12 



theory of alternators. In the most 

 simple and most frequently occurring 

 case of a coil with straight sides, it is 

 obvious that the dimension of the pole- 

 piece, which is parallel to the active part 

 of the coil, would have to vary according 

 to a sine-function as shown in Fig. 2, 

 where ABCD represents the coil, and PP 

 the mean pitch line through the poles 

 straightened out. The shaded areas N 

 S are the poles, which are shown on 

 opposite sides of the pitch-line to mi 

 their sinoidal contour at once apparent. 

 Poles of this shape are, however, not 

 used in practice ; the poles are either 

 trapezoidal, circular, or rectangular. In 

 the Mordey alternator the poles have a 

 trapezoidal face, and those succeeding 

 each other on the same side of the 

 armature are of the same sign, and are 

 placed in line with those of opposite sign 

 on the other side of the armature. Tims 

 along the mean pitch line, uniform fields 

 alternate with spaces in which there is 

 no induction; this arrangement when 

 straightened out may be represented by 



