THE DESIGN OF ELECTROMAGNETS 61 



where B y = flux density at end near yoke, 



and B p = flux density at end near pole pieces. 



The total ampere-turns for the magnet of Fig. 21 can now be 

 calculated by using the B-H curve of Fig. 3 which is supposed to 

 apply to the particular quality of magnet steel or iron which it 

 is proposed to use. The calculation can conveniently be put in 

 tabular form as shown below. 



FIG. 21. Horseshoe magnet. (Modified design.) 



Thus, it is necessary to have not less than 9,600 ampere-turns 

 on the two bobbins. 



Calculation of Windings. The formula (26) of Art. 10 can be 



written, 



mean length of turn (inches) X SI 

 (m) = 



