Metallurgical characterization of materials for the reference mass standard of the French watt balance experiment

Abstract

Watt balance experiments are intended to link the mass unit to the Planck constant. Therefore, in these experiments, a reference mass standard is necessary. However, in the French apparatus, the platinum–iridium alloy that forms the international prototype of the kilogram cannot be considered because of its magnetic susceptibility. Consequently, new materials were investigated, according to stringent specifications. This led to the selection of alloys based on precious metals. The stability of the materials used in mass metrology is strongly dependent on bulk homogeneity and cleanliness: after the investigation of different production modes and heating treatments, microstructural and analytical studies were performed. The critical properties of the different materials were examined and discussed in comparison with the platinum–iridium alloy.