PARTS OF A DYNAMO-ELECTRIC MACHINE 61 



and Fig. 33 shows the complete brush rigging of a small 

 c-c. generator. 



Superiority of Carbon for Brushes. It has been said 

 that carbon brushes are used on all machines except those 

 designed for electroplating and this in spite of the fact 

 that copper is a better conductor than carbon and only 

 requires about one-quarter as much contact surface for a 

 given current. There are two important reasons why 

 carbon is preferred to copper for brushes: 1st, the mechan- 

 ical wear on the commutator is much less with carbon 

 than with copper; 2d, it is practically impossible to obtain 



FIG. 32. Brush Holder for Railway Motor. \\Vstmghouse Elec. 

 and Mfg. Co. 



sparkless commutation when copper brushes are used on 

 any but low voltage machines. 



Sparkless Commutation. The commutation is said to 

 be sparkless, or " black," when no sparking takes place 

 at the contact surface between the brush and commutator. 

 It is very important that sparkless commutation be 

 obtained because under the action of sparking at the brush 

 contact, the commutator very quickly roughens, which 

 makes the sparking worse and so the machine is soon 

 rendered unfit for service. The explanation of this effect 

 (i.e., sparking produced by copper brushes) will be taken 



