The kilogram

The unit of mass, the kilogram, is the last of the seven base units of the International System of Units (SI) to be defined in terms of a material artefact rather than by relation to an invariant of nature. Progress is being made towards a redefinition of the kilogram in terms of the Planck constant (h), realised via the watt balance and Avogadro experiments. The International Committee for Weights and Measures (CIPM) has targeted the 2014 meeting of the General Conference on Weights and Measures (CGPM) for the ratification of the redefinition and in order for this to be feasible, a practical means of linking the new definition to the current mass scale must be in place. The work outlined in this JRP will develop the procedures necessary to provide this link and to ensure the continuity of the worldwide mass scale. Without this work the uncertainty with which the Planck constant can be fixed will be compromised and furthermore the practical implementation of a redefined kilogram will not be possible.

A practical means of traceability

The development of a practical means of traceability between a new definition of the kilogram and the (current) mass scale is necessary both for the initial fixing of a value of the Planck constant with relation to current scale (the International Prototype Kilogram) in order to redefine the mass unit and for the dissemination of the unit following redefinition with reference to the fundamental realisation (in vacuum). Both the watt balance and Avogadro experiments are designed to realise the kilogram under vacuum conditions. The current definition of the unit is realised via the International Prototype Kilogram (IPK), which is kept and used in air. In order to set the value of the Planck constant with relation to the current mass scale a means of linking the mass of the IPK in air with a mass in vacuum must be realised. Once the Planck constant is fixed and the kilogram is redefined the unit of mass will be realised in vacuum via the watt balance and Avogadro experiments. It is therefore also necessary to consider traceability from the realisation in vacuum to the mass scale in air. Both the fixing of h and the subsequent dissemination of the redefined unit must be achieved at the lowest possible level of uncertainty to maximise the benefit of the redefinition to the mass user community.

Joint Research Project

This Joint Research Project (JRP) will address both these requirements for the implementation of the redefinition of the kilogram by developing artefacts and protocols which will allow traceability to a vacuum realisation while minimising the uncertainty in the redefinition (the value of the Planck constant) and in the subsequent dissemination of the unit.