Equilibrium curve and entropy difference between the supraconductive and the normal state in Pb, Hg, Sn, Ta, and Nb

The thermodynamical treatment of supraconductivity (Gorter and Casimir 1934a, b) is based on the fact that at the transition from the normal into the supraconductive state the induction becomes zero and it was assumed that the magnetic threshold curve, i. e. that curve in the (H, T) diagram that indicates the disappearance of the electric resistance, is also the equilibrium curve between the supraconductive and the normal state. It has to be remembered, however, that zero resistance and zero induction in the supraconductive state are not causally connected by electrodynamic theory, but that their coincidence must as yet be considered as a purely empirical fact. It is therefore not possible to deduce the existence of zero induction from the observation of zero resistance and vice versa. It is, for example, known that zero induction and zero resistance do not coincide in supraconductive alloys (Mendelssohn 1935) and preliminary experiments on tantalum (Mendelssohn and Moore 1936) have shown that even in pure metals the threshold values obtained in conductivity experiments cannot be used for an evaluation of thermodynamical quantities. In two previous communications (Keeley and Mendelssohn 1936; Mendelssohn 1936) two methods (later called 1 and 2) for the determination of the induction in supraconductors were described. With these methods the equilibrium curves of extremely pure Pb, Hg, Sn, Ta, and Nb were determined in the temperature interval between 1·5° and 4·2° K.