62 ELECTRICAL MACHINERY 



up in a later chapter. It must be borne in mind here, 

 however, that the sparking with copper brushes is not 

 due to such causes as rough commutator, etc.; no matter 

 how smooth the commutator may be or how well fitted the 

 brushes may be, this sparking cannot be eliminated. 



FIG. 33. Complete Brush Rigging of a Small c-c. Machine. West- 

 inghouse Elec. and Mfg. Co. 



Pressure of Brushes. The springs on a brush holder are 

 adjustable so that the pressure exerted by a brush on the 

 commutator may be varied as desired. If too little pressure 

 is used, the contact is not good and the electrical resistance 



