Tait Coefficients and λ Transition of Helium I and II

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Abstract
Helium I and helium II are both found to obey Tait's law. The constant J is constant, within experimental error, with temperature for He I making this substance a fluid of the first kind. Helium II is a fluid of the second kind inasmuch as J is constant along an isotherm, but J varies with the temperature. The isotherms which cross the λ transition can be fitted with two straight lines, one for the He I region and one for the He II region. Plots of J, L, and J/L vs temperature are given. While the behavior of the J and L curves is abnormal, apparently the abnormalities are parallel in both constants, since the J/L curve is much more regular. The number average degree of association, the number of particles, and the volume of holes is calculated along the 2.00°K isotherm. There is a sudden jump of these quantities at the λ point. In going from He I to He II the degree of association increases; the number of particles and the volume of holes decreases. Apparently the structural change that occurs is an inversion. In He I the structure is that of a normal liquid like water, where it consists of larger particles joined by defects consisting of holes and smaller particles. At the λ point due to the lowering of the pressure the defect ``continuum'' becomes tenuous due to the increase in the volume of holes, and the bonds suddenly break. The skeleton of the structure of He II then becomes one of the large particles forming a loose network with the remains of the small particles which formerly formed the ``continuum'' occupying the free space in the network. These small particles are then the superfluid component of He II.

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