531 



Tliese equations are analogous to the equations in lieat conduction : 



da 1 



K 



1 Ö d 



V Or Ot 



(51) 

 (61) 



wliere (9 is tlie temperature, F the flow of heat, AT is the conduct- 

 ivity for heat, and 6' is tlie specific heat. The electrical problem 

 is the analogue of the heat problem for a substance having unit 



specific heat and a conductivity for heat ^ — =zz—. Thus a perfect 



snpra-conductor corresponds to /ir=0 i. e. to a perfect insulator 

 for heat. Tliis is another expression for the fact (hat the shielding 

 properties of the supra-conductor are perfect. 



\n view of the difticultj- of treating 

 the cylindrical case accurately we 

 shall specialize the problem by in- 

 vestigating it within the ap|)roximation 

 (3'') i.e. neglecting the curvature of the 

 surface within the depth of penetration, 

 this makes the problem an essentially 

 unidimensional one. 



The shaded region on the right of 

 the plane AB (see Fig. 2) is occupied 

 by the metal. The axis OX is perpen- 

 dicular to AB. The changes in the field 

 are produced from the left side of ^i^. 

 H is positive when vertical and up- 

 ward. E is positive when into the plane 

 ot the paper. The relations between 

 E and H are : 



F/a. e. 



dH 



= ^E. 



dE 



dH 



and hence 



d'H dB 



d«' 



dt 



(7) 

 («) 



(311) 



We shall consider several problems all of which are similar 



35* 



