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
Fig. 4.
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
