Abrikosov and the author have separately suggested that a new type of reversible magnetic behavior is to be expected in superconductors whose coherence length is small compared with their penetration depth. The treatments of these two authors are shown to lead to similar results for pure metals and to only slightly different results for alloys. The experimental evidence in favor of a new type of reversible magnetic behavior in superconductors is discussed.

Tandis qu'un calcul simplifié démontre qu'un transformateur supraconducteur aurait un encombrement petit, par rapport à celui d'un transformateur classique de même puissance, des travaux récents montrent l'importance des pertes à l'état mixte

Contents 1. Introduction . 2. The Ginzburg-Landau equations . 2.1. Introduction . 2.2. The range of coherence and the penetration depth 2.3. T h e upper critical field Hc2 . . 3. Vortex lines. 4. The reversible magnetic behaviour of type I1 superconductors . 4.1. The vortex line tension and the initial penetration field H,, . 4.2. The magnetic behaviour at low fields and the evidence for the existence of vortex lines . 4.3. The magnetic behaviour near Hc2 and the thermodynamic properties of the mixed state at arbitrary temperatures , 4.4. T h e influence of the paramagnetism of the normal state. 4.5. The superconducting sheath : supercooling . 5.1. The static irreversible magnetic behaviour 5.2. Flux motion . Acknowledgments . References .

The atomic heats of vanadium, in the normal and superconducting states, have been determined from just above the transition temperature, r c =5.03°K, down to 1.1 °K. After corrections to the 1948 temperature scale had been made, the normal state atomic heat could be represented by C n = yT-{-(12/5)Tr i R(T/®) z , with 7= (9.26rfc0.03)X10-3 joule mole^1 deg^2 and 0 = 338±5°K. The entropy difference, S n~Ss , between the normal and superconducting states, extrapolated to 0°K, was found to vanish, in accordance with the third law of thermodynamics. The critical field values deduced from S n -S 8 gave #0=1310 oersteds; at higher temperatures they were in agreement with initial penetration fields previously reported.The most interesting result of this work was that below about .0.7T c the electronic contribution to the atomic heat of the metal in the superconducting state could be represented by an exponential expression of the form C es /yT c =ae~b Tc,T with a -9X1 and 6 = 1.50; such an exponential relation is consistent with a single-electron model of a superconductor involving a gap of the order of kT c per electron in the spectrum of available energy levels.

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