Experiments at very low temperatures obtained by the magnetic method II—New supraconductors

Abstract

In a previous paper (which we shall refer to as I) we discussed the experimental technique of obtaining very low temperatures by the magnetic method and gave some theoretical considerations connected with it. We showed also that the entropy changes involved in demagnetizing the paramagnetic salts used for this method are of such an order that it should be easily possible to cool appreciable amounts of additional sub­stances together with them. This paper deals with experiments of this kind. The chief point in such experiments is the thermal contact between the cooling salt and the substance to be cooled. The ordinary way of making such a contact would be by means of a gas (which has the advantage that the contact can easily be broken again); but that is, of course, out of the question in this temperature region. A very effective way of making the heat contact would be to use liquid helium, letting the liquid make contact between the salt and the additional substance, and this contact could be broken by moving one or both of the substances with respect to the surface of the liquid. Having done this there would be no contact at all, as the vapour pressures at the temperatures concerned are so small that no heat can be transmitted through the gaseous phase of the helium. The cooling down of the helium would not appreciably reduce the efficiency of the cooling, as can be seen from the following data. The entropy change involved in bringing 1 cu cm of iron ammonium alum into a field of 10,000 gauss at 1° is 5·10 -3 cal/degree. The total entropy of 1 cu cm of liquid helium at 1°, on the other hand, amounts to 1·10 -3 cal/degree. Thus it would easily be possible to use even equal volumes of salt and liquid helium. This way of making heat contact seems very promising; but of course it complicates the apparatus to a certain extent, and so, in the present stage of experimental technique, we made use of a simpler arrangement which had not all the advantages of the other but was sufficient for our present purposes. We used direct mechanical contact.