The experiments to be described in this paper arose from a suggestion by M. von Laue that it would be of interest to examine more closely the behaviour of simply and multiply connected supraconducting bodies in an external magnetic field. If a closed circuit be taken wholly within a supraconducting body, sufficiently far from the surface, the magnetic flux through the circuit should be constant as long as no part of the body is subjected to a magnetic field greater than the critical field strength. For a simply connected body, if the spontaneous ejection of flux on cooling through the transition point, the so-called Meissner effect, is complete, the constant flux through any circuit should be zero. For a multiply connected body, it should be equal to the value immediately after the body became supraconducting. Only in the case of a multiply connected body, that is, a closed circuit, can there be a resultant current through any cross-section in the steady state. This may be taken as a definition of the current I in the circuit, the so-called persistent current. Let L be the self-inductance of the circuit, calculated for the supraconducting state on the assumption that the current flows entirely in a layer very close to the surface. Let
ϕ
be the calculated magnetic flux through the circuit due to external magnetic field, allowing for the distortion of the field by the presence of supraconducting material. Then, if it can be assumed that the maintenance of the constant flux through the closed circuit is due to a persistent current in the above sense, the law of constant flux can be written in the form LI +
ϕ
=
ϕ
0
. (1)
Characteristics of a Persistent Current Compensator for Superconducting NMR Magnets Using Linear Type Magnetic Flux Pump
