110 Till-; MAGNETIC CIRCUIT [ART. 40 



age length / between the pole-tip and the plane MN, being guided 

 by Fijr. 29. (Do not encroach upon the fringe to the armature.) 

 Then the permeance of the path is, in perms, 



(56) 



If a greater accuracy is desired, subdivide the total path into 

 smaller paths in series and in parallel, and calculate the permeance 

 or the reluctance of each separately. Then the total permeance is 

 found according to the well-known law of combination of reluc- 

 tances and permeances in series and in parallel (Art. 9). When 

 mapping out the lines of force in the air, begin them nearly at right 

 angles to the surface of the pole (see Art. 41 a below) and draw 

 them so as to make the total permeance of the path a maximum, 

 that is, reducing as far as possible the length and increasing the 

 cross-section of each elementary tube of flux. The medium may 

 be said to be in a state of tension along the lines of force, and of 

 compression at right angles to their direction, by virtue of the 

 energy stored in the field. Hence, there is a tendency for the tubes 

 of force to contract along their length and expand across their 

 width. 



(b) Between the opposite pole-cores. In this part of the leakage 

 field each elementary concentric path is subjected to a different 

 m.m.f., that between the roots of the poles being practically zero, 

 while the m.m.f. between the points p and p' is equal to that 

 between the pole-tips. In most cases it is permissible to consider 

 the whole leakage flux as if passing through the whole length of the 

 pole core, and then crossing to the adjacent poles at the pole-tips. 

 Therefore, it is convenient to add the permeance between the pole 

 cores to that between the pole-tips. But the average m.m.f. 

 between the waists is only about one-half of that between the tips, 

 so that the equivalent permeance between the pole cores, reduced 

 to the total m.m.f., is equal to one-half of the actual permeance. 

 If the actual permeance, calculated according to formula (56) is (P, 

 the effective permeance is J(P. The average length and cross- 

 section of the path are easily estimated from the drawing of the 

 machine. 



(c) Between the flanks of the pole shoes. The path extends indefi- 

 nitely outside the machine, and the lines of force are twisted 

 curves, so that it is difficult to estimate the permeance graphically. 

 As a rough estimate, this permeance can be reduced to that of the 



