Measurements of the Velocity of Sound inHe3andHe4Gas at Low Temperatures with Implications for the Temperature Scale

Abstract

Measurements of the velocity of sound in ${\mathrm{He}}^3$ and ${\mathrm{He}}^4$ gas have been made at constant temperature as a function of pressure in the temperature range 1.2 to 3.8°K. The measurements, made using a fixed-frequency, variable-length resonant technique, have an uncertainty of approximately 0.07%. The results have been analyzed in terms of an expansion of the velocity of sound in powers of the pressure to determine (i) the absolute temperature of the experimental system and (ii) the second virial coefficient of the gases in this temperature range. The calculations indicate that the $T_{58}$ ${\mathrm{He}}^4$ temperature scale may be 3 to 6 mdeg K lower than the absolute temperature scale. The results of the second-virial-coefficient calculations agree well with the best previous determinations taken from gas isotherm measurements.