Production of High Magnetic Fields at Low Temperatures

K. Mendelssohn

Editor’s Note

Kurt Mendelssohn here reports an important experimental advance in the generation of powerful magnetic fields at low temperatures. Using superconducting alloys able to bear field strengths up to 22,000 gauss at a temperature of just 2 K, he and colleagues had experimented with a transformer-like arrangement of magnetically coupled coils. A magnetic field of 1,000 gauss created in a conducting primary coil was made to induce current in a secondary superconducting coil. With a dense coil solenoid as part of the second coil, the device could produce a field of 22,000 gauss in a small volume. This achievement, Mendelssohn points out, might be used to create extremely low temperatures by the method of adiabatic demagnetization.　　中文

THE use of a supra-conductor (therefore completely free from Joule heating) has been more than once suggested for the production of magnetic fields at low temperatures. The magnetic field obtainable by this means is limited by the magnetic threshold value at which supra-conductivity ceases. Still, considerable fields can be obtained by the use of alloys (investigated in Leiden1) the threshold value of which is 22,000 gauss at 2° K., a strength which is sufficient for many experiments.　　中文

The chief remaining difficulty lies in the heat conductivity of the leads to the supra- conducting coil. This problem of heat conduction through the leads can be eliminated by transferring the necessary energy for the magnetic field by induction. The suggested arrangement is similar in principle to a transformer, the primary circuit of which is normally conducting and the secondary circuit of which is supra-conducting. The primary circuit consists of a D. C. source and the primary of the transformer; the supra-conducting secondary circuit consists of a secondary with a few turns of large radius and a solenoid with many narrow turns for producing the high field. On closing the primary circuit the magnetic energy transferred to the secondary is shared with the solenoid. With this arrangement, one produces, to some extent, condensation of the lines of force. Calculation shows that for given geometrical dimensions there is an optimum ratio for the number of turns in the secondary coil to that in the solenoid. In this way, within the limits of the usual dimensions of an apparatus, it is easily possible with a primary field of about 1,000 gauss to obtain a field of 22,000 gauss in the space of a few cubic centimetres.　　中文

The method should be specially suitable whenever it is desired to produce fairly high magnetic fields at low temperatures in not too large a volume, as, for example, in the production of extremely low temperatures by the adiabatic demagnetisation of paramagnetic substances2. According to the experiments of Kürti and Simon3, Giauque and MacDougall4 and de Haas and his co-workers5, it should be possible, with the above arrangement, to obtain temperatures below 0.1° K. from a starting point of 1° K. Since heat conductivity along the current leads is eliminated, and since heat capacities at helium temperatures are so minute, a few cubic centimetres of liquid helium should suffice to cool the whole arrangement.　　中文

Experiments with an apparatus embodying the above methods are being made in this laboratory.　　中文

(132, 602; 1933)

K. Mendelssohn: Clarendon Laboratory, Oxford. Sept. 28.

References:

1. de Haas, W. J., and Voogd, J., Comm. Leiden, No. 214b (1931).

2. Debye, P., Ann. Phys. (4) 81, 1154 (1926); Giauque,W. F., J. Amer. Chem. Soc., 49, 1864 (1927).

3. Kürti, N., and Simon, F., Naturwiss., 21, 178 (1933).

4. Giauque, W. F., and MacDougall, D. P., Phys. Rev., 44, 235 (1933).

5. de Haas, W. J., Nature, 132, 372 (Sept. 9, 1933).