Popular Science Monthly/Volume 12/March 1878/The Telephone and How it Works
|THE TELEPHONE AND HOW IT WORKS.|
By GEORGE M. SHAW.
AMONG the innumerable uses of electricity none is more remarkable than its employment for the transmission of sound. The ultimate mystery of the action we cannot, of course, undertake to explain, but the mechanism by which it is produced is by no means difficult to understand.
If a wire, from a galvanic battery such as is shown in Fig. 1, through which a current of electricity is passing, be wound around a piece of steel or soft iron, as represented in Fig. 2, some curious things will happen. If the bar be soft iron, it will be made magnetic, and kept in that condition as long as the current continues to pass Fig. 1. round it, and its ends will then attract and hold bits of iron, but drop them when the battery is taken away. If the bar be of steel, instead of soft iron, it will be magnetized and attract iron just as before; but, unlike the soft-iron bar, it will keep its magnetism and attract the iron even after the battery is removed. Its magnetism will be permanent. Since, however, electricity made the magnet, we can, in turn, make the magnet a source of electricity. Suppose the magnetized steel bar has attracted and is holding on to a piece of iron. We can now take the battery away and join the ends of the wire, as in Fig. 8; then, if the piece of iron be pulled off and stuck on again, a current of electricity will run through the wire every time it is done. Electricity produced in this way is called magneto-electricity, and the current in the wire is said to be an induced electric current. If, now, the wire from bar No. 1 (Fig. 4), be extended to a distance, and coiled around another magnetized bar Fig. 2. (No. 2), the currents induced in it, by making and breaking the contact of the piece of soft iron with the first magnet, will simultaneously affect the magnetism in the distant magnet also. Though the magnets be a mile or a hundred miles apart, the disturbance in one is immediately and equally manifested in the other.
But, what is still more remarkable, these induced currents may be sent through the wire without the actual contact of the soft iron with the steel bar. If this piece of iron is brought very near to one magnet without touching it, and then withdrawn, an electric thrill or wave is induced in the wire which is felt in the distant magnet, just as if the contact had been actually made and broken. And so if we play the piece of soft iron backward and forward, before the magnet, no matter how rapidly or slightly, each motion is felt as an electric pulse in the magnet at the other end. To borrow a metaphor from life, it is as if the close approach and quick oscillation of the piece of soft iron fretted or tantalized the magnet, and sent a series of electrical shudders through the iron nerve.
We have here the fundamental principle of the telephone. No galvanic battery is employed to furnish an electrical current, as in the case of the telegraph; but the currents in the wires are produced by the motions of the piece of soft iron near the magnet. Thus far we have represented these motions in a very rude and coarse way, as if the piece of iron were vibrated backward and forward by the hand; Fig. 3. but what we have really to deal with is something infinitely more delicate than this. The piece of soft iron of which we have been speaking, shown at a, Figs. 3 and 4, represents what is called the diaphragm of the telephone, which is a thin, circular sheet of iron, a couple of inches in diameter, held by its rim, and adjusted so that its centre comes very close to the end of the magnetized bar. And the motions which now concern us are simply the vibrations produced in this iron membrane by the beats against it of agitated air. Everybody knows that sounds are propagated through the aerial medium by waves that travel swiftly from their sources, and that we hear them because the waves strike in rapid succession upon the drum of the ear. It is also well understood that these waves differ greatly in their rates, depending upon the rapidity of vibration in the sounding body; and, moreover, that they are very complex, there being waves within waves of various orders in a single tone. It is the special complexity of these wave-systems, in the different cases, that gives those peculiarities of tone that mark different musical instruments and distinguish the voice in different individuals. These
waves, started by a person talking, beat against the diaphragm of the telephone as they beat against the tympanum of the ear, and throw it into vibrations, which are reproduced in the thrills of the magnet that again excite tremors in the wires, and these, affecting the magnet, at the other end set the other diaphragm into vibration, and this gives out a new set of air-waves which, falling on the tympanum of the listener, reproduces the original sound or voice. The arrangement being the same at each end, the machine, of course, works both ways, so that when a person is talking to the distant diaphragm the direction is reversed, and the sounds are emitted by the diaphragm near by, and thus the original talker gets his response. The action set up in one telephone is instantaneously repeated or copied by the other. And so by this marvelous chain of effects a person discoursing, apparently, to a piece of iron, may be intelligibly heard hundreds of miles away, and a conversation kept up between the distant parties as if they were in the same room. The "ear-shot" from the beginning of the world has been but a few hundred feet; by the invention of this little contrivance it is now extended to hundreds of miles.
So much for the theory of the telephone; let us now note how it may be practically constructed. To make one, procure a tube, bb (Fig. 5), of thin sheet-brass, one inch long, two inches in diameter,
and with a flange one-half inch wide. Then from a ferrotype-plate—the photographer's "tin-type"—cut a round plate, shown edgewise, at c c, to cover the tube bb over the flange. This is the vibrator or diaphragm. Next cut a wooden ring or "washer," d d, the width of the flange, and about one-eighth of an inch thick. Then make a spool, e e, one inch long, of thin sheet-brass again, with one flange wide enough to cover the wooden ring, the tube of the spool being made so as to fit tightly the magnet g, which is a strongly-magnetized steel rod, four inches long and three-eighths of an inch in diameter. The parts will then fit together, and may be screwed firmly through the flanges b b. The least polished side of the plate c c should face the magnet, and it is well to scrape the part opposite the end of the bar, so as to expose the iron. The spool is to be wound with about fifty yards of No. 36 or 38 silk-covered copper wire, the thickness of a bristle. The magnet is then shoved in, till it nearly touches the plate c c. After joining the ends of the spool-wire to the line-wires f f, that run to another instrument just like it, the telephone will be ready for use. It is important to concentrate the voice upon a narrow space at the middle of the plate, and for this purpose a movable wooden mouth-piece, a a, is used, with an opening at the bottom about the size of a dime. This mouth-piece should fit neatly, and reach to within about one-eighth of an inch of the diaphragm. Fig. 6 represents a section of Bell's telephone. E E is the diaphragm, F the tube, B the silk-covered wire wound upon the spool, extending C, to the binding-screws D D, where they are connected with the line-wires. The magnet A has its distance from the diaphragm adjusted by the screw at the opposite end. Fig. 7 represents the form and aspect of the instrument as used. It is about five or six inches long and two and a half inches broad at its widest part. In sending a message, the instrument is held to the mouth,
and the words distinctly spoken in ordinary tones or even a whisper. The instrument is then held to the ear to receive the answer. Instead of this, two telephones connected may be used at each station, so that one may be held to the ear all the time, while the other is used for telephoning, as illustrated in Fig. 8; and this one, too, in hearing a long message, or in a noisy room, may be held to the other ear, and so shut out all other sounds. This also gives two persons a chance to hear at the same time, by giving a telephone to each.
Several telephones may be connected together in one office, so that any number of persons, by having one each, may hear the game message. In singing, each singer has a telephone. At the late fair of the American Institute, we were one evening listening to a quartet of college-boys, uproariously singing "Upidee i-dee-i-da," in the Tribune Building, through six miles of wire, when suddenly all was still. "Hello!" we shouted. "Hello you!" was answered back. "What's the matter?" "Big fire in Leonard Street. The fire is—" "Never mind the fire; go on with the singing," we rejoined, and the singing went on with "The Red, White, and Blue." The impression produced by listening to a communication through this instrument has been aptly described as follows: "The voice, whether in speaking or singing, has a weird, curious sound in the telephone. It is in a measure ventriloqual in character; and, with the telephone held an inch or two from the ear, it has the effect as if some one were singing far off in the building, or the sound were coming up from a vaulted cellar or through a massive stone-wall." The singing or speaking is heard microscopically, as it were, or rather microphonically, but wonderfully distinct and clear in character. The enchantment of distance is there, and one listens as to sounds from fairy-land. The longest distance at which conversation has been carried on, so far, through the telephone, is about 250 miles. With a submarine Fig. 7. cable conversation has been carried on between England and France across the English Channel. Conversation has also been held through the bodies of sixteen persons standing hand-in-hand. The telephone has been regarded as a toy, or a curiosity to be played with; but, while it is undoubtedly extremely interesting as a novelty, it is very much more than this; it is, scientifically and practically, a great success. There are, undoubtedly, difficulties in its use, but, considering that it is a contrivance but of yesterday, the wonder is that it is so perfect. The telegraph was much longer regarded as an impracticable invention, and it is impossible to say how soon the telephone may not take rank among the necessities of common life. If we may trust the analogies of experience, its difficulties are certain to be overcome, although it will probably never meet the exaggerated expectations of many people. Some of the obstacles to the realization of the popular impression of its capabilities have been so well stated in Chambers's Journal, by a gentleman skilled in the working of the telegraph, and who made a series of interesting experiments *on the telephone, that we may fitly close our article by making free extracts from his paper, accompanied by some slight revision: When a telegraphist first gets into his hand this beautifully simple and electrically delicate instrument, his first inclination is to test its carrying-power. This is, of course, a closet experiment, not working with actual telegraph-line, but with 'resistance' equivalent to a telegraph-line of stated length. An experiment of this nature gives better results than could be obtained by a veritable line, because the insulation is, so to speak, perfect. No leakage at undesigned points of contact, or disturbance from unfavorable atmospheric conditions, is felt, and the experiment is entirely under the observer's control. The apparatus used is designed to offer the same labor for the electric current to overcome as would be offered by a stated length of outside telegraph-line. This artificial resistance is nicely graduated, and, as the method of testing was suggested by Ohm, a German electrician, the unit of resistance is termed an 'ohm.' Removing the telephone to such a distance that the two observers were 'out of ear-shot,' the test with resistance was tried, and with a resistance of 1,000 ohms—roughly speaking, equal to seventy miles of a well-constructed line—the sound was perfect, although not very loud. Every articulation of the speaker at the other end could be distinguished so long as silence was maintained in the room, or so long as no heavy lorry rumbling over the stones outside sent in harsh noises which drowned the faint whisper of the instrument. The resistance was gradually raised to 4,000 ohms nearly—300 miles—with like favorable results; and for some little distance beyond, articulation could still be made out. But, by the time 10,000 ohms had been applied, putting the speaker at a distance of, say, 700 miles, sound only, but not articulate sound, reached the ear. The tone was there, and every inflection of the voice could be followed; but articulation was absent, although the listener strained every nerve to catch the sound, which the speaker, as
was afterward ascertained, was shouting in a loud, clear voice. The prolonged notes of an air sung could be heard with the resistance named, but again no words could be distinguished."
The next experiment was to join up the telephones in the office with different line-wires in succession going to various distances, and working with different kinds of telegraph-instruments. "When this was done, the real obstacle to telephonic progress at once asserted itself in the shape of 'induction.' The first wire experimented with was partly 'overhouse' and partly underground, and the offices upon it were working A B C, or printing-instruments. It is difficult to render clear to the person ignorant of telegraphic phenomena the idea expressed by the word induction. Briefly, it may be put thus: that, when a strong electric current is passing on a wire, it has the faculty of setting up a current of opposite character in any wire not then working, or working with a feebler current, that may be in its vicinity. The why or the wherefore cannot be explained, but there is the fact.
"In various recent articles on the telephone, mention has been made of 'contact' as the cause of disturbance. This word, however although it has been used by telegraphists, is misleading, and can only be used as an endeavor to express popularly an electric fact. Actual contact of one wire with another would spoil the business altogether. A wire bearing an electric current seems to be for the time surrounded, to an undefined distance, by an electric atmosphere, and all wires coming within this atmosphere have a current in an opposite direction set up in them. This is as near an explanation of the phenomena of induction as the state of telegraph science at present affords. Now, the telephone works with a very delicate magnetic current, and is easily overpowered by the action of a stronger current in any wire near which the telephone-wire may come. To work properly, it 'requires a silent line.'
"In the place where the observations were made, there were a large number of wires traveling under the floor, along passages to the battery-room, and to a pole on the outside, whence they radiate, or out to a pipe underground, where many gutta-percha-covered wires lie side by side. On applying the ear to a telephone joined into a circuit working in such an office, a curious sound is heard, comparable most nearly to the sound of a pot boiling. But the practised ear could soon separate the boiling into distinct sounds. There was one masterful Morse instrument—probably on the wire lying nearest the one on which we were joined up—whose peremptory 'click, cli-i-i-ck, click,' representing 'dot, dash, dot' on the printed slip we read from, could be heard over all. Then there was the rapid whir of a fast speed transmitter sending dots and dashes at express speed by mechanical means; and, most curious of all, the 'rrrrr-op, rr-op, rrrrrrr-rrrrrr-op, rrrrr-op, rr-op' of the A B C, or printing-instrument, the deadliest foe to the telephone in its endeavors to gain admission into the family of telegraph-instruments. There may be reason in this, for as the ABC, or printing-instrument, is the instrument used for private telegraphy, or for the least important public offices, because it requires no 'code' to be learned by the manipulator, so it would likely be the first to be displaced if an acoustic telegraph permanently took the field. So the sentient little ABC opens its mitrailleuse-fire on the intruder, on whose delicate currents, in the words of an accomplished electrician, it plays 'old Harry.' The peculiar character of the sounds we borrow on the telephone from this instrument arises from the fact that, as the needle flies round the dial, a distinct current or pulsation passes for each letter, and the final 'op' we have tried to represent shows the stoppage of the needle at the letters as words were spelled out.
"It must not be understood that the sounds of those various instruments are actually heard in the telephone. What happens is, that the currents stealing along the telephone-wire by induction produce vibrations in the diaphragm of that instrument, the little metal membrane working on the magnet in ready response to every current set up the latter. When it is remembered that the principle of the telephone is that the sound-caused vibrations in the filmy diaphragm at one end create similar but magnetically-caused vibrations in the diaphragm at the other end, and so reproduce the sound, it will be obvious why the rapid roll of the ABC currents, or the swift sending of the fast-speed transmitter, when brought by induction into the telephone-wire, cause disturbances in the sound-vibrations, and thereby cripple the instrument. One instrument of either kind named would have a certain effect, but one Morse would not have any greatly prejudicial effect. But a number of Morses going together, such as were heard in our experiments, would combine to be nearly as bad as one A B C or fast-speed Morse. So delicate is the diaphragm to sound (and necessarily so) that, in all experiments with the telephone itself, every sound from without broke in, giving an effect like the well-known 'murmur of the shell.' "
"Joining up our wire now to a more distant station at some miles along the railway, and having on its poles a number of what are known as 'heavy' circuits, the pot-boiling sound assumed even more marked characteristics. The A B C no longer affected us; but a number of Morse instruments were in full gear, and the fast-speed transmitter was also at work. While we were listening, the circuit to which we were joined began to work, and the effect was literally electrical. Hitherto we had only borrowed currents—or, seeing they were so unwelcome, we might call them currents thrust upon us—and the sounds, though sharp and incessant, were gentle and rather low. But, when the strong current was set up in the wire itself, the listener who held one of our telephones nearly jumped from the floor when an angry 'pit-pat, ’pit-pat, pit-pat-pit' assailed his ear, causing him to drop the instrument as if he had been shot. It was a result none of us had expected, for it did not seem possible that the delicate metal diaphragm and the little magnet of the telephone could produce a sound so intense. Of course, it was only intense when the ear was held close to the orifice of the instrument. Held in the hand away from the ear, the telephone now made a first-rate 'sounder,' and we could tell without difficulty not only the signals that were passing, but found in it a more comfortable tone than that given by the Morse sounder in common use.
"Other experiments of a like character led to results so similar that they may be left unnoticed; and we proceed now to describe one of a different character, designed to test the telephone itself. At a distance of about half a mile, access was obtained to a Morse instrument in private use, and joined to the office by 'overhouse' wire. Dividing our party and arranging a programme of operations, two remained with a telephone in the office, while other two, of whom the writer was one, proceeded with the second telephone to the distant instrument. By an arrangement which a practical telegraphist will understand, the key of the Morse was kept in circuit, so that signals could be exchanged in that way. It may be noticed, however, that this was hardly necessary, as the diaphragm of the telephone can be used as a key, with the linger or a blunt point, so that dot and dash signals are interchangeable, should the voice fail to be heard. As the wire in this instance traveled almost alone over part of its course, we were in hopes that induced currents would be conspicuous by their absence. In this we were, however, disappointed, for the pot was boiling away, rather more faintly, but with the 'plop-plop-plop' distinctly audible, and once more a sharp masterful Morse click was heard coming in now and again. The deadly ABC was, however, absent, so that our experiment proved highly successful. For some reason or another—probably an imperfect condition of the wire, or the effects of 'induction' over and above what made itself audible to us—the spoken sounds were deficient in distinctness; but songs sung at either end were very beautifully heard, and, indeed, the sustained note of sung words had always a better carrying-power than rapidly-spoken words. Every syllable and every turn of melody of such a song as 'My Mother bids me bind my Hair,' sung by a lady at one end, or 'When the Heart of a Man,' sung at the other, could be distinctly heard, but with the effect before noticed, that the voice was muffled or shut in, as if the singer were in a cellar, while it was not always possible to say at once whether the voice was that of a man or a woman.
"In the course of some domestic experiments it was remarked that, in playing the scale downward from C in alt. on the piano, the result to the listener was a 'tit' only for the four upper notes, although all below that had a clear 'ting,' and the octaves below were mostly distinct, although at the low notes of the piano the sound was again lost. The ringing notes of a musical-box were not so successful, but, with close attention, its rapid execution of 'Tommy Dodd' could be well enough made out. An endeavor was made to catch the ticking of a watch, but this was not successful, and the experiment is not recommended, as the near presence of a watch to a magnet is not desirable; and the watch exposed to it in this instance was, it is thought, affected for a short time thereafter, although it received no permanent damage.
"The observations made in the course of these experiments convinced those present that the telephone presents facilities for the dangerous practice of 'tapping the wires,' which may make it useful or dangerous, according as it is used for proper or improper purposes. It might be an important addition for a military commander to make to his flying cavalry; as an expert sound-reader, accompanying a column sent to cut off the enemy's telegraph-connections, might precede the act of destruction by robbing him of some of his secrets. The rapidity and simplicity of the means by which a wire could be 'milked,' without being cut or put out of circuit, struck the whole of the party engaged in the various trials that are described above. Of course, the process of tapping by telephone could not be carried out if the instrument in use was an A B C or single needle, or if the wire was being worked duplex or with a fast-speed Morse, for in these cases the sounds are too rapid or too indefinite to be read by ear. The danger is thus limited to ordinary sounder or Morse telegraphs; but these still form the mainstay of every public system.
"Since the trials above described were made, the newspapers have recorded a beautiful application, by Sir William Thomson, of the electric part of the telephone to exhibit at a distance the motions of an anemometer; the object being to show the force of air-currents in coal-mines. This is a useful application of an electric fact, and doubtless points the way to further discoveries. But it is to be noticed that the experiment, interesting as it is, hardly comes under the head of a telephone, what is reproduced at a distance being not sound but motion.
"Obviously the invention cannot rest where it is; and no one more readily than the practical telegraphist will welcome an instrument at once simple, direct, and reliable. Even in its present form the telephone may be successfully used where its wire is absolutely isolated from all other telegraph-wires. But the general impression is that its power of reproducing the sound must be intensified before its use can become general, or come up to the popular expectation."
The realization of so marvelous a device as the telephone cannot fail to stimulate speculation as to where such wonders will stop. If words may be converted into electricity and back again into words, what is to hinder their being converted into something more lasting than electricity—something that will endure, so that spoken words may be reproduced in the future exactly as spoken now; that persons, though dead, may yet speak? What is to hinder? Nothing! The thing is already done; the spirit of the Phonograph has taken on more than a shadowy form, as will be explained to our readers next month. And what next?—
"Ah! Science, give us one more link,
That we may hear our neighbors think."