OPERATION AND CARE OF ELECTRIC MACHINERY 451 



a revolution and the test repeated. Two apparent grounds 

 will again be detected, one in each path, but it will be 

 noticed that one of the segments is the same as was 

 detected before, while the other is now on a different seg- 

 ment than it was. That ground which persists on the 

 same commutator bar is the only real ground; the one which 

 shifts from one segment to another as the armature is 

 revolved is a phantom ground only, as the winding is really 

 not grounded in this place at all. As the armature is 

 turned in several positions and tested each time, it will be 

 noticed that the phantom ground moves on the structure 

 in the opposite direction to that in which the armature has 

 been moved while the real ground turns just as the arma- 

 ture is turned. 



Use of a Bell-buzzer for Making Tests. Instead of using 

 a voltmeter for these tests a telephone receiver may be 

 employed if the dry cell is connected to the brushes through 

 a bell-buzzer. This is a very convenient way of testing 

 as a suitable low reading voltmeter is not always available. 



Repairing an Armature. Repairing a faulty armature 

 generally requires considerable skill and should not be 

 attempted by the average operator. A repair man from 

 the factory should be employed or, if the armature is a small 

 one, it may be shipped back to the factory for repairs. 



