Low Temperature Measurements of theHe3andHe4Melting Curves

The melting curve of ${\mathrm{He}}^3$ has been measured to 12 m°K in order to see whether the exchange interaction has caused the solid entropy to fall below its completely disordered value of $R\ln 2$. For $12{\mathrm{m}}^{\circ }\mathrm{K}<~T<~600{\mathrm{m}}^{\circ }\mathrm{K}$, the melting curve data are described by $P=29.\mathrm{}107+0.25537{\mathrm{}\left(t\right)\mathrm{}}^2-0.\mathrm{}05700{\mathrm{}\left(t\right)\mathrm{}}^3+0.\mathrm{}01625{\mathrm{}\left(t\right)\mathrm{}}^4$ atm, where $100t=T-299.028$ and $T$ is the absolute temperature in m°K. The data points fit this equation with an rms deviation of 0.06 atm. Using this information and previous results for the liquid and solid molar volumes and the liquid entropy, the Clausius-Clapeyron relation shows that $S_{\mathrm{solid}}=R\ln 2\pm 16\%$ for $12{\mathrm{m}}^{\circ }\mathrm{K}<~T<~320{\mathrm{m}}^{\circ }\mathrm{K}$. This places an upper limit of 3 m°K on the magnitude of the exchange interaction in solid ${\mathrm{He}}^3$ for molar volumes along the melting curve. The ${\mathrm{He}}^4$ melting curve was measured between 12 m°K and 300 m°K and found to have a slope of 0±0.007 atm/°K.