COMPRESSIBILITY OF LIQUID He4

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Abstract
The refractive index n, density ρ, and isothermal compressibility kT = (1/ρ) × (∂ρ/∂P)T of liquid He4 have been measured along isotherms between the saturated vapor pressure and 4.5 atmospheres at 3.0°, 3.5°, 4.0°, 4.5°, and 5.0° K. The liquid was compressed in an optical cell 9.58 cm long in a Jamin interferometer. Changes in density were inferred through the Lorenz–Lorentz equation from refractive index changes measured with a photomultiplier fringe recorder. One fringe corresponds to a change in refractive index of (5.699 ± 0.003) × 10−6 in this apparatus. Densities range between 0.0995 g cm−3 at 5.0° K at the saturated vapor pressure and 0.1501 g cm−3 at 3.0° K and 4.5 atmospheres pressure. Compressibilities range between 65 × 10−8 cm2 dyne−1 at 5.0° K at the saturated vapor pressure and 1.16 × 10−8 cm2 dyne−1 at 3.0° K and 4.5 atmospheres pressure. The limiting liquid structure factors for zero-angle X-ray scattering and for coherent scattering of slow neutrons have been calculated from these density and compressibility measurements. In addition, the ratio of heat capacities has been calculated at 3.0°, 3.5°, and 4.0° K where other measurements of the velocity of first sound are available.

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