Entropy of He II from 1.6 K to the lambda line

The fountain pressure of He II was measured and the results were used to obtain the entropy from 1.6 K to near T/sub lambda/ at vapor pressure and from 1.7 K to near T/sub 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 ΔS = ∫/sub T/0/sup T//sub lambda/ (C/sub p//T)dT was added to our measurements near T/sub lambda/ to yield values for S/sub lambda/(P) 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