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 held has a dual use of: 1 ) generating a voltage by moving at some velocity to calibrate the mag netic 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 i ncorporation of three-laser interferometry and a refractometer to impr ove the velocity measurements, temperature control and coil rotation d amping to reduce drifts and stabilize laser and mechanical alignments, and a gravimeter to determine local gravity. Systematic errors and sc atter 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 n ear 0.1 mu W/W, which is the target precision of this present design.