The transition between the supracondutive and the normal state I-Magnetic induction in mercury

Abstract

It has frequently been suggested that the occurrence of supraconductivity could be described as a charge of state which the metal undergoes. The fact that at the transition point no change in the crystal structure can be observed and no anomaly in the specific heat of the order of RT occurs makes it portable that the change of state is purely electronic, although our present theory of free electrons in metals gives no indication yet as to the description of this new state. Meanwhile the purely as phenomenological treatment has been more successful. As early as 1924 Keesom outlined a thermodynamical treatment of supraconductivity assuming that the difference of free energy between the supraconductive and the normal state was indicated by the magnetic threshold in values. This treatment was carried out rigorously and completed in many details by Gorter and Casimir. Supported by the experiment of Meissner and Ochsenfeld, these authors based their treatment on the assumption that the magnetic induction is zero in the supraconductive state. A great number of experiments has since been carried out, and while some of the results seem to indicate a certain discrepancy with Gorter's treatment others confirm his assumption. Experiments by Keeley and Mendelssohn have shown that very pure substances with undisturbed crystal lattices approximate most closely to the "ideal" supraconductor in Gorter's sense. It was found that mercury showed a reversible change to zero induction when a magnetic field, higher than the threshold value, was applied and switched off again. It was decided, therefore, to investigate in detail the change of induction at the transition between the suparconductive and the normal state in this substance, and to find out whether the transition is a sharp one or extended over a field interval or temperature interval respectively and whether the induction disappear in the same way as it changes to its normal value.