Critical Experiments for Metrology and the International System of Units

The proposed redefinition of the international system of units (SI) will necessitate determining and fixing values for the Planck, Boltzmann and Avogadro constants, and the elementary charge, and linking the SI units to the defined values. A similar approach was taken 1983 when the metre was redefined in terms of the velocity of light in vacuum.

 The major challenge in moving forward concerns the state of knowledge of Planck’s constant (h). At present this is an experimentally determined quantity and great effort is being expended to reduce the associated measurement uncertainty and provide redundancy. Several National Metrology Institutes, especially those of Canada and the United States, are pursuing a route via a “watt balance” which exploits quantum electrical measurement standards and the equivalence between mechanical and electromagnetic forces to link h to the unit of mass. After redefinition these balances may be operated to provide an electrical method for precision realization of the mass unit.

 In a very different but equally challenging experiment an international collaboration of laboratories has been pursuing the determination of h via a precise measurement of the crystal density of silicon. Most recently this effort has been focused on single crystal spheres of specially grown very highly enriched ²⁸Si, which allow the necessary precision in our knowledge of the silicon isotope ratios.

 Each of these experiments is a tour de force of experimental technique, bringing together the simultaneous exploitation of measurements at the highest possible precision in several different areas. I will describe the two experiments, and their associated challenges, and I will outline the present situation regarding the output of each, and our present state of knowledge of the Planck constant.