448 [Journal of the Asiatic Society of Bengal^ [Ji^y? 1907» 



8. We stall suppose tlie meridian section of the coil to be a 

 curvilinear area bounded by confocal ellipses [r = constant) and 

 confocal hyperbolas {6 = constant) so that r and /i may be treated 

 as independent variables. 



This "will enable us to obtain an approximate solution of the 

 problem of induction in a soft iron rod in the form of a very 

 prolate spheroid, due to a coil of small thickness. 



8 (ca) Sc 8a 8c 8a 



/. the potential due to m turns 



....[«oSi4' 4^-=»^^^l -'' 



(2«±1) *• P..U'^0.f,'^fC''^••.''«4n_,.,^^ ,.,_.N* 



.H-x)-^-^»^'''^'^»^^Mfc'-^"^^^-''"^"'"-^^ 



+ (l-/i2)\/a-l)* \dr 



2irmi\ Q(r') 



|l + r^^^cos7i 'r-r\/ra-lj.~v/l-/i« 1 





{ 



(l"/i«)'(ra-lf + (l--ft«)*(r«-"l)' IcZrd/^ 



where a^/dc da. 



^here P, stands for P^Cm). 



But since (l-/i*>) f __/iP„-i-«/x P^ 



