CXXIX. The specific heat of liquid helium II as a function of pressure

The absorption of sound in liquid helium II

The absorption of sound in liquid helium II has been measured in the temperature range down to 0.4° K, at frequencies of 6.0 and 14.4 Mc, and under pressures of up to 25 atm. The results are compared with the theory of Khalatnikov (1950, 1952). Besides accounting for the existence of a maximum in the absorption-temperature curve, this theory predicts the manner in which the magnitude and position of the maximum varies with frequency, and with pressure. It is shown that these predictions are qualitatively correct, although exact numerical agreement is not obtained. The measurements also confirm the observations of Chase & Herlin (1955) that the absorption at the lower temperatures is greater than that to be expected from Khalatnikov’s original treatment. It seems that the additional absorption may arise from processes associated with the kinetic coefficient of thermal transport (Khalatnikov 1952).

The heat conductivity and viscosity of liquid helium II

Measurements have been made in glass capillary tubes of 52 and 108 u diameter, in the temperature range from 1.15 to 2.15 °K. For sufficiently small temperature gradients the thermal resistance is independent of the heat flow and is caused solely by the viscosity of the normal fluid, n n . This viscosity has been calculated from the measured thermal resistance data with the use of the existing entropy values, and the results from the two tubes are in very good agreement except for a small deviation at low temperatures which is attributed to slip. The viscosity at the saturated vapour pressure, when corrected for slip, agrees closely with the results obtained by the rotating cylinder viscometer but does not agree with the oscillating disk experiments. The variation of n n at low temperatures follows the theory of Landau & Khalatnikov if A/ k is taken as 8.9 °K, but the measured values of the slip correction are slightly larger than would be expected from the theoretical phonon and roton mean free paths. In the 52 u tube the measurements have been extended up to the solidification pressure so as to give n n as a function of density. Near the lambda line n n increases rapidly with density, but at low temperatures the variation is quite small and it is found that the contribution from the rotons is almost independent of density.