Adiabatic Magnetization of Super-conductors

THE past few years have seen a large increase in the number of laboratories using liquid helium, and with it interest in methods for obtaining temperatures below 1° K. has been renewed. The only method which has so far been used is the adiabatic demagnetization of a paramagnetic salt; but this is not the only process known by which a cooling can be effected. Two other low-temperature phenomena yield an appreciable degree of cooling and have a satisfactory velocity of reaction even below 1° K., namely, the mechano-caloric effect in liquid helium 1 and the magneto-caloric effect in superconductors 2 . For reasons which have recently been discussed by Simon 3 , we have never contemplated using the first effect as a cooling method; but the second one offers interesting possibilities. Some experiments were, in fact, carried out fifteen years ago 4 in which final temperatures were reached which had been estimated as between 0.2° and 0.3° K. However, lack of data on magnetic threshold values at low temperatures made an accurate determination of the degree of cooling impossible at the time.