Latest results

Since 2005 the BIPM has been working on a room-temperature experiment to demonstrate the feasibility of simultaneous force and velocity measurements. 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 grams. Coil velocity is about 0.2 mm s^-1 with typical excursion of 20 mm.

In early 2010 the BIPM watt balance measured, for the first time, the Planck constant h in air at atmospheric pressure. It demonstrated at a level of parts in 10⁵, that it is possible to realize the two-phase principle originally proposed by B.P. Kibble in 1975 using only one phase by measuring all quantities at once. The reproducibility of the Planck constant measurement, at the present time, is 5.6 × 10^-6. The relative difference between this result and the recommended CODATA 2006 value is -4.8 × 10^-6. The total relative uncertainty of h is currently 4.9 × 10^-5.

Reference: Picard A., Bradley M.P., Fang H., Kiss A., de Mirandés E., Parker B., Solve S., Stock M., The BIPM watt balance: improvements and developments, Proc. 2010 Conference on Precision Electromagnetic Measurements (CPEM), 2011, 66-67.