541 



finite. In fact we have shown that the resistance of tlie layer per 

 cm.' is finite and further (lie current sheet in tiie layer has a finite 



J^ th 



strengtli being-— of the difference in H on the two sides. Thus for 

 4jr 



the Case II the amount of energy dissipated per cm.' is 



\n V t 



8 



Tlie sudden change in temperature which would have to be produced 

 at the surface in order to supply this amount of heat would be 

 given by ' 



% V CK 4.19 X 10' 



and is insignificant. 



Other considerations for periodic alternating fields indicate that 

 heating may be an important factor, the danger being in eddy cur- 

 rents in the part of the conductor having (J^ö,. 



SUMMARY. 



Special cases of the propagation of changes in magnetic field in 

 a supra-conductive metal are discussed. The calculations show that 

 with the assumptions made (treatment of the conductor as a conti- 

 nuous medium) the thickness of the supra-conductive layers involved 

 may be of the order of molecular dimensions during perceptible 

 intervals of time. 



The writer wishes to express his gratitude to Professor Lorentz 

 for his criticism and advice. 



