CHAPTER XXX 

 CURRENT ELECTRICITY 



169. Useful Electricity. Static electricity, produced by 

 rubbing one body with another, though important in some 

 respects, is not capable of doing useful work. Even when it 

 is accumulated on a condenser, such as a Ley den jar, the 

 discharge is practically instantaneous, while for operating 

 machinery and the like, a continuous flow of electricity is 

 required. The current electricity now so extensively used, 

 was not known until long after the effects produced by static 

 electricity were familiar to scientists ; but until its discovery, 

 no progress in adapting electricity to manufacturing, trans- 

 portation, and the like was possible. 



170. The Voltaic Cell. The first current electricity was 

 produced by chemical energy. It was found that when a 

 strip of copper and a strip of zinc were placed in a jar of water 

 containing a small amount of sulphuric acid and their pro- 

 jecting ends connected by means of a wire, a weak current 

 of electricity, produced by chemical reaction between the 

 acid and zinc, would flow along the wire. Such an arrange- 

 ment was called a Voltaic cell. Other metals and non-metals 

 may be used in place of the strips of copper and zinc, and 

 various other solutions may be substituted for the dilute 

 sulphuric acid, but the result is the same in all, namely, the 

 transformation of chemical energy into a current of electricity. 

 Such cells are still used for supplying the current that rings 

 door-bells and operates telegraph instruments, telephones, 

 and the like. The most familiar form is the dry cell which 

 consists of a zinc cup filled with moist chemicals surrounding a 



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