130 



PRINCIPLES OF ELECTRICAL DESIGN 



with OQ. Produce DP to 0' where it meets the perpendicular 

 erected on AD at the point B. Flux lines of which the length 

 and direction will be approximately correct can now be drawn. 

 All the linevS from points on armature surface lying between A 

 and B will be considered perpendicular to the armature surface, 

 i.e., verticals erected on the datum line. Between the points 

 B and D the lines will be considered straight, but with a slope 

 determined by the position of the point O r through which they 

 will pass if produced beyond the pole face. From the point F 

 the equivalent flux line will be the arc of the circle described 

 through the point Q with the radius OQ, and lines from points 

 between D and F must be put in by the eye so that their curva- 

 ture shall be something between the circle through F and the 

 straight line through D. One of these intermediate lines has 

 been drawn from the point E. Over the region beyond F all flux 



\ 



a P d 



FIG. 46. Effect of neighboring pole in modifying flux distribution. 



lines will be drawn as circles described from the center and 

 continued beyond the vertical OR until they meet the pole in a 

 direction normal to the surface of the iron. Thus the line from 

 the point G is completed by an arc of circle with its center at 

 the junction of the line OR and the flat surface of the polar 

 projection, while the flux line from H is continued as a straight 

 line (the shortest distance) until it meets the pole perpendicularly 

 to the surface. 



Any desired number of lines can very quickly be drawn in 

 this manner, and they may be thought of as the center lines of 

 "equivalent" tubes of flux of uniform cross-section over their 

 entire length. If, now, the length of any one of these imaginary 

 flux lines be measured in centimeters, the reciprocal of this 

 length will be the permeance per square centimeter between pole 

 shoe and armature at the point considered. It is, therefore, 

 an easy matter to plot a permeance curve similar to the one 



