Melting pressure thermometry

FIGURE 19 (a) The entropy Sof liquid and solid He (upper diagram) measured in units of the gas constant R, plotted as functions of temperature T and the melting pressure Pm of He as a function of T. It may be noted that the minimum in the melting curve occurs at the temperature where the difference between the solid and liquid entropies changes sign. [From values tabulated by Betts, D. S. (1976). Refrigeration and Thermometry Below One Kelvin, Sussex Univ. Press, London.]... 

V = vapor pressure, G = gas thermometry, T = triple point (gas, liquid, and solid in equilibrium), MIF = melting/freezing at 1 atm. 

The metals themselves have remarkable melting points and boiling points. The extreme example, gallium, is liquid at ordinary pressure over a range of two thousand degrees and has been employed for high-temperature (1000 ) thermometry, in a quartz envelope. It has a strong tendency to superfusion and will remain liquid at room temperature for a considerable period. 

Since at the incongruent melting-point (transition point) of Glauber s salt, there are three phases in equilibrium, the system will, under constant pressure (say atmospheric) be invariant. The temperature, therefore, will be perfectly definite. On this account the proposal has been made to adopt this as a fixed point in thermometry. The temperature is, as we have seen, affected only comparatively slightly by change of pressure. 

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