Second Sound inHe3-He4Mixtures below 1°K

An investigation has been made of second sound in mixtures of ${\mathrm{He}}^3$-${\mathrm{He}}^4$ in the temperature region obtained by demagnetization of a paramagnetic salt. Several concentrations of ${\mathrm{He}}^3$ were used ranging between 0.0017 percent and 4.3 percent. The velocity of second sound for the mixtures was found to reach a maximum value of about 35 meters/sec just below 1°K and to approach 20 meters/sec near 0.2°K. In addition, essentially no dispersion of second-sound pulses in mixture below 1°K was observed. This is in marked contrast to heat pulses in pure ${\mathrm{He}}^4$ in this temperature region. Our results show general agreement with the theoretical predictions of Pomeranchuk, provided the effective mass $\mu$ of the ${\mathrm{He}}^3$ atom is allowed to vary from 2.0 ${\mathrm{m}}_3$ at 0.2°K to 3.6 ${\mathrm{m}}_3$ at 1.8°K.