Table of contents

A high-temperature calorimeter has been developed for studying the equilibrium melting behaviour of substances used for realizing the thermometric fixed points. Conventional cells of either 5 cm or 2.5 cm diameter can be accommodated. Using the calorimeter, the equilibrium melting curves of three high-purity silver samples were measured. The samples have narrow melting ranges: two of 1.5 mK and one of about 0.5 mK from 10 % liquid sample fraction to the last measured point near 70 % liquid, yet their liquidus points differ by nearly 5 mK. It was found that 1/F plots cannot be used to determine the deviation of the melting point of real silver samples from that of ideally pure silver.

The Welch-Satterthwaite (W-S) formula is a classical, approximate method for combining several sample estimates of normal population variances. However, the W-S formula is routinely applied to the results of physical measurements even though uncertainties may include those of Type B. This paper considers the validity of the W-S formula as a means of obtaining a confidence interval for the measurand. The classical concept of a confidence interval provides performance criteria that can be used to assess the W-S formula by simulation. Examples presented include the combination of two Type A uncertainties, and the combination of one Type A and one Type B uncertainty. The results show that the W-S formula performs very well. This finding contrasts with the findings of other recent studies which did not apply classical performance criteria.

A method is described to determine the volume of a mass standard from weighings made in air of differing density against a reference volume artefact. Standard uncertainties in volume of 2 parts in 10₅ (at 1 kg) or better are readily achievable with this method, using adequate measuring facilities. The advantages of the method are that volume determination and mass calibration may be carried out simultaneously and that the mass standard is subjected to a variation in air density of only ±5 % or less, compared with conventional hydrostatic weighing methods where the mass standard is immersed in water. Examples are presented of the use of the method for confirming values assigned to mass standards and for full volume determination, together with uncertainty calculations.

We have built an atom interferometer that can measure g, the local acceleration due to gravity, with a resolution of Δg/g = 2 × 10⁻⁸ after a single 1.3 s measurement cycle, 3 × 10⁻⁹ after 1 min and 1 × 10⁻¹⁰ after two days of integration time. The difference between our value for g and one obtained by a falling corner-cube optical interferometer is (7 ± 7) × 10⁻⁹g. The atom interferometer uses velocity-selective stimulated Raman transitions and laser-cooled caesium atoms in an atomic fountain. We extend previous methods of analysing the interferometer to include the effects of a gravitational gradient. We also present detailed experimental and theoretical studies of potential systematic errors and noise sources.

A recent paper described the propagation of uncertainty with polynomial or Lagrange interpolation and demonstrated a number of mathematical results that simplify the calculation of uncertainty. This paper extends the analysis to any interpolation that can be expressed as a linear combination of functions. Examples of this form include Gauss, Fourier and linear least-squares interpolations, as well as a wide range of application-specific interpolations employing a combination of functional forms. Application of the results is illustrated by considering a number of the non-Lagrangian interpolation equations of the International Temperature Scale of 1990 (ITS-90). A comparison of Lagrange and least-squares approaches is used to highlight the benefits of the latter.

The fifth in the series of International Comparisons of Absolute Gravimeters (ICAG) was held at the Bureau International des Poids et Measures (BIPM) in November 1997. Fifteen absolute gravimeters participated in the comparison. The mean gravity value obtained at station A (0.9 m) at the BIPM was found to be 980 925 707.8 µGal with a standard uncertainty of 2.8 µGal. This is consistent with the results obtained in previous comparisons at this site. Conclusions based on the analysis of the present results and proposals for future activities are presented.

A recent study identified boron, sodium, aluminium and silicon as the dominant impurities in triple point of water (TPW) cells. These results suggest that borosilicate glass is a less than ideal container as it is the most likely source of the contaminating elements. An estimate for the rate of change of the equilibrium temperature of TPW cells is presented using results recently obtained at the National Research Council of Canada (NRC) and supplemented by those reported in 1982 by Furukawa and Bigge of the US National Institute of Standards and Technology (NIST). An analysis of the two data sets suggests that a decrease of 4 µK per year is appropriate in describing the average behaviour of TPW cells. The conclusion is speculative in that the behaviour of the various cells has not been monitored over a period of time to track the nature of the changes. Rather, the analysis assumes that the changes have occurred continuously and linearly with time.

The feasibility of levitating a mass for generating and/or measuring dynamic force is discussed. Force can be determined by measuring the acceleration of a well-known rigid mass. Under the influence of the Earth's gravity, dynamic force can be accurately generated and/or measured by realizing horizontal linear motion with a sufficiently small external force. A pneumatic linear bearing is suitable for this purpose. The problems of using this method in metrology are analysed and proposals made for their resolution.

International measurements of the molar volume of single-crystal silicon based on fifteen different samples have confirmed the 1986 value of the Avogadro constant recommended by the Committee on Data for Science and Technology (CODATA) but have not confirmed the 1998 CODATA recommended value and a value published recently. The concluding recommendation of this letter, therefore, is not to include the result for the particular sample used to obtain the latter value in any statistical analysis or adjustments of the values of fundamental physical constants.