PARTS OF A DYNAMO-ELECTRIC MACHINE 67 



and for air 





Transposing -r = B = A: 



A. 



or 



. . (19) 

 '20) 



The ampere-turns required per cm. of air gap is there- 

 fore equal to the flux density in the air gap divided by .4x. 

 In calculating the magnetic circuit it is convenient to 

 tabulate the data as shown below: 



Total ampere-turns required 2723 



Ampere-turns to be used per coil = 



2723 



1362 



We will next figure the necessary ampere-turns for a 

 small modern bipolar machine with a slotted armature and 

 double yoke. In this yoke the flux from the pole divides, 

 half going one way and half the other. The flux in going 

 through the armature teeth is very dense and although the 

 length of path is small it is better to figure this as a separate 

 part of the magnetic circuit, instead of treating the teeth 

 as part of the armature core. 



