62 ELECTRICAL ENGINEERING 



If an iron bar of length I and permeability /* is placed in the 

 solenoid the reluctance of the path through the solenoid is re- 

 duced in the ratio - and the reluctance of the return path is no 

 M 



longer negligible in comparison to it and its value must be calcu- 

 lated. (Fig. 51.) 



(3) In Fig. 52 a solenoid of nl ampere turns is wound on an iron 

 ring of permeability JJL and a section of length k is cut from the 

 iron. If li is the mean length of the path through the iron and 

 its sectional area is A sq. cm. the reluctance of the path through 

 the iron is 



the reluctance of the path through the air is 

 and the total reluctance of the path is 

 The flux through the path is 



AH A 



The flux density in the iron is 8 = -j and is the same as in the 



air. 



The magnetizing force or m.m.f. per centimeter in the iron is 



and in the air it is 



<f> 



CIT vi 



CH^ = CA3 = r 



A. 

 The m.m.f. consumed in the iron is 



MI = cJCi/i = -T- Zi 



and the m.m.f. consumed in the air is 



-r-^2' 



