THE TELEPHONE AND HOW IT WORKS. 



561 



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 ; 

 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, Fig. 3. 



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. Every- 

 body 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 sound- 

 ing body; and, moreover, that the}' 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 instru- 

 ments 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 tym- 

 panum 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 

 vol. xii. 36 



