Second Sound in<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msup><mml:mrow><mml:mi mathvariant="normal">He</mml:mi></mml:mrow><mml:mrow><mml:mn>3</mml:mn></mml:mrow></mml:msup></mml:mrow></mml:math>-<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msup><mml:mrow><mml:mi mathvariant="normal">He</mml:mi></mml:mrow><mml:mrow><mml:mn>4</mml:mn></mml:mrow></mml:msup></mml:mrow></mml:math>Mixtures

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\ifmmode^\circ\else\textdegree\fi{}K and 1.95\ifmmode^\circ\else\textdegree\fi{}K. The velocity of second sound was found to increase markedly with increasing concentration; e.g., at 1.26\ifmmode^\circ\else\textdegree\fi{}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.

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