OPERATION AND CARE OF ELECTRIC MACHINERY 433 



the cells C and D. Even though there is no load on the 

 battery, the load circuit being open, there will still be 

 current flowing in the different cells as indicated by the 

 arrows. The amount of current which will thus flow in 

 the different paths of the armature depends upon the 

 inequality of the voltages in the different paths and upon 

 the resistance of the armature. In large machines this 

 resistance is very small and so, on large, high voltage 

 generators, it is very important that the brushes be spaced 

 exactly right. 



If the brushes on a commutating-pole machine are 

 unequally spaced, it is impossible to eliminate sparking 

 at all brushes because when one brush or set of brushes 

 is in the correct position with respect to the commutating 

 poles, some of the other brushes cannot possibly be placed 

 properly, because we have supposed the brushes unequally 

 spaced while the commutating poles are always spaced 

 equally from each other. We have previously shown that a 

 commutating pole machine is very sensitive with regard 

 to the proper placing of the brushes with respect to the 

 commutating poles. 



Testing for Spacing. The easiest way to obtain equality 

 in spacing is to measure the total circumference of the 

 commutator by laying a tape or strip of paper around 

 the commutator surface under the brushes. This distance, 

 divided by the number of brush studs, is the proper spacing 

 to use in measuring from the toe of one brush to the toe of 

 the next brush. 



140. Fitting the Brushes. The brushes must be thor- 

 oughly " sanded " to fit the commutator. This task 

 requires a deal of time on the larger machines and must 

 be done carefully. The proper method is shown in Fig. 284. 

 Sandpaper (never emery cloth) must be used for this 

 purpose. The sandpaper is torn into strips slightly wider 

 than one brush and a strip is inserted between the brush 

 to be fitted, and the commutator surface, the rough side 



