158 ELECTRICITY 



then in the other. Such a current is called an alternat- 

 ing current. The alternations, or changes of direction, 

 occur with great rapidity. 



Alternating currents may be used for some purposes, 

 and their use is coming to be more important. For 

 some uses, however, the current must be made to flow 

 always in one direction. This is done by a simple 

 device on the armature called a commutator. The cur- 

 rent flowing steadily in one direction is called a direct 

 current. 



182. Kinds of Dynamos. Electro-magnets are used in 

 dynamos that are meant to furnish strong currents, 



because they can be made 

 more powerful than per- 

 manent magnets. The cur- 

 rent to supply the coils of 

 these electro-magnets may 

 be taken from the dynamo 

 itself, or from a separate 



PIG. 115 ni -, 



generator called an exciter. 



An exciter is a small dynamo, the magnets of which are 

 permanent. Fig. 115 shows a permanent-magnet dynamo 

 whose armature is turned by hand. Such devices are 

 used in telephones to ring the call bell, and in a few 

 other ways. Their current is not great, though stronger 

 than the usual battery currents. 



183. Uses of the Current. Dynamo currents are 

 used in motors, electric lights, furnaces, electric cars, 

 electroplating and electrotyping, and for other purposes 

 where powerful currents are needed. For lighting and 



