Experiments on the superconductive transition

Abstract

The change of electrical resistivity at the transition between the superconductive and the normal state in a longitudinal magnetic field has been investigated systematically. By successive elimination of a number of disturbing factors an experimental procedure has been developed which yields consistent and reproducible results. The metals lead, mercury and tin have been investigated in this manner. Contrary to the accepted view it has been found that under ‘ideal’ conditions the transition is not discontinuous. There exists a range of temperature and magnetic field in which the resistance changes gradually from normal to zero and vice versa. The extent of this transition region was found to grow' rapidly with increasing absolute value of the magnetic field. Under conditions closely approximating the ‘ideal’ longitudinal case transitions with hysteresis were never observed. However, it could be shown that resistance hysteresis and discontinuous resistive changes are produced by deviation from the longitudinal case owing to unsuitable geometrical shape of the specimen. The general conclusion has therefore been reached that the resistance of a pure superconductor in a longitudinal field changes continuously in transition between the superconductive and the normal state. The significance of the results has been discussed and further experiments have been proposed.