III. Investigations on paramagnetism at low temperatures. — Part II

Abstract

In a series of papers the author, partly in collaboration with the late Prof. H. Kamerlingh Onnes, has recently published the results of some investigations on paramagnetism at low temperatures. Included in this work were the measurements of the principal susceptibilities of two crystals (cobalt ammonium sulphate and nickel sulphate) at temperatures ranging from about 300° K. (atmospheric temperature) down to the lowest temperature obtainable with liquid hydrogen, 14° K. These data, with the exception of those of Foëx for siderose, are the only ones yet obtained for the principal susceptibilities of paramagnetic crystals at low temperatures over any extended range of temperature. The results showed that, at not too low temperatures, the principal susceptibilities Z 1 , Z 2 , Z 3 follow the law Z n (T + Δ n = C, ( n = I, 2, 3), in which T = absolute temperature, Δ n is a constant, and C is the Curie constant which has the same value for each of the principal magnetic axes of the crystal. The constants Δ n are intimately connected with the structure of the crystal, being a function of the “spacing” of the paramagnetic atoms in the corresponding directions in the crystal. The precise connection between these quantities could not, however, be deduced, firstly, because of the present scantiness of the data, and, secondly, because the accuracy with which the Δ’s could be determined was small. The susceptibilities themselves were determined with an accuracy of about 1 per cent., but, since Δ is only an additive constant, the error in its determination is greater than that of the determination of the susceptibility. Other interesting points were raised by the results, and a continuation of the research seemed likely to give results of considerable theoretical importance and interest. The present work was undertaken in continuation of that just mentioned. It was decided to carry out the measurements over a range of temperature of from atmospheric temperature down to the lowest obtainable with the aid of liquid air, and to aim at reaching an accuracy of one part in a thousand in the measurements. For this purpose the apparatus described in the following pages was designed and constructed. I he desired accuracy was not attained in the first measurements given later in the present paper, the accuracy of these being about 1 per cent. As experience was gained with the apparatus, it was seen that the desired accuracy was probably attainable when special attention was paid to the working conditions, in particular the constancy of the temperature.