INDUCTANCES AND RESISTANCES 



'air gap'. One effect of this is to make the flux for a given K^NI rather in- 

 dependent of variations of jjl which may occur in the iron, whilst retaining 

 the valuable property of iron as a flux-guide. 



The magnetic path-length in the arrangement of Figure 4.3 is not far from 

 40 cm, whilst the length of the air gap is 2 mm. Let the ^ of the iron be 

 known to lie in the region 800-1,200: the permeability of air is unity: 

 then we have 



m.m.f. 



^ Reluctance 



10 cm 



Figure 4.3 



If there were no gap, the reluctance would be proportional to the length of 

 the flux path and inversely proportional to the iron permeability 



r> 1 . T^ ^iron „ 40 Cm 



Reluctanccno gap = K^ = Ag 



/^iron jMiron 



but with the air gap, by analogy with resistances in series 

 Reluctanccwith gap = ^2 It^^ + ^^^ 



, t^ivon 



/"gap! 





8 cm 0-2 cm^ 



^K, 



39-8 



but ^air = 1 



, f^iTon 



+ 0-2 



If, for some reason, fj, changes from 800 to 1,200, the change in reluctance 

 without a gap would be 



40 



K, 



800 



Ko 



40 

 T2OO 



X 100% - 100% = 50% 



but if fi changes from 800 to 1,200 with the air gap, the change in reluctance, 

 and therefore of flux, is now only 



/39-8 \ 



^2 800+0-2 



^ — V X 100% - 100% 



K, 



/ 39-8 

 \1200 



+ 0-2 



0-05 + 0-2 

 p-033 + 0-2 



55 



X 100% 



- 100% ^11% 



