THE OPERATION OF THE DYNAMO AS A MOTOR. 121 



to give access to the controller spindle and contact fingers. This 

 swinging pole piece carries twelve plates of a compressed asbes- 

 tos insulating compound which serve as fire -proof partitions 

 between the various pairs of sectors and fingers. The object of 

 the electromagnet is to " blow out " the arcs which form between 

 the sectors and fingers when connections are broken by turning 



i it,. 



the spindle, and the asbestos plates serve to prevent the arcs 

 from flashing from sector to sector or from finger to finger. The 

 apparatus as a whole is called a series parallel controller. Fig. 

 90 shows a series-parallel controller of the General Electric 

 Company. 



Fig. 91 shows a typical street railway motor. The motor, 

 commutator and all, is enclosed in a steel case to protect it from 



