CURRENT ELECTRICITY 



209 



breaks the circuit, the magnet ceases to act, and the spring 

 forces the armature back into its original position, where it 

 again closes the circuit and causes the process to be repeated 

 again and again. 



175. Electric Light and Heat. There is no substance known 

 which will conduct electricity without offering some resistance 

 to its passage. Good conductors, therefore, are simply 

 those that offer the least resistance. The size of the conduct- 

 ing body also has an effect on the resistance, for the smaller 



. FIG. 78. Electric generator. 



the wire the greater the resistance. Since heat develops as 

 the result of resistance, a smaller wire may be heated white 

 hot by this means, as for example in the filament of the in- 

 candescent lamp. When it is attempted to send currents 

 over wires too small to carry them, they may become so hot 

 as to set fire to objects with which they are in contact. To 

 protect buildings from this danger, a fuse or strip of metal is 

 inserted in the circuit where it enters the building. Before 

 enough current can be sent over the wires to overheat them, 



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