24 



PRINCIPLES OF ELECTRICAL DESIGN 



all dimensions in this and subsequent examples being expressed 

 in centimeters. 



Case (6). Flat Surfaces of Equal Area Subtending an Angle 0. 

 The assumption here made is that the lines of induction in 

 the air gap are circles described from a center on the axis 

 where the planes of the two polar surfaces meet. Let I = length 

 of polar surface at right angles to the plane of the section shown 

 in Fig. 7. The sum of the permeances of all the small paths such 

 as dr is then, 



- 



> 



X 



360Z , r 2 

 2^ loge n 





H'h 



j 



FIG. 6. Permeance between FIG. 7. Permeance between non- 

 parallel surfaces. parallel plane surfaces. 



For the special case when 8 = 90 degrees, 



r> 2* i r 2 

 P=-loge- 



(8) 



For the special case when 6 = 180 degrees, and the two surfaces 

 lie in the same plane, 



P-^log^ (9) 



Case (c). Equal Rectangular Polar Surfaces in Same Plane. 

 This is a case similar to the one last considered, but the formula 

 (9) is not applicable when r\ is large relatively to r z because the 

 actual flux lines would probably be shorter than the assumed 

 semicircular paths. With a greater separation between the polar 



