Fixed points on the scale of high pressures: The freezing pressure of mercury at 0 C

There is now an increasing tendency to use fixed points, identified by phase changes or polymorphic transitions of pure substances, as a basis for agreement on a practical scale of high pressures, on rather similar lines to the use of fixed points for the definition of the International Practical Scale of Temperature. It was first proposed by Bridgman that the pressures corresponding to the liquid-solid equilibrium of pure mercury at various temperatures offer convenient fixed points in the pressure range up to the order of 20 kb (2000 MN m -2 ). The present paper, which is one of a series dealing with investigations of fixed points at the National Physical Laboratory, gives the results of a determination of the freezing pressure of mercury at 0°C, measured directly in terms of the Laboratory's pressure balance standards. The value obtained is 756·92 MN m -2 (7569·2 bar, 7718·5 kgf cm -2 ) with estimated limits of accuracy to ± 0·12 MN m -2 . Comparison with the results of other investigations and possible sources of error are discussed.