13 



a succession of rectangles, as in Fig. 3, 

 placed above and below the line PP, the 

 height of the rectangles representing 

 half the induction between the poles. 

 The analogue arrangement of poles in a 

 direct-current machine is shown in Fig. 4, 

 where a cylindrical armature, provided 

 with an iron core, is completely sur- 

 rounded by its pole-pieces. In the latter 

 class of machines the exact shape of the 

 poles is only in so far of importance as 

 it affects the leakage of magnetism, but 

 for a given total induction through the 

 armature the shape has no influence on 

 the electromotive force. Hence for a 

 direct-current machine, apart from the 

 question of leakage, Fig. 3 is as good an 

 arrangement as Fig. 1, but for an alter- 

 nator it is not so good. A simple calcu- 

 lation, which need not be reproduced 

 here, shows that the electromotive force 

 of the arrangement represented by Fig. 3 

 is 



e—^nzr (5). 



In this case the electromotive force is 

 therefore only twice that of the direct- 



