Properties of Superconducting Colloids and Emulsions
Proceedings of the Royal Society A
Introduction 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 reason able, 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).
... ondon (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 measure ments 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, in volve certain assumptions) are necessary to infer anything about the details of the field penetration. A much more direct method is to measure sus ceptibilities of small specimens, since if there is any appreciable pene tration 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. Experimentally, such a method of investigation is difficult because the magnetic moment is proportional to the volume and so becomes ex ceedingly small if the linear dimensions are reduced sufficiently to produce penetration effects. One way of overcoming this difficulty is to measure the total magnetic moment of a colloid or emulsion, where the small Properties of superconducting colloids and emulsions Vol. 175. A. [ 49 ] 4 on July 19, 2018 http://rspa.royalsocietypublishing.org/ Downloaded from * I have to thank Dr Casimir for pointing this out to me. Properties of superconducting colloids and emulsions 1 1 1 S 1 1 § •! M © © * T h e d a t a fo r H g B is o m itte d a s it is to o ro u g h to b e o f a n y u se ( p a r tly b e c a u se th e h y s te re s is m e a s u re m e n ts o n H g B w e re m a d e w ith a m o re p r im itiv e te c h n iq u e ). on July 19, 2018 http://rspa.royalsocietypublishing.org/ Downloaded from Properties of superconducting colloids and emulsions * It should be noticed that the lack of particle size uniformity in Hg A is irrelevant here, provided that there is not an appreciable volume percentage of particles with r> A present. Also (5) is probably valid for any shape, and so it is immaterial whether the particles in Hg A are spherical or not.