16 



as their fields can be made on the " iron- 

 clad principle," so that no magnetism is 

 lost in external leakage, and only one 

 coil of exciting wire is required. 



The type of field most commonly used 

 in modern alternators is, however, one 

 in which N and S poles succeed each 

 other around the armature, and in its 

 simplest form such a machine is repre- 

 sented by Fig. 5, where an iron-cored 

 cylindrical armature revolves between 

 poles which do not completely surround 

 it. The portion of the armature not 

 embraced by the poles may be taken as 

 equal to that embraced, and Fig. 6 shows 

 this disposition in a multipolar machine. 

 PP is the mean pitch-line of the poles 

 straightened out, and the poles are rep- 

 resented by the shaded rectangles which 

 have a width equal to half the pitch. 

 The mean electromotive force of this 

 arrangement is 



e = — ~n z t (6), 



or 183 per cent, greater than that of a 



