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ELECTRICAL MACHINERY 



mutated, when all the current flowing through the left 

 side of the armature has to go through coil B and lead c 

 to reach the brush and so the external circuit. When the 

 brush is in position 2, this current has two paths by which 

 to reach the external circuit; through coil B and lead c 

 as before or else not through coil B at all but directly through 

 the lead 6. The division of the current between these 

 two paths depends upon their relative resistances and 

 practically all the resistance of either path is in the brush 

 contact. Now as the brush moves from position (2) to 



JP IG 67 Backward Motion of Brush Corresponds to Forward Motion 



of Armature. 



position (3) it is seen that the brush contact area diminishes 

 for one path and increases for the other. The original 

 path through coil B and lead c contains the brush contact 

 whose area is continually diminishing. But if the area is 

 diminishing the resistance is increasing in this path; at the 

 same time the resistance of the second path (directly 

 through lead b and not through coil B) is decreasing. 

 Therefore, during the time the coil is short circuited this 

 variation of brush contact resistance tends to stop the flow 

 of current through coil B. 



We shall now see how the current is started through 

 B in the other direction by the same effect. 



