CHAPTER VII. 



ELECTRIC GENERATORS. 



Of the sources of electricity we have men- 

 tioned two: Friction, and Galvanism or chem- 

 ical action. There are hundreds of forms of 

 the latter species of apparatus for generating 

 electrical energy, so we will mention only a 

 few of the more prominent ones. It is not our 

 intention to go into the chemistry of batteries. 

 There are too many exhaustive works on this 

 subject lying on the shelves of libraries that 

 are accessible to all. All galvanic batteries act 

 on one general principle the generation of 

 electricity by the chemical action of acid on 

 metal plates; but the chemistry of their 

 action is very different. In all batteries the 

 potential energy of one element is greater 

 than the other. The acid of the battery dis- 

 solves the element of greater potentiality, and 

 its energy is freed and under right conditions 

 takes on the form of electricity. The poten- 

 tial of zinc, for instance, is greater than that 

 of copper, and the measure of the difference 

 is called the " electromotive force," the unit 

 of which is the " volt." Electromotive force is 

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