Entropy of He II from 1.6 K to theλline

Abstract

The fountain pressure of He II was measured and the results were used to obtain the entropy from 1.6 K to near ${T}_{\ensuremath{\lambda}}$ at vapor pressure and from 1.7 K to near ${T}_{\ensuremath{\lambda}}$ at various pressures up to 29 bar. The data have a precision of \ifmmode\pm\else\textpm\fi{}0.1% and an accuracy of \ifmmode\pm\else\textpm\fi{}0.5%. There is excellent agreement with recent calorimetric measurements at vapor pressure and good agreement within the combined error estimates with some, but not all, previous fountain-pressure measurements. The small quantity $\ensuremath{\Delta}S=\ensuremath{\int}{{T}_{0}}^{{T}_{\ensuremath{\lambda}}}(\frac{{C}_{p}}{T)}\mathrm{dT}$ was added to our measurements near ${T}_{\ensuremath{\lambda}}$ to yield values for ${S}_{\ensuremath{\lambda}}(P)$ which were fitted to an empirical equation with a scatter of \ifmmode\pm\else\textpm\fi{}0.1%. The increased accuracy of our new measurements will reduce considerably the experimental errors in the determination of the superfluid fraction from the second-sound velocity near the superfluid transition line.