MAGNIOTIC DKSIGX OF RELAYS 45 



reluctance of the comhinat ion is: 



It follows that the reluctance of the combined gap is e(iui\alent to 

 that of a simple gap with an effective pole face area given 1)}': 



A a I (v> 



(18) 



Exact: (R = 



bin 



a 



I 



2/-0/ 



Approximate: (R = — 

 ab 



Lridis of apj)roximatioii : 



Less than 1 per cent if a < ,^/o 



Less than Id per cent if a < /o 



Fig. 10 — Reluctance between inclined plane surfaces. 



aud/or x < -I'o 



In 



Exact : 



(R = 



be 



Where b = length parallel to axis 



A])proximate: (ft = , — 

 bi-od 



Errors of approximation: 



Less than 1 [jer cent if /'s < 1.4ri' 



Less than 10 per cent if /••j < 3r,' 



Fig. 11 — Reluctance of a cylindrical gap. 



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