THE DYNAMO. 65 



shows a carbon brush holder manufactured by the General Elec- 

 tric Company. The current is led from the carbon block to the 

 metal part of the holder by the flexible cable of fine copper wire 

 which is called a pig tail, thus relieving the spring from doing 

 duty as an electrical conductor. The use of the spring as a 

 conductor would heat it and cause it to lose its temper. The 

 small lever at the back serves to adjust the pressure with which 

 the spring presses the carbon block against the commutator. 



36. Bipolar and multipolar dynamos compared. In the early 

 days of electrical engineering comparatively small sized dynamos 

 only were used, and the bipolar field structure was almost univer- 

 sal. The increase in size of the machines involved in recent elec- 

 trical engineering developments has led to the adoption of the 

 multipolar field structure. The advantages of the multipolar over 

 the bipolar field structure for large-sized machines are as follows : 



(a) Economy of iron. The magnetic flux is available for the 

 production of electromotive force only where it passes across the 

 air gap between the pole faces and the armature core. Therefore 



Fig. 57, 



> 



that distribution of flux around the armature is most desirable 

 which gives the shortest possible paths in the armature iron from 

 5 



