CHAPTER XVII 



GENERATION OF ELECTRICITY ON A COMMERCIAL 



BASIS 



226. Generating Large Amounts of Current. We have 

 studied how electricity is generated by chemical means in 

 batteries and by friction. These two forms of electrical 

 energy are very valuable for commercial purposes where a 

 small current is sufficient, such as is necessary for ringing 

 electric bells, etc. The current generated by these two 

 methods is not, however, strong enough to drive large 

 machines or to light lamps. The commercial method of 

 generating electricity on a large scale is by means of a 

 machine called a dynamo or generator. The principal parts 

 of a dynamo are: (1) the magnetic field, produced by per- 

 manent magnets or electromagnets; and (2) the armature, 

 which consists of a moving coil or coils of wire wound on a 

 revolving iron ring or drum. 



227. The Principle of a Dynamo. The generation of 

 electricity by a dynamo is based on a principle of magnetism 

 called induction. When the lines of force that pass from the 

 north to the south pole of a magnet are cut by a wire there 

 is produced or induced in the wire a current of electricity. 

 That is, if we take a loop or coil of wire which has no current 

 in it and a magnet which also has no current, and move the 

 loop or coil between the poles, as shown in Fig. 83, a momen- 

 tary current is produced. If a series of loops or coils are 



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