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THE POPULAR SCIENCE MONTHLY. 



of producing electricity is, roughly speaking, about twenty times as 

 cheap as that of generating it by batteries. 



The present way of generating large quantities of electricity re- 

 quires, then, an engine and boiler for converting the chemical energy 

 of burning coal into mechanical energy, and a device whereby this 

 mechanical energy is made to move magnets in the vicinity of coils of 

 wire or coils of wire in the vicinity of magnets, so as to convert the 

 mechanical energy into electrical energy. Such a device is called an 

 electric machine, or, ordinarily, a dynamo-electric machine ; and this 

 term is usually abbreviated into " dynamo." 



A dynamo of a type in considerable use, and one of the earliest 

 and best forms, is shown in Fig. 1. In this dynamo, coils of wire are 

 wrapped about the long "armature" shown in the center, which is 

 revolved between the poles of the large magnet (A) by a belt coming 

 from a steam-engine, and going around the armature-pulley seen at 



Pio. 1. 



the rear. The approach to and recession from the poles of the differ- 

 ent coils of wire of the armature generate a succession of currents 

 which are collected by " brushes," and sent out into the circuit as a 

 constant current. 



But a most beautiful example of the truth of the theory of the 



