100 PROPERTIES OF CIRCULAR CURRENTS. 



If we put / = - - , which gives -j- = , we may write 

 b + x ax r 



.. 



ax u 2 2.4 2.4.6 



From this can be deduced the successive values of the differentials 

 which enter into the expression for X a ; it is sufficient to change the 

 sign of x, to have that of X 6 . 



Neglecting the fourth powers of the ratio - , we thus find 



6.7* 2 



(42) 



} 



The component X in respect of the coil will be obtained by 

 integrating this expression from r-c to r + c, after having multiplied 

 by n^dr-^ and replacing r by the mean radius a. 



Disregarding again fourth powers of the ratio - , we get 





(43) - 



