Progress on the BIPM Watt Balance

Since beginning the construction early in 2005, considerable progress has been made on the watt balance of the Bureau International des Poids et Mesures (BIPM). We have continued the development of a room-temperature version to test the feasibility of simultaneous force and velocity measurements. Preliminary measurements of the voltage-velocity ratio have been carried out with a reproducibility of the order of 1 part in 10⁴. The coil suspension has been improved to reduce the undesired coil movement in the 5 degrees of freedom different from a perfectly vertical movement. Further work on the current source has reduced its long-term drift to a few parts in 10⁹ per minute with a satisfactory short-term stability. A technique needed to separate the voltage induced in the coil from the voltage drop due to the current flow based on the use of a second noninductive coil has been tested. The second coil is now being integrated into the apparatus. Work has started on the geometrical and magnetic characterizations of a large precision solenoid that will become the reference for the magnetic field alignment. The collaboration with a university on the fabrication of our magnetic circuit is being continued. This paper briefly outlines the main ideas of the BIPM watt balance and reports the progress to date.