COMPOSITION OF SOUND WAVES. 287 



an electric current in the wire of B. These currents are 

 transmitted to the coil of the connected telephone, at a 

 distance of, perhaps, several miles, and there produce, in 

 the diaphragm of the instrument, vibrations exactly like 

 the original vibrations produced by the voice of the speaker. 

 These vibrations of the second diaphragm send out new air 

 waves that are very faithful counterparts of the original air 

 waves that fell upon the first diaphragm. The two sets of 

 air waves being alike, the resulting sensations produced in 

 the hearers are alike. Not only different words but also 

 different voices may be recognized. The arrangement 

 being the same at both stations, the apparatus works in 

 either direction. (See Appendix M.) 



(a.) The reproduced sound is somewhat feeble but remarkably- 

 clear and distinct. The second telephone should be held close to 

 the ear of the listener. Sometimes there are, in the same circuit, 

 two or more instruments at each station, so that each operator may 

 hold one to the ear and the other to the mouth ; or the listener may 

 place one at each ear. When the stations are a considerable dis- 

 tance apart, one binding post of each instrument may be connected 

 with the earth, as in the case of the telegraph ( 395). 



(&.) It is to be distinctly noticed that the sound waves are not 

 transmitted from one station to the other. "The airwaves are 

 spent in producing mechanical vibrations of the metal ; these create 

 magnetic disturbances which excite electrical action in the wire, 

 and this again gives rise to magnetic changes that are still further 

 converted into the tremors of the distant diaphragm, and these 

 finally reappear as new trains of air waves that affect the listener." 



447. The Phonograph. This is an instrument 

 for recording sounds and reproducing them after any 

 length of time. (See Appendix N.) 



(a.) The receiving apparatus consists of a mouth-piece and 

 vibrating disc like those of the telephone. At the back of the 

 disc is a short needle or style for recording the vibrations upon a 

 sheet of tinfoil moving under it. This tinfoil is placed upon a metal 



