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Re-definition of the Standard of Mass

Recent measurements on the International Prototype Kilogram have re-emphasised the need to have the SI unit of mass defined in terms of invariants of nature. This has led to an increased interest in current methods of re-defining the kilogram, ranging from a definition in terms of Avogadro's Constant through to a definition in terms of the Quantum Hall Resistance and Josephson Volt. We are investigating methods of defining the kilogram in terms of the electrical units through a comparison of electrical and mechanical work.

Stability of Mass Artefacts following Cleaning of Surfaces

Recently the mass of the International Prototype Kilogram (the international standard of mass) was re-defined in terms of its mass immediately following cleaning, in order to compensate for the steady change in mass with time due to surface adsorption of material. We are investigating methods of cleaning stainless steel artefacts with the aim of finding a method which gives the best mass stability.

Improved Methods of Mass Comparison

Establishing a scale of mass based on a 1 kg standard involves comparisons of masses ranging from 1 mg to 20 kg. Methods of mass comparison are quite sophisticated to account for comparator drift but also need to be efficient because of the large number of comparisons normally involved. We have developed the "circular weighing method" for efficient and practical comparison of up to 5 weights at a time.

Methods of Analysis of Mass Comparison

In recent years mass metrologists have come to realise the elegance and ease of analysing mass comparisons by matrix methods. We are working on extending the scope of matrix analysis by incorporating a proper calculation of effective degrees of freedom.

Automatic Weighing Systems

An automatic weighing system which can be adapted to most electronic analytical balances has been developed and is currently being adapted for possible use in our routine calibration work. Such a system will lead to new possibilities for efficient weighing methods.