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 x kg/m3. 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 lo6.
The principles for designing a calibration scheme for a set of mass standards are discussed. On these principles, a particular scheme has been chosen as being convenient and reasonably efficient; it is pointed out that although it has not been possible to decide whether the selected scheme is optimum, it can be said that it is more efficient than most. In an actual calibration based on this scheme, the accuracy decreased from 1.5 in lo7, to which the Materials Research Laboratories 1 kg standard is known, to 1.6 in lo6 for the 1 g mass; such a decrease in accuracy is shown to be a property of all such schemes.
The preparation of narrow single-line absorbers of (GdxSc1-x)H2 and sources of PdEu and SmPd3 for use with 155Gd Mössbauer spectroscopy is described. The methods have been developed for use with the small quantities typical with isotopically enriched materials, and should be of general applicability in rare-earth Mössbauer effect studies.
A new manometer to measure gas pressures between vacuum and one atmosphere has been developed and is capable of easily tracking a continuously changing pressure at rates up to 600 Pa/s. It consists of a U-tube manometer in which the mercury surfaces are the reflectors of a Michelson interferometer. The difference in heights of the mercury surfaces is detected and measured by use of laser interferometry techniques with a He-Ne laser. The manometer is sensitive to changes in pressure of less than 40 mPa, but the ultimate accuracy of 180 mPa at a pressure of 101 kPa (2 parts in 106) is limited by the temperature uniformity and stability (10 mK).
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