ELECTRICAL MACHINERY 485 



purpose is generally a coat of rust or a sheet of tissue 

 paper. 



650. Hysteresis. Another source of loss in an arma- 

 ture is due to the fact that every time the current alter- 

 nates the polarity of the magnetism is reversed. If the 

 armature is making 2,000 revolutions a minute and there 

 are two alterations in each revolution there would be 

 4,000 alterations of the magnetism. This causes heat in 

 the armature which is not accounted for in the external 

 circuit, hence is a loss. Not only is there loss by heat 

 in the armature, but the heat acts on the coils and in- 

 creases the resistance in them and creates another loss. 

 The loss in an armature due to these alterations of mag- 

 netism and the heat produced thereby is known as 

 hysteresis loss. 



651. Insulation of an armature. The insulation of an 

 armature is probably the most essential part of a dynamo. 

 After it is put on in the various places where it is needed 

 it must be baked and all moisture evaporated out of it. 

 After an armature is thoroughly prepared for use it is 

 generally tested for poor insulation. The potential dif- 

 ference for the test is about eight times as much' as the 

 armature is expected to carry. If there is any place where 

 the electricity breaks through the insulation it is detected 

 by means of a sensitive galvanometer. 



652. Capacity of dynamos. 

 It would seem that the 

 amount of current that a dy- 

 namo could produce might be 

 indefinite if enough power be 

 supplied. This is true in a 

 certain sense, but there is a 

 limit and this will appear in 

 FIG . 366 one of the following ways: 



