A brief review is presented on the ac losses of superconductors in the low-frequency, high-frequency, and weak-field regions. Special emphasis is given to the hysteresis loss which is the main part of the low frequency losses. Existing data of the low-frequency losses are collected to be shown in a single figure and it is shown that most of these can be explained by the theories developed up to now. Descriptions are also given on flux-flow losses which become dominant in the high-frequency region. It is noted that losses exist even in the field lower than Hc1 in type-II materials (or Hc in type-I materials), and possible mechanisms for these losses are suggested. Lastly some remarks are mentioned on the applications of superconductors to the ac uses.
A new method to get reasonable cooling down to below 1°K using paramagnetic substances is discussed. Preliminary experiment gave a promising aspect and encouraged us to develop the method. The idea is essentialy the same as that of Adiabatic Demagnetization. In some special substances, it is possible to increase amount of magnetic entropy by applying external magnetic field. Magnetic cooling can, in such cases, be followed by adiabatic magnetization process which is just contrary to adiabatic demagnetization. One of the substances used in the experiment, is Cu(NO3)2⋅2.5H2O. Starting from Ti=1.6°K in the course of adiabatic magnetization, the lowest Tf=0.5°K was reached at 36kG where the entropy maximum was expected.
Hitachi constructed the largest helium liquefier in Japan in 1968. Its capacity was confirmed 150l/h as liquefier and 500W as refrigerator. The 500 hours continuous operation was concluded successfully early in 1969. The features of the plant are as follows. (1) Turbine inlet pressure is high (14.5kg/cm2g). (2) Turbine circuit pressure is the same as liquefaction circuit pressure.