25G 0,1 the Maximum Magnetic Effect of Electromajnets. 

 and 



Putting 





d8 ^' 

 we have 



which equation expresses implicitly the value of 8 which 

 makes the magnetic effect a maximum. 



Let us put ^=/^; then 



This expression for S contains /x, itself a function of S ; but 

 a very simple artifice suffices to get over this difficulty. First 

 suppose fjb = 0, and solve the equation; the result will be an 

 approximate value of 8, namely that which it would have were 

 there no insulatinD- coverino- to the wire. 



Then^ employing this approximate value of S, calculate /^= ^> 



and recalculate the value of 8, using this value of fi. 



By repeating this process, which involves very little trouble 

 if logarithms be employed, any desired degree of accuracy 

 may be attained. 



From the above expression for 8 we see that, so long as /j, 

 is not = 0, the diameter of the wire (without its covering) will 

 always be less than it would be were there no insulating co- 

 vering. 



The expression for the resistance of the bobbin may be 

 written 



and supplying its value for 8\ we find 



from which it is seen that, so long a3 ^ is not = 0, the 



