MAGNETISM AND ELECTRICITY 207 



Telegraph operators have learned to read these messages 

 by interpreting the clicks, representing the dots and 

 dashes as letters of the alphabet. 



The Telephone. In 1875 Alexander Graham Bell 

 demonstrated to the world that the sound of a human 

 voice could be transmitted by electricity. He invented 

 what we still use and know as the Bell receiver. The 

 essential parts of this receiver are a permanent magnet 

 wound with fine wire and a disk of thin sheet iron held 

 in place by a hard rubber case. Figure 182 shows a simple 

 arrangement of the telephone parts and how they work. 

 At each end of the line is a bar magnet, surrounded by 



Receiver Receiver 



FIG. 182. Telephone System and Battery Circuit. 



a coil of fine wire and a thin iron disk. The disk A is 

 set in vibration by the sound waves whenever such waves? 

 are produced in front of it. As A moves back and forth 

 in the magnetic field, currents are induced in the coil B. 

 These currents are transmitted to the similar coil B' at 

 the other end of the line and there produce changes in 

 the magnetic field similar to the changes that produced 

 the current at B, The disk A' is set in motion and 

 vibrates in exactly the same way as A, with the result 

 that the sounds which caused the vibration at A are re- 

 produced at A'. 



The modern telephone uses a transmitter and a re- 

 ceiver, and also a second wire as shown in the above 

 diagram. This second wire is necessary because of the 



