60 ELECTRICAL ENGINEERING 



and the flux inside the ring is 



M 0. 



r 

 lines. 



R I/A 



The flux density in the ring is 



< 

 9S = -j lines per square centimeter, 



and the magnetizing force is 



. c $ 0.4 irnl 



and is the m.m.f. per centimeter. 



If the solenoid is wound on an iron ring of permeability M, the 

 reluctance is reduced and becomes 



the flux is increased to 



QAirnl 



and the flux density or induction density is increased to 



the magnetizing force remains the same as before, 



_. c&! $1 0.47Ttt7 



eft = - -- -; --- = - 



(2) Fig. 50 represents a solenoid of n turns carrying a current of 

 I amperes; the m.m.f. is M QAirnl gilberts. If I cm. is the 

 length of the solenoid and A sq. cm. is its sectional area, the 

 reluctance of the path through it is 



The m.m.f. required to drive the flux $ through this reluctance 

 is 



MI = $ -T- gilberts. 



