6 4 



ELEMENTS OF ELECTRICAL ENGINEERING. 



of compact bundles of thin metal leaves or wires, so as to insure 

 many points of contact with'the commutator. Metal brushes are 

 however objectionable for two reasons : (a) It is very difficult to 

 keep the surface of the commutator smooth when metal brushes 

 are used, inasmuch as the soft copper bars and the soft metal 

 brushes do not wear well mechanically. There is always a ten- 

 dency for soft metals to become rough when rubbed together, 

 especially if any extraneous cause, such as the slight sparking at 

 the brushes of a dynamo, starts the roughening process. 



Fig. 56. 



(b) The tendency of a dynamo to spark at the brushes is greatly 

 lessened by using brushes made of comparatively high resistance 

 material, such as carbon, instead of low resistance material like 

 metal. Dynamo brushes are now almost universally made of 

 hard blocks of graphitic carbon. Such brushes wear well 

 mechanically without other lubrication than that supplied by the 

 graphite of the brush. 



The function of the brush holder is to press the solid blocks 

 of carbon which are used as brushes firmly against the commu- 

 tator, and at the same time to give sufficient freedom to the car- 

 bon blocks to enable them to seat themselves snugly against the 

 commutator so as to ensure a large area of contact. Fig. 56 



