THE DESIGN OF ELECTROMAGNETS 



59 



and the total leakage flux is 



3>i = 9,350 + 330,000 = 339,350 maxwells. 

 The leakage factor is 



170,000 + 339,350 , 

 170,000 



which is greatly in excess of the permissible value, unless the 

 cross-section of the core under the windings is increased to keep 

 the flux density within reasonable limits. The simplest way to 

 reduce the amount of the leakage flux is to shorten the magnet 

 limbs, and although the long limbs with no great depth of winding 

 may lead to economy of copper, it is seen to be necessary in this 

 design to increase the depth of winding, t, in order to reduce the 

 length, I, of the exciting coils. The dimension t will have to be 

 more than doubled. Let us make this \y in. and at the same 

 time retain the full section of 2 in. square under the windings; 

 that is to say, the square section bar will be carried up through the 

 coils without being turned down to a smaller section as in the trial 

 design. 



Using formula (36) l to calculate the current density, we have, 



80 X 1,273,000 



whence 



180 X 1.75 X 0.5 

 = (say) 800 



' 10,230 



* ~ 2 X 1.75 X 800 X 0.5 

 = 7.3 in. 



Let us try I = 7 in. 



Allowing still a separation of 2 in. between the outside surfaces 

 of the windings, the distance between the two parallel magnet 

 cores of square section will now be 5.5. in. The permeance 

 between the opposite faces is 



_ 7.25 X 2 X 6.45 

 Pl ~ 5.5 X 2.54 



and between the sides of the magnet cores (by formula 10, page 

 25) 



P 2 = 2 X ~ X 2.3 logio 



1 This formula and also the correct formula (35) are applicable to rec- 

 tangular as well as to circular coils. 



