The NIST watt balance: progress toward monitoring the kilogram

The National Institute of Standards and Technology (NIST) watt balance is an experiment to compare measurements of the watt using electrical references (volt, ohm) to those using mechanical references (length, time, mass). A coil within a radial magnetic field has a dual use of: (1) generating a voltage by moving at some velocity to calibrate the magnetic flux density, and, (2) generating a force with electrical current to balance the gravitational force of a mass. This experiment has had several improvements made to it in the last year. These include the incorporation of three-laser interferometry and a refractometer to improve the velocity measurements, temperature control and coil rotation damping to reduce drifts and stabilize laser and mechanical alignments, and a gravimeter to determine local gravity. Systematic errors and scatter in long-term measurements have been greatly reduced in the last year, but statistically significant deviations relative to within-run uncertainty still persist. The source of these deviations has not yet been identified. Recent within-run standard deviations are generally near 0.1 pW/W, which is the target precision of this present design.