50 



THE BELL SYSTEM TECHNICAL JOURNAL, JANUARY 1954 



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\ \ \ \ \ \ \ w , 



USEFUL GAP FIELD 



• » — CORE LEAKAGE FIELD 



* — ARMATURE LEAKAGE FIELD 



Fig. 16 — Distribution of the field of an electromagnet, 

 surfaces may be found as 



(P =^+ C,f2+ C2d, 



where values of Ci and C2 are given in curve form, and the equivalent 

 value of d is as shown in Fig. 17. When the structure has two return 

 paths, the reluctance is assumed to be one-half the value given by this 

 last equation. 



External Reluctance of a Bar Magnet 



The relations of Figs. 13 through 17 suffice for the evaluation of leak- 

 age in most ordinary electromagnets. In special structures, particularly 

 in polar relays, cases occur where the return path is predominantly an 



10 



8 



6 



5 



U 4 



TWO LEGS (perimeters P| 8. P2) 

 P1 + P2 



ZZZZDICjIZJPs 



COIL_ D' 



^__OP 



D=D'+c| 



+ C,£2 + C2d 



THREE LEGS (PERIMETERS P| & P2 ) 



D/d 



D~aP2 



D' 



2.7T 



yU^i^Y []p, D = D'+d 



' ^^"""^^"^ ~^' (P=2(^'+C,£2 + C2d) 



DPa 



Fig. 17 - — Effective reluctance between parallel bars joined at one end. 



