Ultrasonic Measurements Near the Critical Point ofHe4

Low-frequency sound-velocity measurements in the range 1.5-50 kHz have been made along several isochores near the critical point of ${\mathrm{He}}^4$ using a resonance technique. The adiabatic compressibility along the critical isochore diverges as ${(T-T_c)}^{-\alpha }$ with $\alpha$ larger than zero. The data are consistent with $\alpha \simeq \frac{1}{8}$. The temperature dependence of the measured dispersion agrees with a recent calculation by Kawasaki.