IMPEDANCE. 55 



the self-induction of the armature. Hence the impedance of 

 the armature when the fields are unexcited is considerably higher. 

 The difference between the two curves would be still greater 

 if the armature resistance had not been so high. The impedance 

 was so largely composed of resistance that variation in self- 

 induction makes comparatively little difference to the total 

 value of the impedance. 



In order to make sure that the difference between the 

 two curves was not due to induced currents in the field 

 winding, the voltage at the terminals of the winding was 

 measured when open. No induced voltage was found. In 

 most types of alternators this effect may bg noticed. It is 

 referred to later. 



Another effect which has an important influence on the 

 self-induction and impedance of the armature must be 

 mentioned. This is the currents which are induced in the 

 pole-faces, poles, and field-windings by the alternating flux 

 set up by the current in the armature. This is referred 

 to at greater length in the chapter on alternators. 



