THE ELECTRIC TELEGRAPH. (continued 2)

Fig. 302a.—Reiss’ Musical Telephone.

Fig. 302b.—Bell’s Musical Telephone.

A further step towards the speaking telephone may be illustrated by an earlier invention of Mr. Graham Bell, a native of Scotland, who had settled in the United States. Mr. Bell’s inventions, it may be mentioned, were by no means the results of fortunate accidents or of unsought and spontaneous flashes of conception, but rather the outcome of long, patient and systematic studies. His father, Mr. Alexander Melville Bell, of Edinburgh, had assiduously cultivated acoustic science, and had in 585conjunction with his son, undertaken special researches into the mechanism of the organs of speech, the elements of articulate speech in different languages, and the musical components of vocal sounds. When Graham afterwards pursued these studies in the light of the fuller investigation carried out by Helmholtz, he was naturally led to the application of electricity to acoustic transmission. After some experiments in the production of vowel sounds by combinations of electric tuning forks, he invented a telephone for reproducing musical sounds at a distance, which was a great improvement on that of Reiss, and involved another principle, which indeed is the same as that utilized in his more mature invention of the speaking telephone. As a like explanation of the action would apply in both cases, the reader will find his advantage in following the observations we have to make on the earlier instrument. This consisted of what was virtually two sets of electric tuning forks, each set being acted upon by one electro-magnet. Fig. 302b will suffice to show the general form of the arrangement. A plate of steel is bent twice at right angles longitudinally, and is magnetized so that any transverse slice of it would constitute an ordinary horse shoe magnet. This is seen endways in Fig. 302b at M, and N. and S. will indicate the north and south poles respectively. To each limb of this broad magnet is attached a plate of 586steel, T, cut into teeth, just in the same way as the steel plate in a common musical box or mechanical piano, except that the teeth are not pointed. These are tuned to give severally in pairs the notes of the musical scale when thrown into vibration. Between the prongs of the series of tuning forks thus formed is an electro-magnet, L, made of a bar of soft iron, I, wound longitudinally by a coil, one end of which makes an earth connection at E and the other is connected by the wire, W W´, to complete the circuit through the coil of the distant apparatus. It will be observed that the receiving and transmitting instruments are exactly alike. Now, suppose one of these teeth is struck or otherwise thrown into vibration, the result will be, since the free ends of the teeth are magnetic poles, that alternating electric currents will be generated in the coil of the electro-magnet (see page 509), and these will flow through the entire circuit, including the coil of the distant instrument, where the magnetism generated will alternately attract and repel the polar extremities of the teeth in the steel plate. It will be understood, of course, that the fellow prong of the fork will vibrate also, and will simultaneously approach to and recede from the soft iron core, so that being of opposite polarity, the effect on the electro-magnet will be doubled. The action on the distant electro-magnet will be a rapid series of reversals of the polarities of the core, and hundreds of times in every second the ends of the steel teeth will be alternately attracted and repelled. But not all of these will thereby be thrown into vibration—only the one pair which were tuned into unison with the former can and will respond to that particular series of impulses, and the consequence will be that the same note will be emitted by the receiving instrument. If two or more notes of the transmitter be simultaneously thrown into vibration, the same notes will be heard from the receiver, for each series of currents will flow along the wire independently, just as if the other did not exist, and each will produce its particular effect on the transmitter. In this way an air played on the one instrument is heard also from the other, with all its accents and combinations. But more than this, if a tune be played on a musical instrument near the sender, or if a song be sung, the air will be reproduced by the distant receiver. The reason of this is that the steel tongues take up, or are thrown into movement by, the vibrations that have the same periodicity. The manner in which a vibratory body responds to impulses of its own periodicity may be easily shown by exposing the wires of a piano and raising the dampers, when, if a note be sung near the instrument, it will be found that a number of the wires respond, namely, those that are capable of vibrating synchronously with the constituent vibrations of the voice, for neither a voice nor a sounding wire gives forth one simple system of vibrations, the audible effect being due to the superposition or composition of several diverse elementary systems. With the same arrangement another experiment may be made, as an illustration of a matter important for our subject. Let the different vowels be sung to the piano-wire on the same note or pitch, and in the responses to each a difference of the quality of the sound will be noticed, although the piano will not distinctly give back the vowel itself. It would, however, do so if a number of its wires were strung with certain definite relations in pitch to that of the fundamental note and in unison with the voice components of the vowel sound.

Fig. 302c.—Superposition of Currents.

It has been said above that two systems of electrical currents of different periodicity would flow along one wire independently of each other, but it should be explained that this takes place by a composition of 587the currents, for it is evident that at any given instant the wire can only be in one of three conditions, viz.: (1) with no current flowing; (2) with a current in the positive direction; (3) with a current in the negative direction. Such must always be the case, and, therefore, it should be clearly understood how this is consistent with the superposition of currents of different periodicities, a matter which the diagram, Fig. 302c, is intended to illustrate. Suppose the flow of time to be represented by the dotted lines from a to b, the whole length of which we may call 1 100th of a second, and that the current passing through the wire is represented in intensity and direction by the plain lines; the intensity by distance above or below the dotted line; the direction being positive where the plain line is above, and negative when it is below the dotted straight line, and of course no current at all occurs at the instant when the change of direction takes place. The line A will thus represent alternating currents, rising and sinking in intensity, and changing from one direction to the other, going through 600 regularly recurring phases in one second of time. Similarly, B may represent another series of currents, having here a periodicity of 500 in one second of time. These are here supposed to have greater intensity than the former. If the two currents are sent through one wire their effects are superposed, so that the actual electrical state of the wire would be represented by the curve C, which is compounded from the two others, and where it will be observed the rise and fall of the current, its maxima and minima, no longer recur at regular intervals within the space of the 1 100th of the second, the whole of that period being taken up by a less regular series of changes, the cycle being repeated only 100 times in the second. The same diagram might serve to illustrate the motions of, say, a particle of air or the drum of the ear in acoustic vibration, the distances above and below the straight line being taken to represent the displacements from the position of rest on one side and the other. If the sounds of an organ or piano consisted of only these primary vibrations, B would roughly^[8] represent the movements of the 588wires, the air and the drum of the ear, when the note si₃ was sounded alone; A when the note re₄ was more faintly sounded alone, and then C, if these notes were sounded together, would correspond with the movements of the drum of the ear. The movements it actually makes when we hear speech, or even a single musical note, are, however, a thousand-fold more complex, for no musical instrument gives out a note with a single set of vibrations, the fundamental one being always accompanied by other sets diversely related to it, according to the class of instrument. In some cases, fifteen or sixteen sets of vibrations have been distinguished along with the fundamental note, without exhausting the possible number. Of a like order of complexity will be the currents which the wire of a speaking telephone must convey, and the difference between the undulatory nature of the currents in Bell’s musical telephone and any produced by mere make and break contacts, as in Reiss’ arrangement, will be obvious, and recognized as an important step towards the solution of the problem of transmitting speech. When Mr. Bell invented his instrument, he was seeking for a method of simultaneously transmitting by one wire several messages by audible signs merely; and by the method used in his musical telephone this is practicable, for all that would be required would be pairs of transmitters and receivers, each adjusted to one single particular note. Another point that should be noted is that in the Bell musical telephone no battery is used, for the currents are those generated by magneto-electric induction, and the circuit through the wires and coils are completed by earth connections.

Fig. 302d.—Bell’s Speaking Telephone.

In passing from the invention of the musical to that of the speaking telephone, Mr. Bell passed from the more complex to the more simple instrument, for of all apparatus by which communication can be carried on at a distance, the Bell speaking telephone is one of the simplest. He had only to make its vibrating disc of Scott’s phono-autograph into a magnetized body, capable of producing currents in an electro-magnet coil in the same way as did the vibrating plates in his musical telephone. The Bell speaking telephone was publicly exhibited for the first time at Philadelphia, in 1876, and was shown the same year to the British Association by Sir William Thomson, who pronounced it the wonder of wonders. For the first time in England, the instrument in a still simpler form was exhibited by Mr. Preece, at the Plymouth meeting of the British Association in 1877, and of nearly the same construction as is still often used, although, as we shall presently see, for battery telephones the transmitting apparatus is now made of larger dimensions, of a different shape and on a different principle. We shall describe the simple form in 589which transmitter and receiver are identical, each consisting externally of a small cylindrical wooden or ebonite box, and with a handle three or four inches in length of the same material. Fig. 302d is a section of the instrument where N S is a cylindrical steel magnet, on one end of which is wound the small coil B, made of fine silk covered copper wire, the extremities of which pass through the handle M at f f, and are connected by the binding screws I I´ with the line wire C C´. Close to the coil covered end of the magnet is a very thin diaphragm of iron, L L´, and when this is thrown into vibration by the voice speaking into the trumpet-mouth opening, R R´, its movements produce currents in the coil according to the principles that have already been explained, for it will be observed that the iron disc is magnetized by the inductive action of the permanent magnet N S. These currents passing through the coil of the receiving instrument raise or lower the intensity of the magnetic force in it, so that the distant disc reproduces the vibrations of the transmitter. Such is at least an obvious explanation of the action of this very simple arrangement; but from a number of experiments and observations that have been made with modifications of the instruments, it would appear that other and much more complex phenomena concur in producing the effects. It has indeed been suggested—and the idea is supported by numerous experiments—that, in these telephonic transmissions of speech, vibrations are concerned which are not at all of the mechanical kind we have been dealing with in these explanations, but are molecular.

The Bell telephone is used by speaking distinctly before the mouth-piece of the transmitter, while the listener at the other end of the line applies the mouth-piece of his instrument to his ear, and one wire is sufficient with good earth connections, although sometimes a second wire is employed to complete the circuit. It is also found advantageous to have two instruments in the circuit at each end, so that one may be held to the ear while the operator is speaking through the other. In this way, a rapid conversation can be carried on with the greatest ease, or again, an instrument may be held at each ear, by which arrangement the words are more distinctly heard. It is not necessary to shout, as this has no effect, but to speak with a clear intonation, and some voices are found to suit better than others. The vowel sounds are best transmitted, except that of the English e, which, with the letters g, j, k, and q, are always somewhat imperfectly transmitted. A song is very distinctly heard, both in the words and the air, and the voice of the person singing is readily recognized. Several instruments may be included in one circuit at different stations, so that half a dozen persons may take part in a conversation, and questions and answers may be understood even when crossing each other. If two distinct telephone circuits have their wires laid for a certain distance (two miles) near each other, say a foot or more apart, and without any connection whatever, listeners at the end of the one line will hear the conversation exchanged through the other line. Other forms of the instruments have been arranged, by which a large audience may hear sounds produced at a distance, as, for instance, when a cornet-à-piston was played in London, it was heard by thousands of people assembled in the Corn Exchange at Basingstoke.

It would be impossible within our limits to even briefly describe the great number of improvements and modifications of Bell’s system that were devised by various persons soon after the invention was brought out, and many additional complications were introduced into some of the 590arrangements. Advantage was also taken to a greater or less extent of another principle affecting the strength of electric currents, to which we have now to call the reader’s attention, and to exemplify by one of the simplest instruments, leaving detailed accounts of the various forms in which it has been applied to be found in special treatises. The reader should first turn back to page 400, where he will see an expression of the strength of a battery current. It will be observed that the current may be increased or diminished by diminishing or increasing R, the external resistance, without changing the other terms. Now M. Du Moncel discovered, as far back as 1856, that an increase of pressure between two conductors in contact, and conveying a current, caused a diminution of the electrical resistance, and this discovery was utilized for telephonic purposes by Mr. Edison in his invention of the carbon transmitter (1876). In this there is no magnet, and a stretched membrane may take the place of the metallic plate, although a circle of photographers’ ferro-type plate gives better results. A pad of india-rubber, cork, or other material is fixed on the plate, and rests upon a carbon disc, which again is in contact with a metallic conductor. Between the latter and the carbon the current from a constant battery passes. When the plate is thrown into vibration by speaking into the mouth-piece, the variations of pressure conveyed to the carbon cause variations in the resistance of its electrical contact, and thus a series of undulations are produced in the current, and these affect the electro-magnet of a Bell receiving instrument in the circuit as before, so that the sounds are reproduced. It is now time to say a word about the share in the invention of the speaking telephone which has been claimed by Mr. Elisha Gray, also of the United States, who, at the time Mr. Bell applied for the patent for his instruments, produced drawings and descriptions of a plan he had devised for transmitting speech by undulating electrical currents, and it has been admitted that the plan he had conceived was perfect in principle. He proposed to use a battery current, and his receiving instrument was nearly the same as Bell’s. The undulations of the current were also determined, as in Edison’s telephone, by changes in the external resistance, but this was effected in a different, though equally simple manner. To a membrane stretched across the lower end of a short wide tube that formed the mouth-piece of the transmitter, and was placed vertically, was attached a piece of platinum wire, conveying the current and dipping into a liquid of moderate conductivity, but not quite touching another platinum electrode fixed at the bottom of the vessel containing the liquid. The space of liquid traversed by the current being thus varied by the oscillations of the membrane, the resulting variations of the resistance produced the requisite undulations in the intensity of the current. Both Mr. Bell and Mr. Gray applied for patents on the 14th February, 1876, but the American Patent Office recognized the claim of the former as prior.

Fig. 302e.—Mr. Hughes’ Microphone. (B and R are merely diagrammatic.)

Du Moncel’s observation was applied by Mr. Hughes in the construction of an instrument, which he named the microphone. This was in the same year that Edison had brought out his carbon telephone, and a certain similarity, resulting from the identity of the principle employed, led to an acrimonious controversy on what were supposed to be rival claims. But the microphone differs so much in arrangement and performance from the other instrument as to constitute a distinct invention. The instrument, if it may be so called, is simplicity itself, in the form represented in Fig. 302e, which is one of the most sensitive. There, 591C and C´ are two small blocks of carbon, fixed on a small upright piece of wood. Two cup shaped cavities are hollowed out in the carbon blocks, and these serve to hold loosely, in a nearly vertical position, a small rod of gas retort carbon pointed at the ends. This rod is only about one inch in length, and the lower end merely rests on the bottom of the cup in C´, while the other is capable of moving about in the upper cavity, the vertical position being nearly maintained in a state of unstable equilibrium. The carbons are in the circuit of a voltaic cell or small battery, B, in the line through a Bell receiving instrument, which may be at a distance. When the microphone is to be used, it is placed on a table with a cushion or several folds of wadding beneath its base. If the receiver be applied to the ear of a listener, he will distinctly hear every word pronounced by one speaking near the microphone, even in a low tone; but a loud voice may be heard when the speaker is 20 or 30 feet from the instrument. The minutest vibrations conveyed to the stand are perceived at the receiver as loud noises. The tread of a fly walking over the board, S, is heard like the tramp of a horse, and the ticks of a watch are audible in the receiver when the ear is several inches away from it. The slight touch of a feather on the stand is distinctly audible, and a current of air impinging upon it is reproduced as the noise of a stream of water. The microphone is, in fact, the most sensitive detector of vibrations that is known, and its employment as a transmitter has brought the telephone to its present perfection. It has been constructed in an endless variety of forms, according to the purposes for which it is intended, and its simplicity is as wonderful as its extreme sensitiveness. We will further illustrate these qualities by an experiment of Mr. Willoughby Smith’s on the same principle. Instead of the two carbon blocks, he laid on the table, in parallel positions, two small rat tail files, and completed the circuit by a third file, laid across the others at right angles. This arrangement constituted so sensitive a transmitter that the listener at the distant Bell receiver could hear even the faint sound of the speaker’s breathing. Even three common pins, similarly crossed, make an effective transmitter. The feebleness of the variations in the current requisite to make the Bell 592receiver produce sounds is extraordinary, and a very weak battery current is sufficient, even under the circumstances of ordinary practical use. Still more remarkable is the fact that in favourable conditions the microphone is capable of transmitting sound without any battery at all, but merely with connections to earth, when the ticking of a watch placed upon the stand has been distinctly heard at the distance of nearly one-third of a mile, and speech, also, has been transmitted with unusual distinctness with the battery left out and merely a few drops of water placed at the carbon contacts; indeed, it is said that, even without the water, the voice may be heard. This effect has been attributed to the carbons and water forming a battery themselves, and in the latter to the moisture of the speaker’s breath supplying the fluid element. But, again, the microphone will not only transmit speech, but, under certain arrangements, it will reproduce it (when one of the carbon electrodes is attached to a membrane), although the result is less distinct than with the Bell receiver. It is, however, not so easy to explain how mere variations of current intensity can produce the effect where there can be no magnetic attractions and repulsions. We must, no doubt, look for the cause in some other property of electric currents. The transmitters used in various lines of telephonic communication, erected by the Post Office or by companies in Great Britain, are generally applications of the principle of the microphone, and not of that of either Mr. Bell’s or Mr. Edison’s original instrument. But more recently, Mr. Edison has most ingeniously adapted variations of sliding friction, as modified by the action of the undulatory current on a liquid electrolyte between the sliding surfaces to the production of a loud speaking telephonic receiver—that is, one by which the sounds are made audible to a large assembly. From this instrument, the notes of a cornet-à-piston, played in Brighton, have been distinctly heard throughout a large hall in London.

Another curious transmitter is formed of a fine jet of water traversed by an electric current. Acoustic vibrations are easily set up in the jet, and these modify its conductivity so as to produce corresponding undulations of current intensity.

It would take long to point out the many scientific applications of so sensitive an instrument as the microphone with its Bell receiver. As a medium for conveying speech to a distance, whether for purposes of peace or war, its use is sufficiently obvious. Some curiosities of musical transmission have been noticed, and such experiments are repeated from time to time with increasing success. It has been applied to many purposes in surgery and medicine. In many cases of deafness it has made conversation easy. Even the passage of the molecules of gases, when diffusing through porous partitions, Mr. Chandler Roberts has by its means made audible. The distances to which speech can now be transmitted are considerable, as conversations have been carried on by persons nearly 300 miles apart.

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