuit is disconnected or should by accident the circuit to the motor field windings be broken. In either event the handle H Hies back to the starting posi- tion by the tension of a .spring attached to the bearing of the handle, and thus interrupts the circuit to the armature.
60
PRACTICAL WIRELESS TELEGRAPHY.
The General Electric Company's hand starter differs slightly from the type just de- scribed. A complete diagram is shown in Fig. 72 where the starter is connected to a simple shunt wound motor. It is to he noted in this diagram that the release magnet M is shunted across the D. C. line, and has a coil of fixed resistance R-l connected in series. In all other respects, the wiring is the same as the first type and the starter performs the same
110 VOLT D.C.
Fig. 73 — Circuit of Automatic Starter for Marconi l/2 K. \V. 120 Cycle Set.
functions. If the source of power is cut off, magnet M releases the handle of the starter whereupon it returns to the zero position, breaking the circuit to the armature.
56. Automatic Motor Starters. — It is c.ften essential to install a motor generator at a point remote from the wireless cabin in order that the noise from its operation may not interfere with the reception of wireless signals. In instances of this kind automatic motor starters are employed, which are con- trolled from a distant point by pressing a small button or closing a small switch. Such starters possess the advantage that the acceleration of the starting handle is uniform, and there is, therefore, no danger of burning out the armature or melt- ing the fuses during the starting of a motor.
1 10 VOLT D.C
SHUNT
[AAAMAMJ J *!
D.C. BRAKE RES.
VW\AM '
Fig. 74— Circuits of Automatic Motor Starter for Marconi ^ K. W. 500 Cycle Set.
There are numerous types of automatic starters on the electrical market but only those used in modern Marconi sets can be given consideration here.
The complete circuit of the single step automatic starter is shown in the diagram of Fig. 73. It is employed in connection with the l/2 K. W. 120 cycle Panel transmitters of the Marconi Company. A single resistance unit R-l is connected in series with the brushes
MOTOR GENERATORS.
61
G and F of the armature. The solenoid winding S connected in shunt to the motor arma- ture draws up the plunger C which in turn shunts the coil R-l through the contacts A and B. When the main D. C. line switch is closed, current flows to the motor armature through R-l and as the counter E. M. F. of the armature increases, the solenoid winding becomes more strongly magnetized, drawing up the plunger, which cuts out the resistance coil. Now when the plunger of the solenoid is in the full running position, contacts K and L are forcibly opened and the resistance unit R-2 is connected in series with the solenoid winding. This is to prevent the magnet winding from overheating and consequent injury. The circuits of the automatic starter employed in the ^ K. W. 500 cycle transmitting sets of the Marconi Company are shown in Fig. 74. When the starting switch 2 is closed, the solenoid 3 is connected in shunt to the D. C. line. The flux from this solenoid attracts the lever 4 making contact with points 5, thereby closing circuit from the D. C. line to the motor armature through the resistance coil 6. Simultaneously the solenoid 7 is connected in shunt to the D. C. line (through the lever 4) which attracts the lever 9 making contact with point 10, thus cutting the resistance 6 out of the armature circuit, whereupon the motor is connected directly to the main D. C. line. It is apparent that the lever of solenoid 3 opens and closes the main power circuit while the lever of solenoid 7 cuts out the re- sistance in series with the motor armature. The solenoids 3 and 7 have the resistance coils 14 and 8 respectively, which are connected in series with their respective windings auto- matically by the levers 4 and 9. These resist- ances prevent the solenoid winding from over- heating.
The automatic starter also includes the ele- ments of an electrodyna- mic brake. When the starting switch 2 is open, •lever 4 drops back, also lever 9, followed by con- tact being made between points 11 and 12 con- necting the resistance coil 13 in shunt to the
ttipJHBS * K^B motor armature and the series winding. The
pS§ fcj"4 1 lj|B| motor armature thus temporarily becomes a
i jpj generator and owing to the power expended in
tfS&BJ setting up a current through the resistance 13,
a powerful braking action is set up against the armature, bringing it to a quick stop. The re- sistance coil 15 is the motor field rheostat, by means of which the speed of the motor can be regulated over certain limits.
The starting switch 2 is usually one of the snap type placed conveniently for the wireless operator and near to the aerial changeover switch. In some installations the starting circuit opens and closes through the latter switch, stop- ping the motor whenever the aerial switch is in the "receiving" position. In case it becomes necessary to install the motor generator in the operating room, it is essential that the motor stop immediately after the sending period, to permit the reply from a distant radio station to be deciphered without interference.
The circuits of the automatic starter supplied with the 2 K. IV. 500 cycle transmitting sets of the Marconi Company are shown in Fig. 75a. In addition to acting as a motor starter it performs the func- tions of a main line circuit breaker through the medium of an overload relay switch. The starter has three resistance units connected in series with the motor armature instead of the single resistance unit described in the two previous types. It will be observed from the drawing that the field winding of the motor is connected in shunt with the D. C. line through the regulating field rheostat 23. As resistance is increased at 23, the speed of the motor increases, and consequently, the frequency of the alternator.
Fig. 75 — English Marconi Company's one-half
Kilowatt Vertical Type Motor Generator with
Synchronous Disc Discharger on Shaft
62
PRACTICAL WIRELESS TELEGRAPHY.
p/WWVn
rrr \
ii?" T
I! i .
rr — n
mr .—^— .——.—_ i II l3, wJ I
d i
- — - — i
vwM/"
-4-^c
o
OC §
LU •£
£<
f^ "o
rt
O bo
— rt
II
Z) u
MOTOR GENERATORS.
63
The generator field winding is connected in shunt to the D. C. line through the low power resistance 24 and the voltage regulating rheostat 25. The field circuit continues to the con- tacts of the antenna switch 62 and 63 through the control switch 26 and finally to contact 5 of the automatic starter. By this connecion the circuit to the generator field winding re- mains open until the bar 6 attached to the plunger A of the automatic starter has touched point 5. When the bar of the automatic starter makes contact with point 4, the D. C. arma- ture is connected directly to the main D. C. line.
By increase of resistance at the rheostat 25, the voltage of the A. C. generator drops but it may be increased correspondingly by the reduction of resistance. Low values of voltage may be secured at the terminals of the alternator by an external fixed resistance 24 con- nected in series with the generator rheostat. This is shunted by the switch indicated in the drawing.
The overload relay employed in conjunction with the automatic starter has the magnet winding 20, which may be called the tripping magnet, and the second magnet winding 22, which may be called the holding magnet. Winding 20 is in series with the D. C. armature on the negative side of the line. If more than a predetermined number of amperes flow through this winding, the lever 14 is drawn up, breaking the circuit of the solenoid winding 11 through the contacts 13 and 14. Immediately afterward the circuit through winding 22 is closed through contacts 14 and 21. This causes the lever 14 to be held in that position until either the main D. C. line switch or the starting switch 17 is opened.
SERIES
FIELD KA>1
RHEOS. ^V\
SHUNT \. ^
60 TO 500 CYCLES
110 VOLT D.C.
Fig. 76 — Hand Starter Connected to Motor Generator with Differential Motor Winding.
One terminal of the solenoid winding 11 is connected to the positive pole of the D. C. line at point 12. The circuit continues through the fixed resistance 9, shunted by the switch 10, through the contacts 13 and 14 of the overload relay, through contacts 15 and 16 of the antenna switch, to a terminal of the winding 20, which is of negative polarity. Hence it is readily observed that the solenoid winding is connected in shunt to the D. C. line when either contacts 15 or 16 or the starting switch 17 is closed.
The switch 10 in shunt to the resistance 9 is automatically opened by the plunger A of the automatic starter when it is in the full vertical or running position.
The resistance coils of the motor starter, connected in series with the D. C. line to the armature, are progressively cut out of the circuit at contacts 1, 2, 3 and 4 by the bar 6. When the circuit to the solenoid 11 is closed, the plunger A with the bar 6 moves in a vertical posi- tion, the acceleration being regulated by a piston drawn through a vacuum chamber. When contact is made between the bar 6 and point 1, the circuit to the armature includes the entire set of resistance coils.
When the circuit to the winding 11 is interrupted, either at point 17 or at the serial switch contacts 15 and 16, the plunger A drops downward and through the medium of con- tacts 6 and 7, the resistance coil 8 is connected in shunt with the D. C. armature. At this stage of operations the momentum of the armature causes it to become temporarily a D. C. generator and current of large value flows for a few moments through the resistance 8. The magnetic field* thus set up by the armature causes a powerful dragging action on the field
64
PRACTICAL WIRELESS TELEGRAPHY.
C-l
C-*
EARTH
Fig. 77 — Protective Condensers.
poles bringing the armature to a quick stop. Reviewing the foregoing: When the handle of the type SH aerial changeover switch (or any other type) is thrown to a transmitting position, the motor generator is automatically started, provided the main D. C. line switch is closed. It will be brought to a quick stop when the antenna switch is placed in the receiv- ing position, provided the switch 17 remains open. If the switch 17 is closed, the motor generator can be kept in a continuous state of operation during the receiving period.
The speed of acceleration of the starter arm can be very closely regulated by means of an adjusting screw attached to the bottom of the vacuum chamber. It usually requires 12 seconds to bring the starter up to the full running position.
When it becomes necessary to make repairs or adjustments to the generator or the A. C. power circuits, the generator field switch 26 should be open. When the motor generator is to remain idle for an indefinite period, the main D. C. line switch should be opened to break the circuit to the field winding of the motor.
In the diagram of Fig. 76 the complete circuit of a differentially wound motor coupled to a. simple alternating current generator is shown, including the connection of the field rheostats and the hand starter. The student should give this diagram careful con- sideration as it serves to show the complete funda- mental circuit of various types of motor generators in the Marconi service. This diagram should be used in answer to the Government examination query regarding the fundamental circuits of the motor generator.
57. Protective Condensers. When a wireless telegraph transmitter is in operation, a powerful electrostatic field is set up in the region about the aerial wires. // the power apparatus is installed in such a manner that the low voltage wires leading to the motor generator or other apparatus, lie parallel or in proximity to the antenna wires, currents of very high potential will be induced in the power wires which may puncture the insulation. A path is then afforded for the low voltage current which may cause an arc, com- pletely short-circuiting the windings of a motor generator. In other words, this induction sets up a difference of potential between the various windings or between the windings and the frame of a motor generator which may result in a disastrous burnout. The low voltage wires can be well protected by installing them in iron con- duit, the latter in turn, being thoroughly connected to the earth. The induced currents will flow on the surface of the pipe and be neutralized by the earth connection, and thus do no harm to the power wiring. The power wires of commercial radio in- stallations are either installed in iron conduit or in lead-covered cables, but in addition to this protec- tion, protective devices known as protective con- densers or protective resistances are employed.
Protective condenser units consist of two one-half microfarad condensers connected in series mounted on an insulating support as in Fig. 77. The middle connection is extended to the earth and the remaining terminals connected across the field or armature windings of a motor generator or between these windings and the frame. Differences of potential that may be induced in such windings are thereby neutralized and reduced to zero through the earth connection.
Carbon or graphite rods of high resistance are often employed for protective purposes as shown in Fig. 78. A single graphite rod having resistance of about 5,000 ohms is mounted on an insulating support and connected to earth at the middle point. The two remaining terminals are connected to the windings of the motor generator to be protected. These rods
EARTH
Fig. 78— Protective Resistance Rod.
MOTOR GENERATORS.
65
— PRO COND
have sufficient resistance to prevent appreciable leakage of the low voltage current but possess sufficient conductivity to pass the induced current of high voltage.
Protective rods or protective condensers are connected :
(1) In shunt to the motor armature.
(2) In shunt to the generator armature.
(3) In shunt to the field windings of the motor.
(4) In shunt to the field windings of the generator.
The diagram of Fig. 79 shows how protective condensers are attached to the motor gen- erators in modern Marconi sets. Condensers A, B, C and D are of l/2 or 1 microfarad ca- pacity each. One terminal is connected to a binding post and the other terminal to the frame of the motor generator. The frame of the motor generator is connected to the earth at binding post E or at any other convenient point. These condensers are generally mounted in a containing rack on the top of the motor generator and pro- tected from injury by a cast iron case.
In naval radio systems, fuses are con- nected in series with the protective con- densers to protect the power mains in case of puncture of the dielectric.
58. Care of the Motor Generator.
—When first coming in contact with a motor generator of any type, the stu- dent should note particularly how the brushes are held in the rocker arm and how the connections are attached to and between the various brush holders. He should also note the connections inside the frame from the motor to the generator. Particular observance should be made of the thrust bearing mounted on the end of the shaft to take up the "end play." In the case of the 2 K.W 500 cycle motor generator, Fig- 79- the method of attaching the rotary spark gap to the end of the generator shaft should be carefully gofte over.
Proper care of the motor generator is assured if the following general rules are observed:
(1) Keep motor brushes clean and free from carbon dust. Use sand paper only, avoid emery cloth.
(2) Clean commutator occasionally with a fine grade of sand paper.
(3) Oil bearings frequently. Open up petcocks occasionally, to assure that oil container has the necessary supply.
(4) Make sure that all petcock valves are tight so that they will not loosen by vibration.
(5) Wipe off frame of motor generator, brush holders, and rocker arm occa- sionally to prevent accumulation of carbon dust and grease.
(6) Do not overspeed motor. Normal speed can be observed by the reading of the frequency meter or by applying a speed indicator to the end of the motor generator shaft. Observe either wattmeter or ammeter occasionally to insure that the normal load of the generator is not exceeded.
(7) When removing armature from motor generator, it is generally more con- venient to take off the generator end plate.
(8) Be careful not to injure commutator by scraping against the field poles.
(9) See that protective condensers are at all times properly connected.
(10) Punctured condensers should be removed or disconnected from the circuit.
H^rc^f^rCGelTteorrS,are C°"
66
PRACTICAL WIRELESS TELEGRAPHY.
(11) In the case of the 2 K. W. 500 cycle motor generator, adjust overload relay for 35 amperes.
(12) If a single resistance coil in either the hand starter or the automatic starter burns out, close the circuit by placing a jumper around the burned out portion.
(13) If field rheostat burns out, close the circuit by a jumper. If burned beyond repair, substitute 3 or 4 16 C. P. lamps, connected in parallel.
(14) Tighten up all connections frequently. These should be gone over once per month.
59. How to Remove Motor Generator Armature. — In case it becomes necessary to remove the armature of the 2 K. W. 500 cycle motor generator for the purpose of repairs, it is necessary first to remove the casing of the spark gap. Follow this by taking
Fig. 79a — 1 K. W., 60-cycle Motor-generator (Two Bearing L'niu.
out the wedge-shaped key in the end of the generator shaft. If the rotary disc is given a slight tap with the hammer, the key will be released and the disc may be removed from the shaft. After this, the bearing bracket can be removed from the generator end. The brushes should then be removed from the commutator and the collector rings. After these operations have been gone through, the armature can be pulled out and a new one inserted, if necessary. When the armature is replaced, the oil rings should .be held up to permit the shaft to pass through the bearings. Care should be taken to' see that the oil rings are working properly and that the bearings are thoroughly oiled for the initial test. Before starting the motor generator careful inspection should be made to see that all parts are properly secured and in working order.
It should be noted that the mica of the commutator of this machine is undercut about 1-32 inch, and before it gets flush with the commutator bars, the mica should be cut out again.
PART V.
STORAGE BATTERIES AND CHARGING
CIRCUITS.
60. THE NECESSITY FOR A STORAGE BATTERY IN A RADIO INSTAL- LATION. 61. GENERAL CONSTRUCTION AND ACTION. 62. THE CHARGING PROCESS. 63. THE FUNDAMENTAL ACTIONS OF A STORAGE CELL. 64. THE ELECTROLYTE. 65. THE HYDRO- METER. 66. How THE CAPACITY OF A STORAGE CELL Is RATED.
67. FUNDAMENTAL FACTS CONCERNING THE STORAGE CELL.
68. How TO CHARGE A STORAGE CELL. 69. How TO DETERMINE THE VALUE OF THE CHARGING RESISTANCE. 70. LAMP BANK RESISTANCE. 71. THE USE OF THE AMMETER AND THE UNDER- LOAD CIRCUIT BREAKER. 72. THE AMPERE HOUR METER. 73. OVERCHARGE. 74. How TO CHARGE A BATTERY WHEN THE VOLTAGE EXCEEDS THAT OF THE CHARGING GENERATOR. 75. How TO DETERMINE THE POLARITY OF THE CHARGING GENE- RATOR. 76. DETERMINATION OF THE STATE OF CHARGE AND DISCHARGE OF A BATTERY. 77. KEEPING THE LEVEL OF THE ELECTROLYTE. 78. PROTECTING THE CE^LS FROM ACID SPRAY. 79. GENERAL INSTRUCTIONS FOR THE PORTABLE CHLORIDE TYPE OF ACCUMULATORS. 80. GENERAL OPERATING INSTRUCTIONS FOR THE EXIDE CELL. 81. TlIE EDISON STORAGE BATTERY. 82. THE CHARGE AND DISCHARGE OF THE EDISON CELL.
60. The Necessity for a Storage Battery in a Radio Installation. — The International Radio-Telegraphic regulations require that an auxiliary source of direct or alternating current be available for operation of the motor generator or a low powered emergency transmitter in case of an accident to a vessel at sea which might put out of action the ship's generator.
The United States regulations (Act of August 13, 1912) require that the auxiliary radio transmitter be capable of transmitting to a distance of 100 miles. A small A. C. or D. C. generator operated by a gasoline or oil engine is permissi- ble as a source of current supply under the United States statute, but the general custom is to employ a battery of storage cells for direct operation of the motor generator or emergency transmitter.
Two general types of storage cells are used in connection with emergency transmitters — the lead plate, sulphuric acid cell such as the "chloride" and "exide" types manufactured by the Electric Storage Battery Company and the Edison nickel iron-alkali cell. Certain fundamental facts concerning the charge and dis- charge of storage cells and the standard circuits for their use will now be con- sidered, which is to be followed in another chapter (Part X) by a complete de- scription of the charging panels employed in commercial marine radio instal- lations.
68 PRACTICAL WIRELESS TELEGRAPHY.
61. General Construction and Action. — It is not really electricity which is stored up in a storage cell, but the flow of current from a direct current dynamo through the cell from plate to plate performs a certain amount of chemical ivork. Whenever required this stored up chemical energy can be released in the form of an electric current which will pass from plate to plate through an external circuit. The common type of lead cell comprises a set of prepared lead plates immersed in a dilute solution of sulphuric acid, but a certain electrochemical process known as "charging" must be gone through in order that the cell may deliver a current of electricity.
There are two general methods by which the lead plates for a storage cell may be prepared :
(1) A paste of litharge or oxide of lead mixed with a dilute solution of sulphuric acid may be applied to perforations in a lead grid, and then by means of a current of electricity and a suitable electrolyte the surface of some of these plates may be coated with peroxide of lead while other plates become simply spongy.
(2) Large lead plates may be immersed in a certain electrolyte and connected to the terminals of a dynamo. By repeated charge and discharge, some of the plates may be coated with peroxide of lead while the others simply be- come spongy.
Reviewing the development of the storage battery, we find:
(1) That the earlier types of storage cells comprised two lead plates immersed in a dilute solution of sulphuric acid. The terminals of the plates were con- nected to a direct current dynamo for a period of several weeks. By repeated charges and discharges, the surface of the plates received a coating of so-called "active material."
(2) Later it was determined that the formation of the plates could be hastened by chemical means previous to the charging process and the manufacture of the plates was accordingly cheapened.
(3) In certain types of present day cells, for instance, the exide lead cell, the active material is applied to the plates mechanically, in the form of a paste.
62. The Charging Process. — In general the charging process of a storage cell is as follows :
When two ordinary lead plates or sets of plates are placed in a dilute solution of sulphuric acid of the correct proportion and a direct current of electricity from a dynamo passed from one plate through the solution to the other, the resulting chemical decomposition deposits a coating of peroxide of lead on one plate while the other plate becomes gray and spongy or porous. When one set of plates is fairly well coated with lead peroxide and the other set becomes spongy, the cell is said to be "charged." If the terminals of these plates are now joined together by a conductor (the charging generator having been disconnected), a cur- rent of