Uses of Electromagnets 217 



Its power depends on two factors, the number of turns of wire 

 in the helix and the strength of the current. 



Although electromagnets are manufactured in different shapes, 

 with reference to the use for which they are intended, the horse- 

 shoe shape well illustrates their general properties. This shape is 

 considered especially useful because the same amount of current 

 through the same number of turns of wire will produce a stronger 

 magnet in this shape than in a straight bar. 



A horseshoe electromagnet consists of two parallel soft iron 

 bars, the cores, joined across their tops by a third iron bar, the 

 yoke. Around each core is wound a coil of insulated wire, the 

 two coils being wound in opposite directions and the top of one 

 connected to the bottom of the other by a single wire. When a 

 current is run through these coils the difference in the directions 

 of winding produces opposite effects in the two cores, making the 

 free end of one a N. pole and the free end of the other a S. pole. 



In places where much iron and steel are handled, giant elec- 

 tromagnets are often used on cranes and derricks in place of 

 hooks or other fastening devices. When the current is turned 

 on, these magnets cling to any iron or steel object that they 

 touch. So powerful is the attraction of some electromagnets that 

 pieces of iron or steel of more than 150 tons have been lifted 

 by them. 



The Electric Bell. The electric bell is a familiar applica- 

 tion of electricity. A circuit is necessary in order to have the 

 bell ring. This circuit requires a battery of two or more cells, 

 a bell, a push button, connecting wires, and an electromagnet. 

 Across the ends of the magnet, but not touching them, is a 

 soft iron bar called an armature. This armature forms part of 

 the circuit. When the button is pressed the circuit is completed, 

 and a current of electricity immediately passes from the bat- 

 tery through the coils of the magnet. This causes magnetism, 

 which draws the armature toward the magnet, and the hammer 

 attached to the armature strikes the bell. When the armature 

 moves, the circuit is broken, and the current no longer reaches 

 the magnet The magnet then ceases to attract the armature, 

 which immediately springs back and again completes the circuit. 



