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 the 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 wires, 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 which 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 arrangements. 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.)