The Joule-Thomson Effect in Helium

Important changes in the apparatus previously used in the measurement of the porous plug coefficient, $\mu$, for air are described in some detail. The effect of air as an impurity in helium has been investigated by direct experiment and shown to be quite negligible even at 10 percent of air but increasing rapidly thereafter with increasing proportion of air. It appears that an experimental study of $\mu$ in mixtures of ${\mathrm{N}}_2$ and He is desirable. The helium was purified by contact with activated charcoal at liquid air temperatures. $\mu$ for helium has been measured from -190 to 300°C and from 1 to 200 atm. In this range $\mu$ is negative, is independent of pressure except at the lowest temperatures, and is small both in magnitude and in variation. Significant trends of $\mu$ at the two temperature extremes have been observed. Applications to theory and to other physical properties will follow in later articles. Reasons are given for supposing that helium will not approximate with high accuracy to a perfect gas at any temperature.