The BIPM Watt Balance: Improvements and Developments

This paper summarizes the various improvements to and new developments for the Bureau International des Poids et Mesures (BIPM) watt balance, which have been implemented over the past two years. The main distinctive feature of the BIPM apparatus is its capability for simultaneous force and velocity measurements. Our first campaign of measurements in air at atmospheric pressure has been carried out with simultaneous force-velocity measurements. The results obtained among 11 series showed a relative reproducibility of the Planck constant h of 5.0 × 10^-6. The relative difference between our Planck constant result and the CODATA 2006 recommended value is -4.8 × 10^-6, with a relative uncertainty of 4.8 × 10^-5.