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The mercury melting line has been determined for pressures up to 1200 MPa. The change of electrical resistance in the mercury sample was used for detecting the equilibrium between the solid and liquid phases. Pressure measurements were made with highly stable manganin gages calibrated against two controlled clearance piston gages. Temperature measurements were made in the constant temperature bath by means of platinum resistance thermometry. Systematic errors in pressure and temperature were evaluated for all the measurements as well as the scatter due to the resolution of the equilibrium determination between the two hases of mercury. The mercury melting point at 0 C is 756.84 k 0.16 MPa which is in close agreement with the value obtained by Dadson and Greig. The experimental results are compared with previous melting lines. There are systematic differences when compared to Bogdanov's equation up to 1200 MPa but there is very close agreement with recent data obtained by Houck and Morris over the pressure range they covered. The experimental data were fitted to a third order polynomial; this equation fits the melting line data much more closely than the Simon type heretofore recommended and can be used up to 1200 MPa to increase the accuracy of a practical pressure scale based on the melting line of mercury.

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Rotational dissipation in a low-pressure pneumatic dead weight piston gauge has been measured for four gases: He, H, , N, and SF,. Significant differences in the rotational dissipation were observed between the four gas species. Even larger differences were observed between two operational modes (gauge and absolute). The measured results are interpreted by a model for the rotational dissipation due to the gas in the annular region between the piston and cylinder. Good agreement was found between the measured and modelled results for all four gas species with essentially no adjustable parameters.

NIST pressure calibration services with nitrogen are now based on two transfer standard piston gages for which the effective areas have been determined by calibration with the manometer developed at NIST for gas thermometry. Root-sum-squared three sigma uncertainties for the areas for the two gages are 3.05 ppm and 4.18 ppm.