Persistent Currents in Supraconductors

K. Mendelssohn and J. D. Babbitt

Editor’s Note

Physicists in the early 1930s were struggling to understand superconductivity. Here Kurt Mendelssohn and J. D. Babbitt explored the surprising recent discovery by Walther Meissner and Robert Ochsenfeld that a superconductor expels magnetic flux from its interior. That finding confounded the expectation that persistent electrical currents in a superconductor would instead “capture” magnetic flux and sustain it even if the external magnetic field were removed. Yet the experiments reported here showed that the flux expulsion wasn’t perfect: some magnetisation remains inside superconducting tin. This was the first evidence of persistent “supercurrents”, although their value was only one-sixth of that predicted earlier.　　中文

UNTIL recently it was generally assumed that it was possible to predict, by the ordinary electromagnetic equations, the persistent current produced in a supraconductor cooled below the transition point in a constant external magnetic field after the field was switched off. Thus H. A. Lorentz1 calculated the current induced in a supraconducting sphere, that is, the effective magnetic dipole when an external magnetic field is established.　　中文

According to results recently published by Meissner and Ochsenfeld2, the matter is not so simple as might at first sight appear. Instead of the lines of force being “frozen in” as had been previously assumed would happen when a supraconductor was cooled below the transition point in a magnetic field, it appeared that the field increased in the neighbourhood of the supraconductor, which behaved as a body of zero permeability. If this were so, the flux of induction in the supraconductor should be zero and one might expect, in contradistinction to the old view, that no persistent current or effective induced dipole would be produced by switching off the external field.　　中文

The following experiments seem to show that although supraconductors do not conform to the older theory, neither do they behave as though they had zero permeability.　　中文

(1) A solid tin sphere of 1.5 cm. radius was cooled from 4.2°K. to 2.5°K. (the liquid helium was produced in a liquefaction apparatus utilising the expansion method of Simon) in a field of 70 gauss. When the field was switched off, the magnetic moment of the sphere was observed with a test coil. Its magnitude was about one sixth of that calculated according to the Lorentz equation.　　中文

The magnetic moment remained almost constant whilst the temperature of the sphere rose from 2.5° to 2.9°; with a further rise in temperature it decreased steadily, becoming zero at 3.7°, the normal transition point of tin. Plotting the magnetic moment against the temperature, one obtains a curve of similar shape to that found for the magnetic threshold values.　　中文

(2) The same sphere was cooled to 2.5° without any external magnetic field, a field of 230 gauss (higher than the threshold value at this temperature) was switched on and immediately switched off. The magnetic moment thus produced in the sphere at 2.5° was 8 percent greater than that produced in the previous experiment using 70 gauss, but as the temperature rose it decreased and at 2.9° it reached the same value as the magnetic moment at this temperature in the previous experiment. From 2.9° to 4° the curve coincided with that found in experiment (1).　　中文

(3) Similar experiments to those described above were carried out with a hollow tin sphere of the same radius, the spherical space in the middle being equal in volume to one half the volume of the sphere. The magnetic moments produced in the hollow sphere were two to three times greater than those obtained with the solid sphere.　　中文

In all these experiments the magnetic field was produced by a cylindrical coil in the middle of which the sphere was placed, all iron being excluded. Although the field near the sphere was thus fairly homogeneous, we think it possible that the observed phenomena may be influenced by slight inhomogeneities of the external field. In a completely homogeneous field it would seem possible that the method of cooling might affect the results. In order to test this, we cooled the spheres from the poles and also from the equator. This did not seem to make any difference, the magnetic moment observed being of the same order of magnitude in either case.　　中文

As a result of these experiments, it seems certain that the effective permeability of substances when they become supraconducting decreases, as observed by Meissner and Ochsenfeld. On the other hand, it appears clear that under our experimental conditions the permeability does not vanish entirely, as might be expected in view of the almost infinite conductivity, or if it does vanish, it only does so in certain regions and not throughout the whole volume of the supraconductor.　　中文

In conclusion, we would like to express our thanks to Mr. T. C. Keeley for his advice and assistance in various phases of the work.　　中文

(133, 459; 1934)

K. Mendelssohn and J. D. Babbitt: Clarendon Laboratory, Oxford, Feb. 17.

References:

1. Comm. Leiden, Suppl., Nr. 50 b, 1924.

2. Naturwiss, 21, 787; 1933.