The second-sound velocity of three dilute ${}_{}{}^3{}_{}{}^{}\mathrm{He}$-${}_{}{}^4{}_{}{}^{}\mathrm{He}$ mixtures was measured with high precision down to a temperature of 10 mK and in a magnetic field of 93 kOe. For the most dilute sample, which had a molar concentration of 0.001, the maximum spin polarization of the ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ atoms was estimated to be 36%. The measured relative change in the secondsound velocity was 3%. This is considerably larger than the change predicted recently by Bashkin and Meyerovich. An explanation for this discrepancy is presented.

• Received 2 February 1981

DOI:https://doi.org/10.1103/PhysRevLett.46.1292