Velocity of Sound at 1 MHz Near theHe4Critical Point

Abstract

The velocity of 1-MHz sound waves has been measured in ${\mathrm{He}}^4$ along several isotherms just above the critical temperature. The results indicate that along the critical isotherm, the quantity $\frac{\rho }{u^2}$ diverges approximately logarithmically as a function of $|P-P_c|$. Such a singularity at the critical point is to be expected in view of the corresponding singularity in $C_v$. Additional velocity measurements were made in the saturated liquid, along isotherms several tenths of a degree above $T_c$, and along the 4.5 and 5.0°K isotherms in the liquid. The specific-heat ratio has been calculated along the latter two isotherms and along the critical isotherm.