Towards the redefinition of the kilogram: a measurement of the Planck constant using the NPL Mark II watt balance

Abstract

After the elimination of significant mechanical problems, the NPL Mark II moving-coil watt balance made an initial series of measurements in the period from October 2006 to March 2007. Incremental improvements were made to the apparatus in the period from June 2007 to November 2008 and measurements of the Planck constant h were made up to June 2009, with the aim of providing further information to the ongoing efforts to redefine the SI kilogram in terms of a fixed value of the Planck constant. The apparatus was sold to NRC-INMS in early 2009 and was dismantled and shipped to Canada in the period between June and August 2009.In June 2009, just prior to the shipment, a possible source of significant type-B uncertainty in the mass/force exchange was discovered. There was insufficient time to fully investigate and correct the effect so a component has been added to the uncertainty budget to account for its estimated magnitude. The standard uncertainty of the apparatus, without allowance for the mass/force exchange problem, is estimated to be 36 nW W−1, which represents an improvement of almost a factor of two from the previously reported uncertainty of 66 nW W−1, but, allowing for the problem, the uncertainty increases to 200 nW W−1. Further work, once the apparatus is rebuilt in Canada, should eliminate the source of the added uncertainty. The value of h calculated from the measurements made from 2006 to 2009 is 6.626 071 23(133) × 10−34 J s, which represents a change of 43 nW W−1 from the value reported in 2007. The corresponding value of the Avogadro constant, NA, is 6.022 139 78(120) × 1023 mol−1.