Critical Current Studies of Flux Trapping in Superconducting Lead

A convenient critical current method for observing the salient features of flux trapping in superconductors is used to study a variety of lead and lead-alloy samples. The critical fields Ht, Ha, Hc for type I and Hc2 for type II superconductors are obtained and found to be in accord with the results of other investigators employing different techniques. Because of self-fields due to current asymmetries, the full critical currents of some samples are observed in nonzero applied fields HpI. The critical current versus applied-magnetic-field characteristics of lead pressure contacts and their application to a simple magnetic null detector are discussed. It is found that trapping flux in certain solid specimens can cause a fourfold enhancement of the peak critical current.