NORMAL-FLUID DENSITIES IN LIQUID HELIUM-2 UNDER PRESSURE

Measurements of the normal-fluid fraction n in superfluid helium have been made by observing the period of torsional oscillation of a stack of disks, at pressures up to 25 atm, at temperatures from 1.45 K to the line. At all pressures, the temperature dependence of n is qualitatively similar to the previously observed behavior at the vapor pressure. At all temperatures, n is a monotonically increasing function of pressure. Comparison is made with values calculated by means of the two-fluid model from second-sound velocities and thermo-dynamic data; good agreement is observed, but it is necessary to take into account the fact that a commonly used approximation CpCv is seriously incorrect at high pressures. © 1969 The American Physical Society.