164 



ELECTRICITY 



The speed of the motor and car is governed by adding 

 more or less resistance to the shunt at the controller * 

 ( 166). 



188. The Telephone. Only a very general explana- 

 tion of the telephone can be given here. In Fig. 120, 

 suppose some one talking at A to a person at B. The 

 two instruments, at A and B, may be alike, though they 

 usually differ in appearance. In each, m is a permanent 



c m 



FIG. 120 



magnet and c a coil of wire which is continuous with the 

 circuit ; d is a disk of thin iron. 



As you talk before 0, the sound waves cause the disk 

 d to vibrate. The disk vibrating near the magnet in- 

 duces alternate currents in the coil c in one direc- 

 tion when d approaches c and in the opposite direction 

 when d moves away. These currents go through the 

 circuit to the coil c at B. Their effect is to strengthen 

 and weaken the magnetic field acting upon d (at j5), 

 attracting and repelling d. In this way the disk at B 

 is made to vibrate just like that at A. Its vibrations 

 cause weak sound waves which may be heard by the 

 ear at B. 



Note that the sound waves are produced at B ; alter- 

 nate currents pass through the wire not sound waves. 



