The new Watt Balance Experiment at METAS

The kilogram is the last unit of the international system (SI) still based on a material artifact, the international prototype of the kilogram (IPK). The comparisons made in the last hundred years have clearly revealed a long term relative drift between the IPK and a set of copies kept under similar conditions. Since the long term stability is one of the major conditions set on the SI base units, this situation is no longer satisfactory and a new definition of the mass unit becomes a priority for the metrology community. A promising route towards a new definition based on fundamental constants is given by the watt balance experiment which links the mass unit to the Plank constant h. Today, the kilogram is the last unit of the International System of Units (SI) still based on an artifact, the international prototype of the kilogram (IPK), kept at the Bureau International des Poids et Mesures (BIPM). The IPK prototype that has been machined in 1878, is a cylinder of platinum-iridium alloy (Pt 90% - Ir 10% in mass) whose height (39 mm) is equal to its diameter. Six copies were designated as official copies and are kept in the same conditions as the international prototype. At that time, about seventeen other copies were given to the member states of the meter convention to materialize their national prototype. Since then, other countries have joined the meter convention and new national prototypes have been machined and added to the existing set of international prototype copies. To survey the evolution of the different copies relative to IPK three comparisons have been organized since 1880. The results of these comparisons have clearly put in evidence, a relative drift of this set of masses with respect to IPK. The mean drift of the official copies has been evaluated to 0.5 µg/year. Nevertheless, it is clear that the actual definition of mass does not allow attributing the drift to IPK or to the copies. Moreover, variations of the mass unit directly reflect on the ampere definition and therefore on the whole set of electri- cal units. Since it is now possible to compare two mass standards made out of the same material with an uncertainty of about 1 µg, the instability among the international mass prototypes - including IPK - is a major contribution to the final uncertainty. Clearly, such a situation is no longer satisfactory for one of the base units of the SI. There is now a general consensus in the metrology community that the time for a redefinition of the kilogram has come. To get a better stability, the new definition should be linked to a physical fundamental constant with a relative uncertainty in the order of 10 -8 . During the last decades, several