We present a method to analyse international measurement comparisons, including linking two or more comparisons. This method, which is based on linear generalized least-squares estimation, directly gives the degrees of equivalence required by the Mutual Recognition Arrangement, both relative to the key comparison reference value (KCRV) and between pairs of participating laboratories. The key features of the method are its ability to accommodate multiple travelling standards (including instabilities in the standards), multiple measurements by participating laboratories, multiple comparison loops, and known correlations between measurements (both intra-and interlaboratory). The method can be applied with or without a KCRV (but does not give a value for the KCRV). An analysis of the linked comparisons APMP.M.M-Kl and CCM.M-Kl is presented to illustrate the method. While best suited to comparisons with single-valued travelling standards, the method can be applied to most other comparison types.

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A watt balance relates mechanical power to electrical power by comparing the gravitational force on a mass with the electromagnetic force on a current-carrying coil in a magnetic field. It is one of the favoured approaches to replacing the present artefact kilogram as it will allow the kilogram to be redefined in terms of the Planck constant. In this paper, we propose an oscillatory approach to the dynamic or moving mode of a watt balance experiment as an alternative to the traditional constant velocity approach. Features of an oscillatory dynamic mode include frequency selection and Fourier analysis to reduce noise due to vibration, the option of larger coil velocities (above 2 mm s−1) and smaller coil movement amplitudes (of order ±1 mm) than the constant velocity approach, and direct measurement at the weighing position of the factor γ relating coil current to the gravitational force on the mass. An oscillatory dynamic mode can be used with some of the existing watt balance experiments with the potential to measure γ faster and more precisely than the constant velocity approach.

A novel design for the magnet system of a watt balance is presented. While primarily intended for the watt balance being developed by MSL, this design could be used for other watt balances. It has a radial magnetic field in an annular gap that is screened from external fields. The magnetic field is generated by a single ring-shaped permanent magnet and the geometry is such that the field induced in the permanent magnet by the current in the coil in the weighing mode is zero on average. Hence the magnetic field in the gap is expected to be the same for both the weighing and dynamic modes of a watt balance. From finite element modelling, the magnetic field strength can be uniform to within 40 parts in 10 6 over more than 50 mm (50% of the gap height) and the usable working range for the coil is about 40 mm. Further, the diameter of the annular gap can be relatively small, allowing the coil and its support structure to be kept relatively stiff and light.

The new definition of the kilogram, which is expected to be adopted by the General Conference on Weights and Measures in 2018, will bring some major changes to mass metrology. The most fundamental change will be the replacement of the present artefactbased definition with a universal definition, enabling in principle any National Metrology Institute (NMI) to realize the kilogram. The principles for the realization and dissemination of the kilogram in the revised SI are described in the mise en pratique of the definition of the kilogram. This paper provides some additional information and explains how traceability can be obtained by NMIs that do not operate a primary experiment to realize the definition of the kilogram.