537 



cylindrical ease because the curvature of llie surface can have no 

 influence in the thickness .t'c- Thus if the solution analogous to 



H=H, + (H,-Hc)& 



W/'t) 



can be written down for the cylinder in question the solution for 

 Xc offers no difficult}'. 



As an example let us consider a thin sheet of metal to which 

 the magnetic field is applied from both sides tangentially to the 

 surface. Let the thickness of the sheet be c. The solution given in 

 Weber Riemann (I.e.) Vol. 2, p. 112 formula II applies here. The 



constant a'' is in onr notation - . Thus according to this formula if 



^ 



^ is suddenly changed by an amount He — //, on both sides of the 

 sheet the change in the value of H at a point having a distance x 

 from one of the sides and considered at the time t is: 



(Hc-H,) - f - V L_L e A \ c / sin \- 



( e n„=i n , c 



-) h - -^ ^^ — ~ e A U / sin ^ ' 



c JT (1=1 n c 



This expression must now be differentiated with respect to x 



the value of the derivative with reversed sign at x=iO must be 



D~ jj 



divided by pfj and equated to — ^ -. This leads to 



^.^, = , ^ .... (15) 



where 



»,(0,g)=2[gi +^4 +...J. 



It may be shown that (15) degenerates into (14) if c-^aa. 



The essential difference between (15) and (14) is that according 

 to (15) for sufficiently high values of t the quantity ^^Xc is of the 



order of /'"' while according to (14) /ïj.tv is always of the order 

 yt. The increase in conductivity after a sufficient lapse of time 

 becomes therefore very much more rapid than (14) would suggest. 



