Properties of superconducting colloids and emulsions

Abstract

The characteristic feature of a large superconductor is that it cannot be penetrated appreciably by an external magnetic field lower than the critical field; in other words, the permeability of a superconductor is practically zero and its susceptibility strongly diamagnetic. It is reasonable, however, to expect that actually there is a slight penetration of the field, and theoretical predictions as to the details of this penetration have been made by F. and H. London (1935). Judging from the accuracy with which the permeability has been found zero for large specimens, the depth of penetration cannot be greater than 10 -3 cm., and so to obtain appreciable penetration effects very small specimens must be used. One method of experiment which has been used successfully is that of resistance measurements on thin wires (Pontius 1937) and thin films (Shalnikov 1938; Appleyard and Misener 1938; Appleyard, Bristow and London 1939; Appleyard, Bristow, London and Misener 1939) of superconductors; with this method, however, all that can be learnt is the size dependence of critical field, and thermodynamic arguments (which, as will be seen, involve certain assumptions) are necessary to infer anything about the details of the field penetration. A much more direct method is to measure susceptibilities of small specimens, since if there is any appreciable penetration it must reduce the diamagnetic susceptibility characteristic of the superconductor in bulk; thus, from the size dependence of the susceptibility, information can be obtained about the penetration law. Moreover, by measuring magnetic moments, the critical field can be deduced as well as the susceptibility, so that results of the same nature as from resistance measurements can also be obtained.