MISCELLANEOUS PRACTICAL APPLICATIONS. 



333 



direction, the magnet M will be alternately weakened and 

 strengthened, the force with which the magnet attracts the dia- 

 phragm will vary accordingly, and the diaphragm will be caused 

 to move to and fro in unison with the reversals of current. 



Consider two telephones, A and B, connected in circuits A 

 sound strikes the diaphragm of telephone A and causes the 

 diaphragm to vibrate. Telephone A acts as a transmitter, and 

 telephone B acts as a receiver, as explained above, and the dia- 

 phragm of telephone B is caused to vibrate in a manner exactly 

 similar to the vibrations of the diaphragm of telephone A, and 

 thus the diaphragm of telephone B reproduces the original 

 sound. 



37. The carbon transmitter. The alternating current which is 

 produced by a telephone acting as a transmitter is very weak 

 even when the transmitter telephone is exposed to a loud sound. 

 The carbon transmitter is an arrangement by means of which a 

 vibrating diaphragm may control a strong battery current and 



line 



line 



Fig. 36. 



cause a strong induced current to surge back and forth through 

 the telephone line in unison with the movements of the diaphragm. 

 The arrangement of the carbon transmitter is shown in Fig. 36. 

 The current from a battery passes through the primary P of 

 a small induction coil and through a mass of granular carbon C 

 which lies between a carbon block B and a diaphragm DD. 

 The electrical resistance of the granular carbon varies with the 



