THE CONTINUOUS CURRENT GENERATOR 115 



then might have the form shown by the dotted curve of 

 Fig. 69. In this case at the time when segment g leaves 

 the brush the current B is only 2 amperes (negative); it 

 should be 10 amperes (negative). But if the current 

 flowing in the right-hand part of the armature cannot 

 reach the external circuit through segment g it must go 

 through coil B, and this means that at the instant the brush 

 and segment g separate the current in B must suddenly 

 change from 2 amperes to 10 amperes. The rate of change 

 of current in B is very large at this instant and so a large 

 counter e.m.f. of self-induction is set up in B. 



Brush 



Fia. 70. Place where Spark Appears. 



Cause of Sparking. The current from C has then two 

 possible paths; it may force its way through B against 

 the high counter e.m.f. of B or it may form an arc over the 

 mica insulation and get to the brush without going through 

 coil B. This condition is indicated in Fig. 70. The forma- 

 tion of this small arc depends upon the high counter e.m.f. 

 of coil B and this in turn depends upon the rapid change in 

 current through B when segment g leaves the brush. This 

 rapid change of current is necessary because during the time 

 of short circuit the resistance variation has not been 

 sufficient to reverse completely the current in B (dotted 

 line, Fig. 60). 



