Progress in relating the kilogram to Planck's constant with the NPL Watt balance

The NPL moving-coil Watt balance has been operated regularly for the last two years. The results exhibit good short-term noise and stability but show two significant drifts over periods of 1 to 4 months. The long-term nature of the drifts has hindered the search for their cause which is now thought to be due to interacting mechanical distortions which alter the alignment of the apparatus after evacuation. Modifications are in hand to eliminate the effects of these distortions.