Monitoring gravity for the NIST-4 watt balance

The National Institute of Standards and Technology (NIST) new watt balance (NIST-4) has been constructed for the realization of mass through an exact value of the Planck constant in the new International System of Units (SI). The total relative uncertainty goal for the instrument of a few parts in 10
⁸
requires the determination of the local acceleration of gravity, g, with parts in 10
⁹
accuracy at the location of the mass at the time mechanical and electrical forces are being compared. We describe the procedures used and give results for the measurements of the local acceleration of gravity in the new watt balance facility.