Second sound has been investigated in ${\mathrm{He}}^3$-${\mathrm{He}}^4$ mixtures for ${\mathrm{He}}^3$ concentrations between 0.09 percent and 0.80 percent in the temperature range between 1.24°K and 1.95°K. The velocity of second sound was found to increase markedly with increasing concentration; e.g., at 1.26°K, the velocity with the 0.80 percent concentration was 26.52 meters/second as compared to 19.08 meters/second for pure ${\mathrm{He}}^4$. The results of these experiments are compared with the theoretical predictions of Pomeranchuk.

• Received 31 July 1950

DOI:https://doi.org/10.1103/PhysRev.80.1043