68 . PRINCIPLES OF ELECTRICAL DESIGN 



and the outer shell. Referring to Fig. 22, upon which the 

 approximate dimensions of the leakage paths have been marked, 

 the numerical, values of the two permeances are seen to be, 

 approximately, 



2 X x(10 + 5) X 6.45 

 forpath(2),P, = - - = 43.5 



The flux through path (1) is 



m.m 

 ~2 

 0.4T X 8,260 



m.m.f. 

 *i= 2~ X 



X 72 



2 

 = 374,000 



and the flux through path (2) is, 



$2 = m.m.f. X P 2 



= 0.4 TT X 8,260 X 43.5 

 = 450,000 - 



The total leakage flux is $1 + <> 2 = 824,000 maxwells. The 

 useful flux is 66 X 6.45 X 7,240 = 3,080,000 maxwells, and the 

 leakage factor is 



3,080,000 + 824,000 



3,080,000 



1.27 



The flux density in the cylindrical outer shell, near the yoke, 

 will be 7,240 X 1.27 = 9,200 gausses, and this will also be the 

 density in the central pole if the cross-section is the same, but a 

 higher density would be permissible. The section at every part 

 of the magnetic circuit can be calculated on the basis of an 

 assumed density. As an instance, if it is desired to have a density 

 of 11,000 gausses in the yoke at the section AB, the thickness of 

 the casting at this point, as indicated by the length of the line 

 AB, would be obtained from the equation 



3,080,000 X 1.27 

 ~ABX7rX10X 6.45 



whence AB \Y in. 



In this manner the magnetic circuit may be proportioned. 

 The path of the magnetic flux is indicated by the dotted lines in 



