Since 2005 the BIPM has been developing a watt balance as a means for the practical realization of the expected new definition of the kilogram in terms of the Planck constant. The BIPM watt balance operates using a 0.5 T radial magnetic field, a 250 mm diameter and 1160 turn induction coil, a flowing current of ±1 mA and a test mass of 100 g. Coil velocity is about 0.2 mm s1 with typical excursion of 20 mm.
An improved version of the apparatus is fully assembled and operational in air. The relative uncertainty on the Planck constant determination is currently 3 x 106. The uncertainty is dominated by the residual misalignment of the electromagnetic force due to the limited mechanical stability of the suspension. Measurement in vacuum using a refined suspension is planned in spring 2017 with the objective to reduce the uncertainty to 1 x 107.
Reference: H. Fang, F. Bielsa, A. Kiss, T. Lavergne, Y. F. Lu, L. Robertsson, E. de Mirandés, S. Solve and M. Stock, Progress on the BIPM watt balance, Proc. CPEM 2016, 2016.