The liquefaction of helium by an adiabatic method

Abstract

The liquefaction of helium by Kammerlingh Onnes has led in the past thirty years to discoveries of the greatest importance to the study of the solid state. In spite of this, very few laboratories are now equipped with the apparatus necessary for the production of liquid helium. It is therefore very desirable that the complicated technique necessary for its production should be simplified to allow of its more extensive use. In this paper we shall describe a more efficient liquefier, based on an adiabatic principle, which we hope will considerably simplify the production of liquid helium for scientific work. At present two principal methods are used for the cooling and liquefying of gases. The first method is based on cooling produced by adiabatic expansion where the expanding gas is cooled by doing external work. This phenomenon was observed by Clèment and Desormes in 1819 when they discovered the cooling of a gas in a container when its pressure was reduced by letting out some of the gas through a tap. It can be shown that on expanding, the gas remaining in the container has done work in communicating kinetic energy to the escaped gas, and therefore has been cooled adiabatically. Olszewski in 1895 applied this method to the liquefaction of hydrogen; he compressed the gas to 190 atmospheres and pre-cooled it with liquid oxygen boiling at reduced pressure (-211°C); on releasing the pressure, he observed a fog of liquid hydrogen drops. From this experiment he was able to determine the critical data for hydrogen. This method has also been used recently by Simon for liquefying helium. Simon took advantage of the fact that at very low temperatures the thermal capacity of the container is so small that it practically absorbs no cold from the liquefied helium. The limitations of this method are that it can only conveniently be applied for obtaining small amounts of liquid helium; it is not suited for a continuous output of helium, and also there is necessarily a loss of cold due to the gas which leaves the container. The method is also complicated by the fact that high pressures are required, and that pre-cooling with liquid hydrogen boiling at reduced pressure is necessary.