In our younger days most of us have amused ourselves with a toy telephone consisting of a long piece of string having each end passed through the bottom of a little cardboard box, and secured by a knot. If the string is stretched tightly this arrangement enables whispered words to be heard at a distance of 20 or 30 yards. Simple as is this little toy, yet it is probable that many people would be rather nonplussed if asked suddenly to explain how the sounds travel along the string from one box to the other. If the toy had some complicated mechanism most likely every one would want to know how it worked, but the whole thing is so extremely simple that generally it is dismissed without a thought.
If we strike a tuning-fork and then hold it close to the ear, we hear that it produces a sound, and at the same time, from a slight sensation in the hand, we become aware that the fork is in vibration. As the fork vibrates it disturbs the tiny particles of air round it and sets them vibrating, and these vibrations are communicated from one particle to another until they reach the drum of the ear, when that also begins to vibrate and we hear a sound. This is only another way of saying that the disturbances of the air caused by the vibrations of the tuning-fork are propagated in a series of waves, which we call “sound waves.” Sound is transmitted better through liquids than through the air, and better still through solids, and this is why words spoken so softly as to be inaudible through the air at a distance of, say, 100 feet, can be heard fairly distinctly at that distance by means of the string telephone. The sound reaches us along the string in exactly the same way as through the air, that is, by means of minute impulses passed on from particle to particle.
A more satisfactory arrangement than the string telephone consists of two thin plates of metal connected by a wire which is stretched very tightly. Words spoken close to one plate are heard by a listener at the other plate up to a considerable distance. Let us try to see exactly what takes place when this apparatus is used. In the act of speaking, vibrations are set up in the air, and these in turn set up vibrations in the metal plate. The vibrations are then communicated to the wire and to the metal plate at the other end, and finally the vibrations of this plate produce vibrations in the air between the plate and the listener, and the sound reaches the ear.
This simple experiment shows the remarkable fact that a plate of metal is able to reproduce faithfully all the vibrations communicated to it by the human voice, and from this fact it follows that if we can communicate the vibrations set up in one plate by the voice, to another plate at a distance of 100 miles, we shall be able to speak to a listener at the further plate just as if he were close to us. A stretched string or wire transmits the vibrations fairly well up to a certain distance, but beyond this distance the vibrations become weaker and weaker until no sound at all reaches the air. By the aid of electricity, however, we can transmit the vibrations to a tremendous distance, the range being limited only by the imperfections of our apparatus.
The first attempt at the construction of an electric telephone, that is an instrument by means of which the vibrations set up by the voice or by a musical instrument are transmitted by electricity, was made in 1860 by Johann Philipp Reis, a teacher in a school at Friedrichsdorf, in Germany. His transmitting apparatus consisted of a box having a hole covered by a tightly stretched membrane, to which was attached a little strip of platinum. When the membrane was made to vibrate by sounds produced close to the box, the strip of platinum moved to and fro against a metal tip, which closed the circuit of a battery. The receiver was a long needle of soft iron round which was wound a coil of wire, and the ends of the needle rested on two little bridges of a sounding box. The vibrations of the membrane opened and closed the circuit at a great speed, and the rapid magnetization of the needle produced a tone of the same pitch as the one which set the membrane vibrating. This apparatus transmitted musical sounds and melodies with great accuracy, but there is considerable difference of opinion as to whether it was able to transmit speech. Professor Sylvanus Thompson distinctly states that Reis’s telephone could and did transmit speech, but other experts dispute the fact. We probably shall be quite safe in concluding that this telephone did transmit speech, but very imperfectly. In any case it is certain that the receiver of this apparatus is not based on the same principle as the modern telephone receiver.
Some years later Graham Bell, Professor of Vocal Physiology in the University of Boston, turned his attention to the electric transmission of speech, probably being led to do so from his experiments in teaching the deaf and dumb. His apparatuses shown at an exhibition in Philadelphia in 1876, consisted of a tube having one end open for speaking into, and the other closed by a tightly stretched membrane to which was attached a very light steel bar magnet. The vibrations set up in the membrane by the voice made the little magnet move to and fro in front of the poles of an electro-magnet, inserted in a battery circuit, thus inducing currents of electricity in the coils of the latter magnet. The currents produced in this way varied in direction and strength according to the vibratory movements of the membrane, and being transmitted along a wire they produced similar variations in current in another electro-magnet in the receiver. The currents produced in this manner in the receiver set up vibrations in a metal diaphragm in front of the magnet poles, and so the words spoken into the transmitter were reproduced.
Since the year 1876 the telephone has developed with remarkable rapidity, and an attempt to trace its growth would involve a series of detailed descriptions of closely similar inventions which would be quite uninteresting to most readers. Now, therefore, that we have introduced the instruments, and seen something of its principle and its early forms, it will be most satisfactory to omit the intermediate stages and to go on to the telephone as used in recent years. The first telephone to come into general use was the invention of Graham Bell, and was an improved form of his early instrument just described. A case or tube of ebonite, which forms the handle of the instrument, contains a steel bar magnet having a small coil of insulated wire at the end nearest the mouthpiece of the tube, the ends of the coil passing along the tube to be connected to the line wires. Close to the coil end of the magnet, and between it and the mouthpiece, is fixed a diaphragm of thin sheet-iron. A complete outfit consists of two of these instruments connected by wires, and it will be noticed that no battery is employed.
The air vibrations set up by the voice make the diaphragm vibrate also, so that it moves backwards and forwards. These movements are infinitesimally small, but they are sufficient to affect the lines of force of the magnet to such an extent that rapidly alternating currents of varying degrees of strength are set up in the coil and sent along the line wire. On arriving at the receiver these currents pass through the coil and produce rapid variations in the strength of the magnet, so that instead of exerting a uniform attraction upon the iron diaphragm, the magnet pulls it with constantly varying force, and thus sets it vibrating. The air in front of the diaphragm now begins to vibrate, and the listener hears a reproduction of the words spoken into the transmitter. The way in which the fluctuations of the current make the second diaphragm vibrate exactly in accordance with the first is very remarkable, and it is important to notice that the listener does not hear the actual voice of the speaker, but a perfect reproduction of it; in fact, the second diaphragm speaks.
The reader probably will be surprised to be told that the transmitter and the receiver of a magneto-electric telephone are respectively a dynamo and electric motor of minute proportions. We provide a dynamo with mechanical motion and it gives us electric current, and by sending this current through an electric motor we get mechanical motion back again. In the transmitter of the telephone just described, the mechanical motion is in the form of vibrations of the metal diaphragm, which set up currents of electricity in the coil of wire round the magnet, so that the transmitter is really a tiny dynamo driven by the voice. The receiver is provided with electric current from the transmitter, and it converts this into mechanical motion in the diaphragm, so that the receiver is a little electric motor.
Transmitters of the type just described work well over short distances, but the currents they produce are too feeble for transmission over a very long wire, and on this account they have been superseded by transmitters on the microphone principle. A microphone is an instrument for making extremely small sounds plainly audible. If a current is passed through a box containing loose bits of broken carbon, it meets with great resistance, but if the bits of carbon are compressed their conducting power is considerably increased. Even such slight differences in pressure as are produced by vibrating the box will affect the amount of current passing through the carbon. If this current is led by wires to an ordinary telephone receiver the arrangement becomes a simple form of microphone. The vibrations of the box vary the resistance of the carbon, and the corresponding variations in the current set up vibrations in the receiver, but in a magnified form. The smallest sound vibrations alter the resistance of the carbon, and as these vibrations are magnified in the receiver, the reproduced sound is magnified also. The footsteps of a fly may be heard quite distinctly by means of a good microphone, and the ticks of a watch sound like the strokes of a hammer.
By means of this power of magnifying vibrations a microphone transmitter can be used on a line of tremendous length, where an ordinary Bell transmitter would be utterly useless. The general features of this transmitter, Fig. 31, are a diaphragm and a block of carbon separated slightly from one another, the intervening space being filled with granules of carbon. These are enclosed in a case of ebonite having a mouthpiece in front and two terminals behind, one terminal being connected with the carbon block and the other with the diaphragm. From these terminals wires are led to a battery and to the receiver, which is of the Bell type. The current has to pass through the carbon granules, and the movements of the diaphragm when set in vibration by the voice vary the pressure upon the granules, and in this way set up variations in the current. Carbon dust also may be used instead of granular carbon, and then the instrument is called a “dust transmitter.”
It is usual to have a transmitter and a receiver on one handle for the greater convenience of the user. The arrangement is shown in Fig. 32, and it will be seen that when the user places the receiver to his ear the transmitting mouthpiece is in position for speaking. The microphone with its carbon dust is placed at A, just below the mouthpiece, and the earpiece or receiver B contains a little magnet and coil with a diaphragm in front, so that it is really a Bell instrument. A little lever will be noticed at C. This is a switch which brings the transmitter into circuit on being pressed with the finger.
It is now time to see something of the arrangement and working of telephone systems. As soon as the telephone became a commercially practicable instrument the necessity for some means of inter-communication became evident, and the telephone exchange was brought into being. The first exchange was started in 1877, in Boston, but this was a very small affair and it was run on very crude lines. When one subscriber wished to communicate with another he had to call up an operator, who received the message and repeated it to the person for whom it was intended; there was no direct communication between the various subscribers’ instruments. As the number of users increased it became necessary to devise some system whereby each subscriber could call the attention of an operator at the central station, and be put into direct communication with any other subscriber without delay; and the exchange system of to-day, which fulfils these requirements almost to perfection, is the result of gradual improvements in telephone methods extending over some thirty-five years.
When a subscriber wishes to telephone, he first must call up the operator at the exchange. Until comparatively recently this was done by turning a handle placed at the side of the instrument. This handle operated a little dynamo, and the current produced caused a shutter at the exchange to drop and reveal a number, just as in the electric bell indicator, so that the operator knew which instrument was calling. As soon as the operator answered the call, the shutter replaced itself automatically. The signal to disconnect was given in the same way, but the indicator was of a different colour in order to prevent confusion with a call signal. These handle-operated telephones are still in common use, but they are being replaced by instruments which do away with handle-turning on the part of the subscriber, and with dropping shutters at the exchange. In this latest system all that the subscriber has to do is to lift his telephone from its rest, when a little electric lamp lights up at the exchange; and when he has finished his conversation he merely replaces the telephone, and again a little lamp glows.
We must now see what happens at the exchange when a call is made. Each operator has control of a number of pairs of flexible cords terminating in plugs, the two cords of each pair being electrically connected. The plugs rest on a shelf in front of the operator, and the cords pass through the shelf and hang down below it. If a plug is lifted, the cord comes up through the shelf, and it is drawn back again by a weight when the plug is not in use. Two lamps are provided for each pair of cords, one being fixed close to each cord. The two wires leading from each subscriber’s instrument are connected to a little tube-shaped switch called a “jack,” and each jack has a lamp of its own. When a subscriber lifts his telephone from its rest a lamp glows, and the operator inserts one plug of a pair into the jack thus indicated, and the lamp goes out automatically. She then switches on her telephone to the caller and asks for the number of the subscriber to whom he wishes to speak; and as soon as she gets this she inserts the other plug of the pair into the jack belonging to this number. By a simple movement she then rings up the required person by switching on the current to his telephone bell.
Here comes in the use of the two lamps connected with the cords. As long as the subscribers’ telephones are on their rests the lamps are lighted, but as soon as they are lifted off the lamps go out. The caller’s telephone is of course off its rest, and so the lamp connected with the first cord is not lit; but until the subscriber rung up lifts his instrument to answer the call, the lamp of the second cord remains lit, having first lighted up when the plug was inserted in the jack of his number. When the second lamp goes out the operator knows that the call has been responded to, and that the two subscribers are in communication with each other. Having finished their conversation, both subscribers replace their instruments on the rests, whereupon both lamps light up, informing the operator that she may disconnect by pulling out the plugs.
It is manifestly impossible for one operator to attend to the calls of all the subscribers in the exchange, and so a number of operators are employed, each one having to attend to the calls of a certain number of subscribers. At the same time it is clear that each operator may be called upon to connect one of her subscribers to any other subscriber in the whole exchange. In order to make this possible the switchboard is divided into sections, each having as many jacks as there are lines in the exchange, so that in this respect all the sections are multiples of each other, and the whole arrangement is called a “multiple switchboard,” the repeated jacks being called “multiple jacks.” Then there are other jacks which it is not necessary to duplicate. We have seen that when a subscriber calls the exchange a lamp glows, and the operator inserts a plug into the jack beside the lamp, in order to answer the call and ascertain what number is required. These are called “answering jacks,” and the lamp is the line signal. It is usual to have three operators to each section of the switchboard, and each operator has charge of so many answering jacks, representing so many subscribers. At the same time she has access to the whole section, so that she can connect any of her subscribers to any other line in the exchange.
When a number is called for, the operator must be able to tell at once whether the line is free or not. The jack in her section may be unoccupied, but she must know also whether all the multiple jacks belonging to that number are free, for an operator at another section may have connected the line to one of her subscribers. To enable an operator to ascertain this quickly an electrical test is provided. When two lines are connected, the whole of the multiple jacks belonging to each are charged with electricity, and if an operator at any section touches one of these jacks with a plug, a current through her receiver makes a click, and on hearing the click she knows that the line is engaged. The testing takes an extremely short time, and this is why a caller receives the reply, “Number engaged,” so promptly that he feels inclined to doubt whether the operator has made any attempt at all to connect him up to the number.
In order that an operator may have both hands free to manipulate the plugs, her telephone receiver is fixed over one ear by a fastening passing over her head, and the transmitter is hung from her shoulders so as to be close to her mouth.
In telegraphy it is the rule to employ the earth for the return part of the circuit, but this is not customary in telephony. The telephone is a much more sensitive instrument than the telegraph, and a telephone having an earth return is subject to all kinds of strange and weird noises which greatly interfere with conversation. These noises may be caused by natural electrical disturbances, or by the proximity of telegraph and other wires conveying electric currents. On this account telephone lines are made with a complete metallic circuit. As in telegraphy, protection from lightning flashes is afforded by lightning arresters. The current for the working of a telephone exchange is supplied from a central battery of accumulators, and also from dynamos.
PLATE XII.
By permission of
Craven Brothers Ltd.
LARGE ELECTRIC TRAVELLING CRANE AT A RAILWAY WORKS.
Although the manual exchange telephone system of to-day works with remarkable efficiency, it has certain weak points. For instance, if an operator cares to do so, she can listen to conversations between subscribers, so that privacy cannot be assured. As a matter of fact, the operators have little time for this kind of thing, at any rate during the busy hours of the day, and as a rule they are not sufficiently interested in other people’s affairs to make any attempt to listen to their remarks. The male operators who work through the slack hours of the night are occasionally guilty of listening. Some time ago the writer had to ring up a friend in the very early morning, and during the conversation this gentleman asked what time it was. Before the writer had time to get a word out, a deep bass voice from the exchange replied, “Half-past two.” Little incidents of this sort remind one that it is not wise to speak too freely by telephone. Then again operators are liable to make wrong connexions through faulty hearing of the number called for, and these are equally annoying to the caller and to the person rung up in mistake. Many other defects might be mentioned, but these are sufficient to show that the manual system is not perfect.
For a long time inventors have been striving to do away with all such defects by abolishing the exchange operators, and substituting mechanism to work the exchanges automatically, and during the last few years the system of the Automatic Electric Company, of Chicago, has been brought to great perfection. This system is in extensive use in the United States, and is employed in two or three exchanges in this country. Unfortunately the mechanism of this system is extremely complicated, so that it is impossible to describe it fully in a book of this kind; but some idea of the method of working may be given without entering into technical details.
Each subscriber’s telephone instrument is fitted with a dial which turns round on a pivot at its centre. This dial has a series of holes round its circumference, numbered consecutively from 1 to 9, and 0. Suppose now a subscriber wishes to speak to a friend whose telephone number is 2583. He removes the receiver from its hook, places his finger in the hole marked 2, and turns the dial round in a clockwise direction until his finger comes in contact with a stop. He then removes his finger, and the dial automatically returns to its original position. He then places his finger in the hole marked 5, and again turns the dial as far as the stop, and when the dial has returned to the normal position he repeats the process with his finger placed successively in the holes marked 8 and 3. He now places the receiver to his ear, and by the time he has done this the automatic mechanism at the exchange has made the necessary connexions, and has rung the bell of subscriber number 2583. On completing the conversation each subscriber returns his receiver to its hook, and the exchange mechanism returns to its normal position.
The turning of the dial by the finger coils up a spring, and this spring, acting along with a speed governor, makes the dial return to its first position at a certain definite speed as soon as the finger is removed. During this retrograde movement a switch automatically sends out into the line a certain number of impulses, the number being determined by the hole in which the finger is placed. In the case supposed, groups of two, five, eight, and three impulses respectively would be sent out, each group separated from the next by an interval during which the subscriber is turning the dial.
Now let us see what takes place at the exchange. The subscriber’s instrument is connected to a mechanical arrangement known as a “line switch.” This switch is brought into play by the act of removing the receiver from its hook, and it then automatically connects the subscriber’s line to what is called a “first selector” switch. The group of two impulses sent out by the first turning of the dial raises this first selector two steps, and it then sweeps along a row of contacts connected to “trunks” going to the 2000 section. Passing by occupied trunks, it finds an idle one, and so connects the line to an idle “second selector.” This selector is operated by the second group of impulses, five in number, and after being raised five steps it acts like the first selector, and finds an idle trunk leading to the 2500 section. This places the caller’s line in connexion with still another switch called a “connector,” and this switch, operated by the remaining groups of eight and three impulses, finds the required tens section, and selects the third member of that section. If the number 2583 is disengaged, the connector switch now sends current from the central battery to this instrument, thus ringing its bell, and it also supplies speaking current to the two lines during the conversation, restores the exchange mechanism to its original condition as soon as the conversation is ended and the subscribers have hung up their receivers, and registers the call on the calling subscriber’s meter. If the connector finds the number engaged, it sends out an intermittent buzzing sound, to inform the caller of the fact. All these operations take time to describe, even in outline, but in practice they are carried out with the utmost rapidity, each step in the connecting-up process taking only a small fraction of a second.
For ordinary local calls the automatic system requires no operators at all, but for the convenience of users there are usually two clerks at the exchange, one to give any information required by subscribers, and the other to record complaints regarding faulty working. For trunk calls, the subscriber places his finger in the hole marked 0, and gives the dial one turn. This connects him to an operator at the trunk switchboard, who makes the required connexion and then calls him up in the usual way.
It might be thought that the complex mechanism of an automatic exchange would constantly be getting out of order, but it is found to work with great smoothness. Each automatic switchboard has a skilled electrician in attendance, and he is informed instantly of any faulty working by means of supervisory lamps and other signals. Even without these signals the attendant would be quickly aware of any breakdown, for his ear becomes so accustomed to the sounds made by the apparatus during the connecting-up, that any abnormal sound due to faulty connecting attracts his attention at once. However detected, the faults are put right immediately, and it often happens that a defective line is noted and repaired before the subscriber knows that anything is wrong.
On account of its high speed in making connexions and disconnexions, its absolute accuracy, and its privacy, the automatic telephone system has proved most popular wherever it has been given a fair trial. Its advantages are most obvious in large city exchanges where the traffic during business hours is tremendously heavy, and it is probable that before very long the automatic system will have replaced manual methods for all such exchanges.
The telephone system is more highly developed in the United States than in this country, and some of the exchanges have been made to do a great deal more than simply transmit messages. For instance, in Chicago there is a system by which a subscriber, on connecting himself to a special circuit, is automatically informed of the correct time, by means of phonographs, between the hours of 8 a.m. and 10 p.m. New York goes further than this however, and has a regular system of news circulation by telephone. According to Electricity, the daily programme is as follows: “8 a.m., exact astronomical time; 8 to 9 a.m., weather reports, London Stock Exchange news, special news item; 9 to 9.30 a.m., sales, amusements, business events; 9.45 to 10 a.m., personal news, small notices; 10 to 10.30 a.m., New York Stock Exchange and market news; 11.30 a.m. to 12 noon, local news, miscellaneous; 12 noon, exact astronomical time, latest telegrams, military and parliamentary news; 2 to 2.15 p.m., European cables; 1.15 to 2.30 p.m., Washington news; 2.30 to 2.45 p.m., fashions, ladies’ news; 2.45 to 3.15 p.m., sporting and theatrical news; 3.15 to 3.30 p.m., closing news from Wall Street; 3.30 to 5 p.m., musical news, recitals, etc.; 5 to 6 p.m., feuilleton sketches, literary news; 8 to 10.30 p.m., selected evening performance—music, opera, recitations.” Considering the elaborate nature of this scheme one might imagine that the subscription would be high, but as a matter of fact it is only six shillings per month.
The telephone has proved of great value in mine rescue work, in providing means of communication between the rescue party and those in the rear. This end is achieved by means of a portable telephone, but as the members of a rescue party often wear oxygen helmets, the ordinary telephone mouthpiece is of no use. To overcome this difficulty the transmitter is fastened round the throat. The vibrations of the vocal cords pass through the wall of the throat, and thus operate the transmitter. The receiver is fixed over one ear by means of suitable head-gear, and the connecting wire is laid by the advancing rescuers. A case containing some hundreds of feet of wire is strapped round the waist, and as the wearer walks forward this wire pays itself out automatically.
By the time that the telephone came to be a really practical instrument, capable of communicating over long distances on land, the Atlantic telegraph cable was in operation, and an attempt was made to telephone from one continent to the other by means of it, but without success. In speaking of submarine telegraphy in Chapter XVII. we saw that the cable acts like a Leyden jar, and it was this fact that made it impossible to telephone through more than about 20 miles of cable, so that transatlantic telephony was quite out of the question. It was evident that little progress could be made in this direction unless some means could be devised for neutralizing this capacity effect, as it is called, of the cable, and finally it was discovered that this could be done by inserting at intervals along the cable a number of coils of wire. These coils are known as “loading coils,” and a cable provided with them is called a “loaded cable.” Such cables have been laid across various narrow seas, such as between England and France, and England and Ireland, and these have proved very successful for telephonic communication. The problem of transatlantic telephony however still remains to be solved. Experiments have been made in submarine telephony over a bare iron cable, instead of the usual insulated cable. Conversations have been carried on in this way without difficulty between Seattle, Washington, U.S.A., and Vashon Island, a total distance of about 11 miles, and it is possible that uninsulated cables may play an extremely important part in the development of submarine telephony.