Comparative Measurement of the Density of Mercury

The results of the four absolute determinations of the density of mercury since 1945 are discussed. When reduced to a common temperature scale the values differ by more than the claimed uncertainties. Results are presented of a comparative measurement of the density of two further samples of mercury in terms of a reference sample. The density difference is determined by measuring the change in apparent mass of a tungsten carbide block when fully immersed in each mercury sample. The standard deviation of the measured density difference between the samples and the reference was found to be 4 × 10 -4 kg/m 3 . The values obtained show significant differences in density, and the possible cause, i.e. variation in the isotopic ratios between samples, is discussed. We confirm that the density of mercury must be measured if it is required to be known to better than a few parts in 10 6 .