50 



ELECTRICAL MACHINERY 



as it rotates in the magnetic field is an alternating <>"<% 

 and if the ends of the winding are connected to insulated rings 

 on the shaft and brushes bearing on the rings connected to 

 the external circuit, the e.m.f. which such an armature wind- 

 ing impresses in the external circuit is an alternating one. 

 The current which would flow in such an external circuit 



FIG. 24. Use of Fan and Ventilating Ducts to Keep Armature Cool. 



would be an alternating current; the current would flow 

 through the circuit in one direction for a short time, would 

 stop flowing altogether, and would then reverse and 

 flow in the opposite direction, etc. Such an alternating 

 current is useful in many classes of service but for many 



