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SCIENCE OF HOME AND COMMUNITY 



FIG. 138. Bell telephone 

 receiver. 



cause the diaphragm to vibrate back and forth. This vibra- 

 tion across the magnetic field of the magnet induces currents 



in the coil of wire, first in one direc- 

 tion and then another, the strength 

 depending on the distance the dia- 

 phragm moves. These varying 

 currents travel to the other end 

 of the line and cause the magnet 

 to attract the diaphragm and give 

 it the same motions as those of 

 the first diaphragm which started 

 the variations in the current. In 

 this way the second diaphragm 

 makes the same vibrations as the 

 first, and hence it reproduces the 

 same sounds. 



This form of telephone can be 

 used for only short distances. The modern telephone has 

 been modified in several ways. In the first place, another 

 instrument has been introduced to serve as the transmitter. 

 The receiver is about the same as that first used, except that 

 a U-shaped magnet is used in place of a single magnet. 



Carbon transmitter. The carbon transmitter is the form 

 most commonly used (figure 139). 

 Back of the diaphragm D is some 

 granular carbon G in a little box C, 

 and as the diaphragm vibrates, it 

 causes a variation in the pressure 

 upon the points of carbon. This pro- 

 duces a variation in the strength of 

 the current that passes through the 

 battery. This is connected with an 

 induction coil Ic (figure 140), and 

 produces in the secondary coil varying currents, which travel 

 over the main line. These variations are reproduced in the 



FIG. 139. Carbon trans- 

 mitter. 



