PARTS OF A DYNAMO-ELECTRIC MACHINE 51 



other classes of service, such as electroplating, electric 

 railways, etc., there is required a current which runs con- 

 tinuously in the same direction through the circuit. 



The current through a circuit will not be continuously 

 in one direction (generally called a continuous current, 

 abbreviated c-c., or direct current, abbreviated d-c.) unless 

 the machine which is forcing current to flow through the 



FIG. 25. Partially Assembled Armature Core, Showing Ventilating 

 Ducts, also Lamination Used as Spacer. Westinghouse Elec. 

 and Mfg. Co. 



circuit impresses on the circuit an e.m.f. which continuously 

 acts in one direction. 



Function of the Commutator. The elementary generator 

 having slip rings and brushes connecting its windings to 

 the external circuit, could not do this. The commutator 

 is a device which serves to reverse continually the connection 

 of the armature coils to the external circuit so that, as 

 the e.m.f. in the coils reverses, the connection of the coils 



