OPERATION AND CARE OF ELECTRIC MACHINERY 445 



of the brushes on the commutator (due to the dirty surface) 

 acts just the same as a high resistance in the field rheostat. 

 The commutator may be cleaned by holding a piece of oily 

 rag on the revolving commutator; the oil will generally 

 loosen the sticky, black coating which forms on a com- 

 mutator surface. The commutator should always be cleaned 

 after applying oil to its surface. 



19. The brushes may have been disturbed and then set 

 in the wrong position so that, even if the field were excited, 

 there would be no voltage on the armature terminals. 

 The brush should rest on that commutator bar to which 

 is connected the coil which lies in the interpolar space; 

 whether or not this is so may generally be ascertained by 

 tracing the end connections of the coils. 



20. In discussing the " building up" of a self-exciting 

 generator we showed that some residual magnetism is 

 necessary to start the process. If, due to some jarring, 

 reversed current, or similar cause, the field frame has lost 

 it- rcsi< lu;d magnetism, it is necessary to re-magnetize the 

 field by supplying current to the field coils from some 

 outside source. A few primary cells will generally be 

 sufficient to give the requisite amount of residual magnetism. 

 Before connecting the cells to the shunt field circuit the 

 field must be disconnected from the armature, otherwise 

 the low resistance armature forms a short circuit for the 

 field and the current from the cells, instead of flowing 

 through the field, practically all goes through the armature. 



21. The residual magnetism may have reversed so that 

 what small current is sent through the field circuit by the 

 voltage due to residual magnetism tends to decrease, instead 

 of increase, the field magnetism. In this case the residual 

 magnetism must be reversed or else the connection of the 

 field circuit to the armature may be reversed. This latter 

 procedure is permissible only in the laboratory because 

 it will cause the generator to build up with reversed 

 polarity. If this were done on a railway generator, for 



