540 



Eliminating y and letting 



1 , T 



the resultant equation for « becomes 



2 He 1 1 — (r+l)0(a) 



(17) 



(18) 



H^—Hc l/jr 0(a)[l — 0(a)Jae«' 



Solving (16') for y we obtain 



2 1—0 

 y = -log—-- (19) 



2/7 1 



If T ^=: (18) becomes = — ~ = — z: . This formula is 



readily seen to be in agreement with (12) if in the latter //, =r — He- 



Thickness of Supra- Conductive Lm/ers. 



Formulas (14) (19) enable us to make an estimate of the thickness 

 of supraconductive la^'ers produced b}' the suppression or reversal 

 of a strong magnetic field. Thus according to (14) the quantity « is 



of the order of — . Since /?, is approximately 1, the thickness of the 



layer reached in 1 sec. measured in centimeters is of the order of 

 magnitude of the ratio of the conductivities just above and just below 

 the transition point. This ratio may be 10~^ and thus if formula 

 (14) applies supra-conductive layers the thickness of which is of 

 molecular dimensions are dealt with. 



If the thickness of the slab discussed in (15) is 1 cm., the first 



term of the series ''J,[o, e i^^^'J is 2 «-«"'' (,5, being set = 1). Thus 

 if ; = 10— * sec. (14) and (15) are nearly in agreement and the eff"ect 

 of finite dimensions is not sufficient to tlirovv off the conclusion just 

 drawn because 10~* sec. is a comparatively easily measurable 

 interval of time. 



The thickness of the supra-conductive layer brought about by the 

 reversal of the field is according to (19) and (16) 



y 1/7 2 1/7 1—0 



^ z:z. loq 



and is thus of the same order of magnitude. 



It is also of interest to observe that the amount of heat dissipated 

 by the eddy currents in the microresidually conducting layer is 



