196 Mr. C. Coleridge Farr. Some Expressions for 



Case III. 



Short solenoid : expressions for the components in the region in 

 which r and r are both less than T. 

 Here 



() x = 



(0 Z - 2*yn[2l\og, 



S 

 T 



1 1 



It is easily deduced from (f) that the value of Z at the centre of a 

 circular coil consisting of a few turns all having approximately the same 

 radius T, is 27ryK/T, where K is the total number of turns. 



Case IV. 



Expression for the magnetic force in that region of a solenoid in 

 which S >r>T and />S. 



If 1 c < 1 ; ]-/><!' 



we have 



(2J,) 1(1 -*+!) 



4. r / i WL 

 Z j - i ^ 



Z - 2ir 



' - 2 



