Current effects in cylindrical superconductors

Abstract

Experiments are described which show the effect of current on the transition of cylindrical superconductors to the normal state produced by increasing an external magnetic field. The transition curves were determined with an accuracy of 0·1%. Both longitudinal and transverse fields were used.The following results were found.(a) Transverse field(i) The field strength at which the first traces of resistance appeared was independent of current strength for reasonably small currents (up to about 100 mA. for 0·010 cm. wire).(ii) With increase of current the transition curve at first changed shape, becoming more and more nearly linear, and then was displaced parallel to itself toward smaller values of the applied field by an amount in accordance with Silsbee's hypothesis.(iii) The change in shape of the transition curve was accurately reproducible and was identical for specimens of the same material and diameter.(iv) The results for specimens of different diameter could be accurately correlated if the measured values of the resistance were plotted against the current divided by the specimen diameter raised to the power (l + R/Rn), i.e. I/(diam.)(1+R/Rn), where R/Rn is the ratio of the measured resistance to the resistance just above the transition point.(b) Longitudinal field(i) With increase of current, however slight, the transition curve was displaced parallel to itself, toward smaller values of the applied field, without showing any change in shape.(ii) The displacement of critical field for a given change of current was completely independent of the diameter of the specimen.(iii) The observed shift to smaller fields was much greater than could be accounted for by the additional field produced by the current at the surface of the wire.(iv) If we denote the (extrapolated) value of the critical field for zero current by (Hc) and the difference between this and the observed value for a current (I) by (ΔHc) then I and Hc are connected by the empirical relationwhere A and n are constants depending on the nature of the superconducting metal.when Hc is expressed in gauss and I in milliamperes.