Present status of the different approaches to the re-definition of the kilogram.

The unit of mass, the kilogram, remains the only base unit in the International System of Units which is still defined in terms of a physical artefact. The scope of the work reported here is to describe and review the progress of the three main approaches concerned with providing a re-definition of the kilogram. Following on, areas of research in which NPL' s Mass and Density Standards group can make a significant contribution to the possible future re-defmition of the unit of mass are recommended. The three approaches are the Avogadro project, the Watt balance and the Ion counting method. The aim of both the Avogadro and Ion counting approaches is to relate the kilogram to an atomic mass: the Avogadro project through measurements made on artefacts manufactured from silicon single crystals; the Ion counting method through the accumulation of gold atoms. The Watt balance approach relates the kilogram to electrical units by equating mechanical and electrical power. The main recommendation of this report is that NPL should participate in the Avogadro project through the measurement of the mass of silicon artefacts. This will require the characterisation of the surface properties of these artefacts and its effect on the mass stability in both ambient and vacuum environments. Furthermore, the techniques developed to weigh these artefacts in vacuum could be used in support of the Watt balance project.