MAGNETISM AND ELECTROMAGNETICS 



61 



But the flux going out at one end has to pass, around through the 

 a,ir and in at the other end as shown. The reluctance of this 

 return path is difficult to calculate and is not of great practical 

 importance; its length is greater than I and its sectional area is 

 very much greater than A and so its reluctance is small compared 

 to 3li. Thus in the case of long solenoids of small section the 



00000000000 



FIG. 50. Solenoid in air. 



reluctance of the return path may be neglected and the assump- 

 tion may be made that the whole m.m.f. M is utilized in driving 

 the flux through the reluctance 3li. Thus 



' 



OAirnl 



I/ A 



the induction density in the solenoid is 

 c 3> 0.4 wnl 



and the magnetizing force at any point inside the solenoid is 



3-C = g> = ' dynes, as in equation 94. 



if A 



