The Liquid-Solid Transformation in Helium near Absolute Zero

Abstract

Although helium is normally a liquid to absolute zero, it can be solidified by the application of moderately high pressures. This transformation is of particular interest, since by the third law of thermodynamics the entropy difference between the liquid and the solid must disappear as the absolute zero is approached. This can most easily be shown by the disappearance of the heat of melting. Measurements from 1.0° to 1.8° of the slope of the melting pressure curve and the change in the molar volume of helium on melting are described, from which the heat of melting and the change in internal energy on melting were calculated. These results verify the predictions made by the third law. The measurements are also combined with those due to earlier workers to show the discontinuities in the various thermodynamic functions at the intersection of the $\lambda$-point-pressure curve with the melting pressure curve.