An initial measurement of Planck's constant using the NPL Mark II watt balance

During 2006 modifications were made to the NPL Mark II moving-coil watt balance which eliminated several significant sources of error. The apparatus was run from October 2006 to March 2007 to check its operation and to produce a preliminary value of Planck's constant h. This paper reports the current state of the work and predicts the uncertainty that is expected to be achievable with the existing apparatus. The preliminary value of h is 6.62607095(44) x 10(-34) J s, which corresponds to a value of the Avogadro constant of N-A=6.02213999(40) x 10(23) mol(-1).