154 



SCIENCE OF HOME AND COMMUNITY 



a bar of soft iron, and a current of electricity is sent through 

 the wire, the bar becomes a magnet and will pick up iron. 

 This is called an electromagnet. The strength of the magnet 

 may be increased by increasing the number of turns of the 

 wire or by increasing the strength of the current. 



This form of magnet is very useful because it can be con- 

 trolled. If an ordinary bar magnet is brought near some 

 iron nails, they are taken up by the 

 magnet and remain there unless shaken 

 off. If an electromagnet is brought 

 near iron nails, they are taken up by 

 the magnet and stay there only as long 

 as the current passes through the wire. 

 If the current is broken, the nails drop 

 off at once. So the power of the elec- 

 tromagnet may be controlled simply 

 by opening and closing the circuit. 



We may now refer to figure 54 to 

 see how the electromagnet works in 

 the electric bell. It is bent into the 

 form of a horseshoe, SN. When the 

 button P is pushed down and a current 

 of electricity passes through the wire, 

 the magnet becomes magnetized and 

 draws down the iron bar, A, called 

 the armature, to which is fastened a 



FIG. 54- Electric bell. 



clapper H. When this is drawn down, it strikes the bell. 

 At the same time that the armature is drawn down, the 

 electrical connection at the point B is broken, and so no 

 current passes through the electromagnet. It ceases to be 

 a magnet and so no longer- holds down the iron bar, which is 

 then brought back by the spring 5 to which it is attached. 

 As soon as this happens, the contact is made again, and 

 thus the operation is repeated. 



