Entropy of He II from 1.6 K to theλline

The fountain pressure of He II was measured and the results were used to obtain the entropy from 1.6 K to near $T_{\lambda }$ at vapor pressure and from 1.7 K to near $T_{\lambda }$ at various pressures up to 29 bar. The data have a precision of ±0.1% and an accuracy of ±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 $\Delta S=\int {T_0}^{T_{\lambda }}(\frac{C_p}{T)}\mathrm{dT}$ was added to our measurements near $T_{\lambda }$ to yield values for $S_{\lambda }\mathrm{}\left(P\right)\mathrm{}$ which were fitted to an empirical equation with a scatter of ±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.