19 



(2) Rectangular field, width of poles 

 equal to one-half the pitch, as shown in 

 Fig. 6. 



(3) Eectangular field, width of poles 

 equal to one-third of the pitch. 



As regards the armature, either the 

 whole available space may be covered 

 with coils, in which case the width of 

 each group of conductors forming a coil, 

 or one side of a coil, will be equal to the 

 pitch, and the armature will contain the 

 greatest possible amount of wire ; or, 

 only portions of the available space on 

 the armature may be covered with the 

 active conductors, which plan is generally 

 adopted in modern alternators, because 

 by it the differential action alluded to is 

 diminished. The cases selected for con- 

 sideration are the following : 



(1) Width of coils equal to the pitch. 



(2) Width of coils equal to one-half 

 the pitch. 



(3) Width of coils equal to one-third 

 of the pitch. 



The calculation of the mean electro- 

 motive force, for any combination of field 



