Toward a Final Result From the NPL Mark II Watt Balance

The National Physical Laboratory (NPL) Mk II watt balance made an initial series of measurements of Planck's constant in the period from October 2006 to March 2007. The results of these measurements significantly differ from those recently published by the National Institute of Standards and Technology (NIST). Modifications have been made to the apparatus to reduce the most significant uncertainties that were identified during these measurements. The temperature stability of the working resistors has been improved, most of the optics comprising the interferometer have been replaced, and a new programmable Josephson junction array voltage reference has been built. This paper describes the changes to the apparatus and gives the results of the measurements that were made to monitor the effects of these modifications. The apparatus was not fully realigned during this process, and no significant deviations from the previously reported result were seen. Work on the apparatus will continue until the end of 2008 to reduce the uncertainty associated with the NPL measurement and to further investigate possible sources of the difference between the NPL and NIST results.