The Influence of Magnetic Fields on Persistent Currents in a Closed Superconducting Circuit

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Abstract
A study has been made of the effects of external magnetic fields in preventing persistent currents from flowing in a closed superconducting circuit, in order to determine as closely as possible the field strength at which the first trace of electrical resistance appears. In agreement with earlier work by a less accurate method and with the resistance measurements of de Haas, but in contradiction to the idealized theory of the transition state, it is clearly shown that no resistance appears until the mean magnetic induction within the specimen becomes considerable. It has been found further that the amount of resistanceless current that a cylindrical specimen can carry under these conditions has a well-defined limit. The effects of very small quantities of impurities of different types have also been investigated and have been found greatly to affect the above results. When the persistent current strength was slightly above its equilibrium value, it required a measurable time to decay; the behavior was similar to the relaxation effects observed in the study of magnetic and thermal properties of superconductors.

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