230 



ELEMENTS OF GENERAL SCIENCE 



a battery, the metal bar is pulled down, giving forth a loud 

 click. A spring pulls the bar back into place when the circuit 

 is opened. 



The telephone receiver (fig. 115) consists of two permanent 

 bar magnets arranged to form a horseshoe magnet. A soft- 

 iron pole piece is placed on each end. These pole pieces each 

 have a coil of very fine wire (of 30 ohms resistance) about 

 them. The varying current which flows through the small 

 electromagnets strengthens or weakens the permanent mag- 

 net, and this causes the iron disk near the ear to vibrate, pro- 

 ducing the sound. 



T 1 - A^ ^^ , The electric bell 

 (fig. 116) consists 

 of two coils of 

 wire forming an 

 electromagnet of 

 the U-type. When 

 the current flows 

 through these coils 

 a piece of metal 

 which is attached 

 to the clapper is 



suddenly drawn over, thus ringing the bell. The exact opera- 

 tion of any bell and its wiring circuits should be learned by 

 examination and by the construction of a system of electric bells. 

 Other examples of electromagnets may be found in electric 

 toys (fig. 114), clocks, door openers, annunciators, telephones, 

 arc lamps, motors, automobiles, and street cars. 



236. Uses of large electromagnets. Electromagnets of great 

 strength are required in electric motors and generators. The 

 most powerful machines have very large coils, which are usu- 

 ally made of thick copper rods or bars. A single electromagnet 

 of a huge dynamo may have as much as two hundred pounds 

 of copper in the coil, and the dynamo may contain thirty-six 

 such magnets. 



FIG. 116. Electromagnets of an electric bell 



B, dry cell ; G, bell ; PB and X, push buttons at dif- 

 ferent places, each connecting with the bell ringer, A, 

 by means of its own circuit ; P l and P. 2 , binding posts ; 

 JV, magnets 



