Uses of Electromagnets 



221 



relay, a distant key causes a sounder to make a series of loud 

 clicks. 



The Telephone. The word telephone is derived from the 

 Greek stems phon, meaning sound, and tele, meaning afar off. 

 The telephone, then, is an instrument which carries sound far 

 away. The telephone was invented by Alexander Graham Bell, 

 in 1876, demonstrating to the world that the sound of a human 

 voice could be carried by electricity to far-away places and heard 

 distinctly. 



The important parts of a telephone circuit are the transmitter, 

 the receiver, the connecting wires, and a source of electricity. 

 The transmitter consists of a mouthpiece, a vibrating diaphragm, 

 and, touching the diaphragm, a circular box about the diameter 



American Telephone and Telegraph Co. 



A SIMPLE TELEPHONE CIRCUIT 



1. Sound waves in air. 2. Diaphragm. 3. Sound box. 4. Connecting wire. 5. Eleo- 

 tromagnet. 6. Diaphragm. 7. Sound waves in air. 



of a penny. In the box are loosely-packed coarse grains of 

 carbon. The top and the bottom of the box are made of conduct- 

 ing material and are connected with wires extending to the 

 receiver. We know, from observation of bells, drums, and harp 

 strings, that vibrating bodies are the source of sound waves which 

 travel through the air. When the sound waves caused by the 

 vibration of the vocal cords in our throats strike the diaphragm 

 of the telephone transmitter, they cause it to move to and fro, 

 and the carbon grains in the box are first pressed closer together 

 and then loosened. The firmness or looseness of the contact 

 between these grains varies the resistance in the circuit. This, 

 in turn, varies the electric current which is being sent through 

 the circuit by a battery which forms a part of the circuit. 



