The dielectric constant in liquid and solid⁴He

We report precision measurements of the dielectric constant ε of liquid and solid⁴He at constant density as a function of temperature in the range 0.1<T<1.2 K. For these measurements a capacitance bridge was used, the stability of which permitted a reproducibility of δε / ε ≈ 6×10^-9 in the dielectric constant. We find that the polarizability p, defined by the Clausius-Mosotti equation, is a weak function of temperature. This dependence on T at the low-temperature end diminishes as the density of⁴He is decreased to that at saturated vapor pressure. We also describe briefly attempts at measuring the change of ε at the superfluid transition in⁴He and during the melting process, all corrected to conditions of constant volume, and upper limits for these changes are given. Finally, we present measurements of ε in liquid⁴He at saturated vapor pressure between 0.15 and 1.3 K, from which we deduce the change in density. The results are compared with recent data obtained with a microwave cavity method and good agreement is found. Further comparisons are made with Van Degrift's results at 4 MHz and also with other recent data above 0.7 K.