Gravimetry for watt balance measurements

In a watt balance (WB) the gravity on a known mass is used in the determination of the value of the Planck constant with a precision of <2 × 10−41 J s. To attain this precision the gravity value at the centre of a mass within the WB must be known with a relative uncertainty of <1 × 10−8. This necessitates laboratory gravity mapping in three dimensions, the establishment close to the WB of a gravity reference station, and modelling of the ties to the centre-of-mass location. The self-gravity of the WB must be accounted for, and since WB measurements run over several weeks, corrections for tidal, polar motion and atmospheric effects must be included. We describe the gravimetry carried out at the National Research Council of Canada WB laboratory, and include a discussion of past gravity variations in the vicinity of the WB, which suggest that seasonal variation may be of sufficient magnitude to affect results obtained with the balance.