Physics. — "Transients of Magnetic Field in Sui>ra-conductors" . 

 By G. Bkeit. National Research Fellow U. S. A. (Commu- 

 nicated by Prof. H. A. Lorentz). 



(Communicated at the meeting of June 30, 1923) 



It is known tliat supra-coiiductivity is determined not only by 

 temperature bill also by the magnetic field and tlie current density ')• 



In view of the considerations of Silsbee and Langevin it is 

 probable that the only essential factors are the magnetic field and 

 the temperature '). 



This hypothesis will be adhered to below. The problems to be 

 discussed are the calculations of the manner in which a strong 

 magnetic field im|iressed from the outside on a su|)ra-conductor 

 destroys its siipra-conductivity and the way in which the supra- 

 conductivity is reestablished when the magnetic field is withdrawn. 



If the view proposed by Bridgman ') is correct there is an evolution 

 or an absorption of heat whenever a change in the conductive state 

 takes place. These phenomena being of unknown magnitude, they 

 will be disregarded below. If experiments should fail to confirm 

 the calculations here developed, the source of disagreement may be 

 then looked for in the neglect of Bridgman's latent heat. 



The mathematical difficulty of the problem consists in the existence 

 of two distinct states determined by the magnetic field. The purpose 

 of this paper is to point out some special solutions (particular 

 integrals) of the problem. 



We shall employ the electromagnetic system of units. By H 

 (a vector) and by o we shall denote the magnetic field and the 

 resistivity. The symbol He will be used for the threshold value of 

 the field. The resistivity a may have either of two values o^, <i, 

 according as to whetlier \H\ '^ H,-, or \H\ <^ He- The value a, is 

 the microresidual resistivity and in a special case may be taken to 

 be zero. The electric intensity at any point we shall denote by the 



') H. Kamerlingh Onnes, Proc. Arast. Acad. So. 16, (2j 1914. Leiden Coram. 

 NO. 133, 139. 



') F. B. SiLSBEE. .lournal Washington Academy 6, 597—602, 1916. Bureau of 

 Standards Scientific Paper N». 307 (July 23, 1917). 



') Journal Washington Acad. Vol. 11, p. 455, 1921. 



35 



Proceedings Royal Acad. Amsterdam. Vol. XXVI. 



