Reference / Paper · 1924
Telephony, Including Automatic Switching
Textbook covering manual and automatic switching.
Reference / Paper · 1924
Textbook covering manual and automatic switching.
eas ellie Ge poa= PRT hel ASE i e oseS tieee ie Uae a eies ae e Ree aTe eh So Mid Pocd y ay ws eee ok; Poeees, one - ee pee?HL eee ‘ ey as CYA CS @ve OS I kOe C Roi oS se | S RL ‘ te Bae a : o SSa A S RS | NA I h h FAN ePy y e A A a t a r e H 2 e e G ee Ge ie t o i n t e k e Pot ot Be Sh EL Se ATOR a yr "5 sRyd he a Po ba ae =) TELEPHONY INCLUDING Automatic Switching Telephonic Speech Transmission. The Telephone Exchange. Automatic Switching. MultiOffice and Large City Systems. Private Exchange. Testing and Adjustment. BY ARTHUR BESSEY Electrical alg SMITH Engineer ILLUSTRATED (llores oO / i V/ LO ~ CHICAGO FREDERICK J. DRAKE & CO. PUBLISHERS CopyRIGHT, 1924 BY FREDERICK Printed J. DRAKE in the U. & Co. S. A. PREFACE The widespread interest in the telephone and the usefulness of viewing the art from more than one standpoint are sufficient reasons for the appearance of this book. The telephone world of these days is laying great stress on automatic switching. Like other industries, telephony has varied its point of emphasis from time to time. Once it was the achievement of speech transmission, then it was the manual switchboard. Once it was wires and cables, then it was the multi-office exchange. The present interest in automatic switching is due to the period of economic refinement, which ealls for more efficient ways of doing things. This book may serve as an introduction to telephony, and the space given to automatic switching is roughly indicative of its importance just now. Not every item of telephony is_ covered, because there is not room for all of them. But it is believed that the reader will get an adequate idea of the present status of telephony, and we trust will be led to pursue with thoroughness the mastery of his part in this great and useful industry. : ARTHUR BESSEY SMITH. Evanston, Illinois, January, 1924. — Digitized by the Internet Archive in 2026 https://archive.org/details/telephonyincludi0000arth TABLE OF CONTENTS PAGE Mae MAGNETO RECEIVER. s.r oe sowie cs oes 9 re ois aneter ate a/eiois ss yeheise ce ses es aS COM MONG EAT TREY, ceeyore oicc.8ocnie'e atehe rs teoone erebsie 27 EOCATM ISA TIERY INSTRUMENT...... 3t aDEDie DEVINTHY CLALIT sya «01s ensiretets. ois(e.cte"o lelere, oie elelcvens LocAL BATTERY TELEPHONE 90 THE PMERPHONE , THE MAGNETO EXSXOELANGI.. 2.2% 6/6/45 avo eleie sicicts 105 SWITCHBOARD...........4++04. 108 THE Locan BATTERY SUBSCRIBER LINE AND AERP EFON Mana to seater (Covel Oioyotei ors cualatela: ove Merscoks te 154 Common BatTTeERY SWITCHBOARDS............ 161 ComMon BATTERY SUBSCRIBER LINE AND TELELOIN Hireue arauceay ate eveeo tner Ra sro eas a oriche oe.arete ena'sleie 169 THE MoLrirnm (SWIECEBOARD® «42641 «0 one 187 THe “RRATURE’” SWITCHBOARD... 00.00.0005 194 THE AUTOMATIC SWITCHING OF TELEPHONE TLS Pe yatessiatcatorereiatecdvMeceiesthacaiaes exeailere/s ayuda’arenavere 203 HGNDEED-LINE HIXCHANGE, THE AUTOMATIC THE <ice as cle ovisieie oso 06 TELEPHONE 219 INSTRUMENT..... 248 THOUSAND-LINE EXCHANGE............. 259 TEN THOUSAND-LINE EXCHANGH.......scecees 271 CONTENTS PAGE CHAPTER XVIII Party XIX THE LINES IN AUTOMATIC MULTI-OFFICE City; EXCHANGES...... 280 EXCHANGE................ 291 XX LARGE, XXI Tuer PrivaTE AUTOMATIC SYSTEMSiica XXII POWER PLANTS. cicrestdereste orsleet seree ovis eispare meee 346 XXIII TESTING ADJUSTMENT AND 2 science = eldee srs iaisteee 315 EXCHANGE.......... OF AUTOMATIC APPABATUG © ore cieiiie rie aa doves niece eiece ele apamsuaeaans XXIV TTOLE aoe wie aitharesele oho sess ioie o lon Ne acta eres cee eee XXV ‘Tn PHANTOM) XXVI LOADING XXVII THE XXVIII MEASURED) 371 383 CIRCULD sc cteretetoexs cle sisieiens teerent 407 OF TELEPHONE PRIVATE 337 LINES..............- 412 EXCHANGE..........--; 416 «ce sfaeres cele oe sie eieleaterettene 429 BRANCH SERVICE TELEPHONY INCLUDING AUTOMATIC SWITCHING CHAPTER THE MAGNETO I RECEIVER The electric telephone has so completely won its way into the life of the world, that no eulogy of it is necessary. Since its invention by Dr. Alexander Graham Bell in 1876, it has been developed and improved with great rapidity. America in particular has shown her appreciation of this device, and today depends upon the telephone to a degree unknown in other countries. The desire to understand this means of communication is sufficient reason for writing and for reading this small book. Sound.—Sound consists of vibrations in the air. In still air, these vibrations travel about 332 meters (1,090 feet) per second. Sound has three properties, pitch, loudness and quality. Pitch depends upon the number of vibrations per second. Loudness depends upon the amplitude of vibration of the air, on the to-and-fro motion of each patricle of air as it vibrates. Quality depends upon the combination of various simple tones which make up what is apparently but a single tone. Quality is sometimes called ‘‘timbre’’ or ‘‘tone eolor.”’ Articulate speech is judged 9 by its ability to 10 TELEPHONY convey ideas. Therefore it has but two properties, loudness and clearness. Loudness requires no definition. ©Clearness means the degree of ease with which the listener can understand the words which are spoken. It involves intricate combinations of many frequencies and amplitudes of vibrations, but the listener need not know what they are to judge the clearness. Speaking down into an empty cistern is an example of loudness with little clearness. The speech of a careful person at some distance in a quiet, open space is clear, though it may be weak. —-> Figure 1.—Telephone Receiver Used and Receiver. as Transmitter The Telephone Recewer.—The permanent magnet receiver, Figure 1, may be regarded as the basis of the electric telephone. It is possible to have a complete means of communication by connecting two of them by wires, using each receiver alternately as transmitter and as receiver. In fact, many an early private line was thus equipped, even if it did cause the user to move the receiver from mouth to ear many times during a conversation. The permanent magnet receiver consists essentially of an iron diaphragm, D, in front of the two poles n s, of a permanent steel magnet, N S. The poles are made of soft iron. Each pole carries a winding of insulated copper wire. are so that connected in series The two coils current flowing in THE MAGNETO RECEIVER 11 the coils will either strengthen the magnetism in both poles or weaken it in both poles. The magnetic fiux caused by the steel magnet pulls the iron diaphragm and dishes it slightly. When sound strikes the diaphragm, the latter moves in and out in accordance. with the vibrations of the air. Each time it moves nearer to the poles, the magnetism becomes stronger. Each time the diaphragm recedes, the magnetism in the poles becomes weaker. Whenever magnetism which passes through a coil becomes stronger or weaker, it causes a voltage to be set up in the coil. In the case which we are considering, the increase of magnetism caused by the approach of the diaphragm will cause a voltage to act and to drive current down in the receiver and to the right in the lower wire as indicated by the broken arrow. This current will pass through the other receiver, if the line is not broken or short circuited. When the diaphragm recedes from the poles, the magnetism will weaken, thereby causing a voltage to exist, but in the opposite direction. This will drive current through the other receiver in the opposite direction. Thus the sound striking the diaphragm of one receiver produces an _ alternating current in the line and in both receivers. Thus the receiver which is being used as a trans- mitter is acting as a single phase, alternating current generator. The passage of the alternating current through the receiver at the distant end of the line alternately strengthens and weakens the magnetic flux in the poles. This causes the pull on the diaphragm to vary. In the case being considered, the flow of current to the right in the lower wire will weaken the magnetism, so that the pull on the 12 TELEPHONY diaphragm becomes weaker, and the spring force in the diaphragm will move it a little farther away from the pole tips. When the current reverses, it strengthens the magnetism, pulling the diaphragm Figure Figure 2.—Telephone 3.—Receiver with Receiver. Shell Removed. nearer to the poles. Thus the second diaphragm is set into vibration, and its motions correspond to the motions of the first diaphragm. With wave the receivers which strikes connected the as shown, diaphragm a sound of the receiver THE MAGNETO RECEIVER 13 at the left will cause it to move inward, and this will cause the other diaphragm to move outward and send the sound wave on into the air. But if the wires are reversed, it will not interfere with speech transmission, for the ear is able to interpret sound equally well in either case. The external appearance of an average bi-polar permanent magnet receiver is shown in Figure 2. Figure 4.—Disassembled Parts of Telephone Receiver. When the shell is taken off by unscrewing the ear cap, the diaphragm may be removed and the magnetic assembly slipped out of the shell, as in <A_ still further dissection is shown in Figure 3. The usual receiver has flat soft iron Figure 4. poles on which the wire is wound. A brass cup serves as a frame on which the The permanent rest of the parts are mounted. 14 TELEPHONY magnet is made of the best steel obtainable for the purpose, for it must retain magnetism for a long time. The soft iron poles and the permanent magnet are usually held together and onto the cup by means of a brass bolt which runs through their ends. However, there is one receiver which has the poles welded onto the permanent magnet, in which case the coils are wound on little spools and slipped over the poles after the magnet has been magnetized and the ends of the poles ground to a definite distance from the plane of the edge of the cup. This depth of grinding is from 0.30 mm (0.012 inch) to 0.38 mm (0.015 inch), depending on the maker. The first receivers were adjustable as to this air gap. But exeprience has shown that better results are obtained by fixing it at the best average value, and making the receiver non-adjustable. The diaphragm is made of ferrotype metal, which is a fairly good grade of iron coated with black japan, lightly on one side and heavily on the other. The heavily coated side must always be put outward, so as to be next to the ear of the user, for the greater danger of rust comes from the outside. The ear cap and shell have been made of hard rubber, resin compositions, and phenol fiber. Hard rubber looks well when new, but loses its good appearance, and is easily broken. Resin compositions vary greatly in appearance and strength, but the better ones are exceedingly hard. Phenol fiber seems to be the best of all, for it resists fracture better than the others and presents a good appearance through many years of service. It is marketed under several names, such as Bakelite, Redmanol, ete. CHAPTER LOCAL II BATTERY The local battery telephone is a proper basis for an easy understanding of the telephone. For a long time it reigned supreme, and is still an ideal as to clear, loud, satisfactory transmission, The simplest arrangement of this kind of a telephone, Figure 5, has a receiver, R, an induction coil, P S, a battery of electric cells, usually dry eells, and a transmitter. There is a local or Figure 5.—Local Battery Telephone. ‘‘primary’’ circuit containing the battery, and a line circuit containing the receivers. The transmitter consists of a diaphragm, D, to which is attached a carbon electrode, usually a flat disc. Back of this is another dise electrode which is stationary. The space between the two is partly filled with granulated carbon. The circuit is connected to the two electrodes, so that the battery eurrent will pass through the granular carbon. 15 16 TELEPHONY The two electrodes and the granular carbon form the ‘‘microphone cell’? M. It is a kind of rheostat. If the electrodes are pressed more closely together, the resistance of the cell is lowered. If the electrodes are moved farther apart, the resistance of the cell rises. This kind of action is caused by the diaphragm, when it is under the influence of sound. In this way the sound waves in air are able to make this rheostat vary in resistance. The primary circuit contains the battery, the microphone cell of the transmitter, and the primary winding of the induction coil. As long as no sound strikes the diaphragm, the transmitter resistance is steady, and the battery current is constant in value. But when sound moves the diaphragm, the resistance of the cell rises and falls and the current falls and rises with the same characteristics. The transmitter in effect converts sound waves into electrie waves of the same kind. The granular carbon transmitter of a simple form was invented in 1881 by Rev. Hunnings, an English clergyman. The primary current during speaking is undulating. It rises and falls according to the voice, but never reaches zero. It may be regarded as direct and alternating current in one. Figure 6 may be used to illustrate the idea. Let us plot on cross section paper time to the right and current up and down. Direct current is represented by a horizontal line, parallel to the zero line. As time goes on it remains constant. Undulating current begins when the current begins to rise and fall. The dotted line shows what the direct current would have been if undulation had not started. Now, if the undulating part could be taken out and allowed to flow in a wire by itself, it would be true alternating current. It would flow first in one direction, then die away to zero and begin to LOCAL BATTERY Ale flow in the opposite direction. Hence we may say with truth that the undulating current in the primary circuit of a local battery telephone is capable of being analyzed into direct and alternating currents. The induction coil (applied to the telephone in 1881 by Thomas A. Edison) consists of a straight iron core on which are two concentric windings. The primary winding is made of relatively few turns of heavy wire and has a resistance of 0.5 to one ohm. The secondary winding is made of relatively fine wire and more turns and has a Current Undu lating Alternating Time Figure 6.—Direct, Undulating and Alternating Currents. resistance of 20 to 50 ohms. The core is soft iron, laminated. Norway iron wire was once considered best, but sheet silicon iron is now coming into use. The induction coil is used to keep the transmitter in a local circuit of low resistance. In fact, in this circuit the transmitter is almost the only resistance. The battery 1s well under one ohm, in a new good cell practically negligible. The primary of the induction coil is not over one ohm. The connecting wires have a fraction of an ohm resistance. But the transmitter resistance will have about five to ten ohms, and sometimes more. The induction coil also steps up the voltage of 18 TELEPHONY the alternating component of the primary current, so that it is better able to traverse the line to Notice carefully that it is the distant telephone. not the battery voltage which is stepped up—it is that of the atlernating current only. The primary current, before speech begins, magnetizes the core of the induction coil to a certain amount. When the sound acts on the transmitter, its rheostatic nature causes the primary current to rise and fall. Whenever it rises, the magnetism of the induction coil core becomes stronger. Whenever it falls, the magnetism of the core weakens. Thus the magnetism in the core of the induction coil undulates just as the primary current undulates. The varying of the magnetism in the induction eoil core causes an alternating voltage in the secondary winding. This alternating voltage drives an alternating current through both receivers and the line. The sound which enters the transmitter of one telephone instrument is reproduced with more or less fidelity in the receivers of both telephones. In the sending telephone, the receiver is a handieap to sending speech. In the receiving telephone, the induction coil reduces the reception of speech. But both sending and receiving apparatus must be connected during conversation, because people can not be asked to operate switches. The frequent change in the direction of conversation forbids it. The best telephone instrument is that which has the best balance between sending and receiving, so that each function interferes as little as possible with the other. ‘‘Side tone’’ is the sound which the speaker hears in his own receiver. It is evident that a person ean hear his own voice in the telephone. Most people do not realize it. But if the receiver fails to work, they often complain that the telephone LOCAL BATTERY _ 19 sounds ‘‘dead.’’ They notice the absence of something familar—it is the side tone. This term is also applied to room noises which are heard in the telephone. Sometimes the room noises affect the transmitter so much as to render it difficult for the user to hear what the distant person is saying. The amount of energy required for sound transmission is very small. It is probably the least of all the applications of power which man has made. So small is it, that if all the machinery in the world were made so perfect that it ran with absolutely no sound, the saving in power, the increased efficiency, would be entirely inappreciable. It is with this “‘microscopic’’ energy that telephony has to deal. It enters the telephone transmitter and controls electric force which, although much greater than that of the sound, is itself insignificant compared with the force of commercial electric light and power. At the receiving end, only a tiny fraction of that energy is delivered, yet the speech is intelligible. THE TRANSMITTER The transmitter is perhaps the least understood of all parts of the telephone instrument. The main diaphragm, upon which the sound acts, is fairly well understood. It was acoustically investigated by a number of physicists, and is still receiving a certain amount of attention. But the microphone cell, with its loose contacts of granular carbon particles with each other and on the carbon electrodes, does not readily yield to mathematical treatment, and requires in its design and manufacture the greatest care and precision. The transmitter invented in 1881 by Hunnings used a earbon dise with a stationary electrode be- hind it, the space between being filled with granular 20 TELEPHONY carbon. Since that proved in structure time, it has been greatly imand operation. The granular carbon is now confined to a small cell, Figure 7, with a mica diaphragm closing the front. This makes it practically moisture proof. Moisture ean not be allowed in the microphone eell, for it spoils the delicate action upon which the success of the transmitter depends. Figure 7.—Cross Section of Transmitter. There are four arrangements of main diaphragm and cell which are now in any eonsiderable use. A, The Western Electric type, with one auxiliary diaphragm, stationary cell. B, The split electrode eell. CC, The two-diaphragm cell type, cell moving a little. D, The Kellogg type, with one auxiliary diaphragm, moving cell, integral with main diaphragm. Each of the above types has its advantages LOCAL BATTERY 21 and its adoveates, but more follow type A than any other. Types C and D have for their object a greater microphonic action by a more violent shaking up of the granular carbon. The rear mica diaphragm of type C permits the walls of the cell to vibrate to a certain extent. The incorporation of the cell walls in the main diaphragm itself certainly seems to secure in type D the greatest possible shaking of carbon. Type B, the split electrode (Stromberg-Carlson), aims to vary the contact re- sistance at both anode and cathode and secure a longer path in granualr carbon. Transmission of the voice must not only be loud, but clear. Anything which adds weight to the moving system hinders the transmitter from faithfully following all the little vibrations of sound. For this reason many transmitters which are loud, sound muffled, or hollow. The type A transmitter is illustrated by the Western Electric structure in Figure 7. The front plate A is heavy, for it is the mounting plate for the other parts, and with the back bridge B forms the frame. This frame should not have appreciable vibration of its own—vibratory response to the voice should be confined to the main diaphragm C and the front electrode D with its auxiliary diaphragm E. The main diaphragm is made of aluminum, with a flanged edge which rests on an oiled cloth ring to cushion it a little. Two springs Ff’ press against an enlarged head on the front electrode stem and hold it firmly against the diaphragm. Hence the front electrode will partake of the motions of the diaphragm. The microphone cell consists of a brass cup @G with a back electrode H, a front electrode D mounted on a mica diaphragm F and the space nearly filled with granular carbon. The mica diaphragm is 1252341 TELEPHONY 2B held on by a clamping ring J. The microphone cell is held by the bridge by means of a stem on the cup which passes through a hole in the bridge and is held by a set screw. The mouthpiece J is screwed into the front plate and serves to guide the sound into the space between main diaphragm and front plate. It is made of hard rubber, resin composition, or phenol fiber, the last named being the most popular now. It is always provided with a vent in the form of slots (Figure 8.—Split Electrode Microphone Cell. through the threads, or a little hole near the base. Various reasons are assigned for this venting. Years ago competitive salesmen used to suck on the mouthpiece of a rival transmitter and thereby pack it, reducing its transmitting ability. There is little reason for this safety now. There seems to be a little added clearness of transmission when a little vent is provided, and this is enough to continue its use. The current is led into the microphone cell at two points, the back electrode through the mounting LOCAL BATTHRY 235 stem, and the front electrode through the tension springs, The electrodes are made of the purest carbon obtainable, carefully polished, and kept very clean. The granular carbon is likewise very carefully made and cleaned, screened to uniform size, and carefully measured as to the volume used in each cell. The type B transmitter differs chiefly in that its front electrode is split into two parts, each of which WS JAM TSS LT TT ITNT ATT /, SSARRRDOG TITS) Figure 9.—Microphone Cell pany with Two Diaphragms. is a terminal of the cell. Figure 8. Current arriving by terminal No. 1 goes into electrode No. 1, through part of the granular carbon to the back electrode, down through the electrode a little way, back again through more granular carbon to electrode No. 2 and terminal No. 2. The back elec trode is an intermediate conductor. Some current leaks directly through the granular carbon, without traversing the back carbon plate. The type C transmitter, Figure 9, has two mica 24 THLEPHONY diaphragms, which permit the call wall to vibrate The structure is very simple, to a certain extent. and requires little explanation. The mounting stem is at the right, the threaded stem at the left passes through the main diaphragm. The mica diaphragms are held onto the ring-shaped cell body by clamping rings which are held on by bent-over lugs. The type D transmitter, Figure 10, has its microphone cell in the body of the main diaphragm. The Figure 10.—Kellogg Transmitter Diaphragm and Cell. eell is pressed out of the aluminum sheet. The front electrode is held into it by a stem and nut. The back electrode is mounted on the back stem, which is clamped into the back bridge. The mica diaphragm closes the annular opening around the back electrode, being held onto the diaphragm by means of an aluminum ring, riveted onto the main diaphragm, Current is led into the back electrode through the bridge, and into the front electrode LOCAL by means of a wire BATTHRY fastened 25 to the aluminum dia- phragm by one of the rivets. The support of the edge of the main diaphragm is a matter of considerable importance. Formerly a soft rubber band was slipped over the edge of the disc. This made a soft pad which made the space air tight and prevented extra vibrations. But in time the rubber hardens and resists the vibrations of the diaphragm. Then it must be replaced. Now it is considered better to use a fairly hard ring and to obtain freedom of motion by bending the edge of the diaphragm over into a flange, as shown in Figure 7. The little flat edge which rests on the empire cloth ring may stick fast to the cloth, but even then it will not retard the motion of the diaphragm appreciably. Tension springs were formerly placed at the edge of the diaphragm and part way in. Now the springs are usually made to press as near to the center as possible. Center damping is better because it reduces the possibility of the formation of nodes, and the vibration of the diaphragm in parts. It should vibrate as a whole. The Induction is one form same principles formers which Coil.—The telephone induction coil of transformer. as are handle used our It operates in the lighting and on power power the transcur- rents. But the conditions of use are different and its structure is also not the same. The induction coil must handle weak currents of many frequencies and should treat them alike as much as possible. No one frequency may be unduly weakened, and no one frequency may be unduly strenethened. For this reason the magnetic circuit is not completely iron—part of it is air. Iron tends to make the speech loud, but not as clear as is desired, because there is a tendency to favor 26 TELEPHONY ‘ some frequencies more than Ane: A coil made without iron, an air core, is most clear, but it is too weak. Hence there is a compromise in the use of a straight iron core, with the return magnetic circuit in the air. If the iron in the core is solid, many eddy currents will exist in the iron. The same was found to be true in power transformers. It sapped the strength of the current, so that lamination was employed to reduce the circulation of current in the iron. But even with thin sheets of iron, about 0.35 to 0.56 mm (0.014 to 0.020 inch) thick, there are still some eddy currents left. It has been found that iron with about 3.5 to 4 per cent of silicon in it has a higher resistance to electrical current than good magnetic iron, and therefore has less eddy current. It also possesses higher permeability, therefore produces more magnetism. For these two reasons, silicon iron is now used in the best telephone induction coils. The primary of a local battery induction coil is made of 300 or 400 turns of wire, 0.5 to one ohm with an inductanee of 8 to 10 millihenries. The secondary may have from 1,500 to 2,500 turns, with 150 to 250 wmillihenries inductance. The transformation ratio is about is, the secondary has about turns as the primary. 4 or 6 to one, that five times as many CHAPTER COMMON III BATTERY The use of local battery magneto telephones necessitates considerable maintenance work. To replace dry cells is the greatest part of it, but hand generators come in for their share of attention. For these reasons the operating of telephones by one battery located in the central office of the exchange has become the standard with Figure 11.—Common Battery Bridged Resistance. nearly all operating companies. It simplifies the substation apparatus, gives automatic signalling on manual boards, and easy control over the switches in automatic exchanges. Common battery working was proposed in 1890. During the period 18921899 it was being developed and applied, while by about 1900 it was considered to have arrived. It is true that common battery causes the transmitter to work in a much higher resistance circuit, since it includes the subscriber line, which may 27 28 TELEPHONY vary from five or ten ohms up to 500 or even 700 or 800 ohms. The longer the line, the weaker the sending of speech. But on the whole, it has proven to be the better way of working exchange lines, and by careful engineering and design, common battery has been made to talk remarkably well. There are a number of ways by which common battery service may be rendered. The simplest is to connect two telephones in series with each other and the line and battery. This is done in some short private lines. But it is not feasible in pub- Le Figure 12.—Bridged Resistance Rearranged to Show Action. lie exehanges. Those which follow show the development of the idea, though not in order of time. The Bridged Resistance—If across the two line wires between two telephone instruments, Figure 11, T-1 and T-2, we bridge resistance and battery, we shall have a simple common battery system. Talking into the transmitter of either telephone will cause the current in its loop to undulate, and this will cause the current in the other leop to undulate also. The simple application of Ohm’s law will show that whenever one transmitter lowers COMMON its resistance, it will BATTERY draw more 29: current, and be- eause of the bridged resistance, the other telephone will get less current. This may be more easily seen in Figure 12, where the same circuit has been thrown into the form of a divided circuit. T-1 is in one branch and T-2 is in the other, while the resistances A and B are in the common battery branch. The Stone System.—tThis is made out of the above system by using inductive coils, or impedances, instead of resistances. Figure 13. They greatly increase the effect which one subscriber loop has Figure 13.—Stone System of Common Battery. over the other, because an inductive coil resists any change in the strength of current in it. In this it is like the flywheel of an engine, when the torque inereases, the mass of the wheel prevents sudden increase of speed, and when the torque decreases, the wheel tends to keep the speed up. The two subscriber loops are the intended path of alternating current, the conversation current. The battery current is supreme in the common branch, the inductive coils. There is a little of the alternating current in these coils, but if they are well made, it is negligible. This circuit may be rearranged as in Figure 14 to show a little better the action. As far as the 20 TELEPHONY flow of battery current is concerned, the two telephone instruments are in parallel. If they were of equal resistance, the current would divide equally. Usually the two telephones are not equal, so that one gets a little more direct current than the other. As long as the transmitters are quiet, the battery current is steady, unvarying in value. When one transmitter rises in resistance, the current in that telephone falls. But the inductance of the two coils will resist this change, desiring to keep the current from falling. Hence the current in the other telephone will be increased. This in- igure 14.—Stone System Rearranged to Show Action. crease is more than would occur if the coil were a plain resistance without inductance. When the same transmitter falls in resistance below its former steady resistance, the current in its telephone rises. But the inductance of the coils will resist this increase, and tend to keep the current constant. This deprives the other telephone of some of the current which it would otherwise get. In this way, the varying resistance of one transmitter causes the eurrent in the other telephone to undulate in the same way. The inductive coils give louder transmission than COMMON BATTERY 31 plain resistance coils, but this result is due in part to the rapidity with which the transmitter changes resistance. If a transmitter increased resistance a certain amount, and then held that value indefinitely, the coils would momentarily increase the current in the other telephone, but it would soon sink to the slightly inereased value which it would have if the coil were non-inductive. The Stone system is used in some small private telephone systems, also in every reverting call on a party line. Figure 15.—Condenser-Impedence Battery. System of Common Condenser-Impedance System.—The condenser-impedance system of common battery transmission, Figure 15, has two impedance coils for each subseriber line, and each telephone has its own separate circuit for the supply of direct current. The two lines are connected by means of condensers. The two coils, A and B, are usually wound on one iron core, so as to secure greater magnetic effect and therefore more impedance to the alternating talking current. The coils, C and D, for the other telephone are on another core. In this way each telephone will get all the battery current which it can get in view of its resistance. A 32 TELEPHONY short line can not rob a long line of current, as is the case in the Stone system. As long as both transmitters are quiet, the current in both loops flows steadily, and there is no alternating current flow through the condensers. But if someone talks into the transmitter of T-1, the current in that loop will undulate. This produces an undulating voltage at the terminals of the condensers, C-1 and C-2. Alternating current now flows through the two condensers and causes the battery current in the other loop to undulate. This Figure 16.—Condenser-Impedence to affects the receiver Show System Rearranged Action. of 7-2, reproducing the speech. A better understanding of the relationships and action can be obtained if we rearrange the circuit, as in Figure 16, Here it is seen that we have an extended Wheatstone bridge. Coils A and C are the proportional arms at one end, and coils B and D are the other two arms at the other end. The two telephones, with their lines, close the gap between the two ends. The two condensers take the places of galvanometers across the circuit. If the two telephones and their lines were equal COMMON to each other BATTERY in resistance, there 33 would be no dif- ference of potential across the condensers, and they would not be charged. But the lines and telephones are seldom equal, hence the current in one telephone will be stronger than that in the other, and there will be a little charge on each condenser. But whether there is a charge or not, the condensers function the same. The action of the impedance coils in this system is like their action in the Stone system, except that there is no “‘resistance shunting.’’ The action is purely electrostatic induction through the condensers. When one transmitter, T-1, lowers its resistance, the voltage from a to b is lowered. The magnetic inertia of the coils, A and B, resists any increase of current, so that this lowering of voltage is greater than would result from simple resistance action. The lowered voltage across a-b permits the condensers to discharge some electricity, which flows from C-2, past 6b, through 7-1, a, C-1, c and telephone T-2. When the same transmitter, 7-1, increases its resistance, the voltage from a to 6b is raised. This would tend magnetic lowering to reduce the current flow, but the inertia of the coils A and B resists any of current, and continue to send as much eurrent as ever. The excess, which 7-7 can not take, is forced through the condensers into telephone 7-2 in the direction b, C-2, d, T-2, c, C-1, to a. Thus a complete cycle of decrease and increase of resistance of the transmitter of 7-1 results in a complete cycle of alternating current in telephone 7-2. In this way the mesh composed of the two telephones, their lines, and the two condensers is traversed by alternating current, which is the talking current. There is little alternating current 34 TELEPHONY in the battery branch of the circuit, including the impedanee coils. : Sometimes the condensers are transposed, as in This has been found to have the Figure 17. effect of reducing still more any noise which may come from the battery during the time that it 1s The two condensers form a by-pass being charged. for any such noisy current, and tend to shunt it from the telephones. Figure 17.—Condenser-Impedence System Condensers. with Transposed The Hayes System—The Hayes system of common battery transmission (invented 1892, H. V. Hayes), Figure 18, uses a repeating coil to link the two subseriber lines together. The four windings, A, B, C and D, are on one iron core, so that all act as a transformer. Each telephone has its own separate battery current cireuit, as in the condenser-impedance system. But there is a difference, that the battery is in the alternating current circuit. It must be a very quiet battery, in good condition and of low resistance, so that while being charged COMMON BATTERY 35 the generation noise will not be heard. Usually a special ‘‘telephone’’ generator is used to charge the battery—a generator which produces very little undulation or ‘‘ripples’’ in its current. The condenser-impedance system can stand a much more noisy battery without disturbing conversation, because the battery is not in the path of the alternating talking current. The operation of the Hayes system is very much like that of the local battery telephone with an in- Figure 18.—Hayes System of Common Battery, duction coil. The transmitter by varying its resistance causes the battery eurrent to undulate. This undulating current makes the magnetism of the induction coil vary, which in turn causes an alternating voltage to exist in the coil which are in the other subscriber line. windings This in- duced voltage makes the direct current in that line rise and fall, and actuates the receiver in the re. ceiving telephone. The current in both loops is undulating, and may be resolved into direct current and alternating current. In the sending loop, the current undulates 36 TELEPHONY by the rheostatic control of the transmitter. In the receiving loop, the alternating current is mixed with the direct current, resulting in undulating current. COMMON BATTERY AND LOCAL BATTERY Common battery is displacing local battery in all exchanges of appreciable size. Local battery still holds its own on rural lines in very many places. The latter gives louder, clearer transmission than the former, if its battery is new and strong. But common battery gives better average transmission, because the battery is of constant strength. The common battery telephone instrument is also simpler, as will appear in the description of telephone instruments. CHAPTER LOCAL BATTERY IV TELEPHONE INSTRUMENT The simplest needs of a complete telephone outfit are to be found in a private line with a local battery, magneto telephone at each end, Figure 19. It connects two points so that A can talk to B over the telephone line. <A study of the instruments and of the line will give the reader a good insight into the fundamentals of telephone communication, |A | aalepiere tire |B | Telephone Teiephone : instrument Figure instrument 19.—Simplest Telephone System. Each telephone instrument needs to have talking apparatus, signalling apparatus, and switching ap: paratus. Short lines, often entirely within buildings, are quite easily rung with battery bells, in which case the arrangement of Figure 20 is suitable. The talking apparatus consists of the transmit