Experiments on iron ammonium alum in alternating fields at very low temperatures

It has been pointed out by Simon (1935a ) that calorimetry in the region below 1° K. can be made sensitive enough to measure accurately the thermal energy changes due to the emission and absorption of radioactive radiations. With a view to carrying out investigations of this kind, experiments have been in progress in the Clarendon Laboratory to improve the sensitivity of the method of temperature measurement. Temperatures below 1° K. are determined primarily by extrapolating Curie's law, verified at higher temperatures, which is that the susceptibility of certain paramagnetic substances is inversely proportional to their temperature. Generally the thermometric substance is the actual paramagnetic salt used to obtain the low temperature. There are various methods of measuring the susceptibility; for instance, the substance may be surrounded by two coils whose mutual inductance is measured at the various temperatures. For this purpose A. C. methods of measuring inductance have certain advantages. In particular a null method may be used by balancing with an external adjustable negative mutual inductance, and headphones can be used as the detecting instrument if the frequency lies in the audible ranges. There is also the possibility of compensating the effect of the unavoidable heating of the substance, the normal drift, by some form of automatically increasing mutual inductance.