Discoveries and Inventions of the Nineteenth Century

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 1100th 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 1100th 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. 8. The lines A and B in the diagram have not harmonic ordinates. 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