The NIST watt balance: progress toward monitoring the kilogram

Abstract

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 W/W, which is the target precision of this present design.

Accurate Measurement of the Planck Constant

“…The National Institute of Standards and Technology (NIST) watt balance [7] has been designed to measure the ratio of mechanical to electrical power, linking the artifact kilogram, the meter, and the second to the practical realizations of the ohm and the volt derived from the quantum Hall effect (QHE) and the Josephson effect (JE), respectively. Josephson voltages, U J ͑n, f͒, and quantized Hall resistances, R H ͑i͒, can be realized by using the following equations:…”

Programmable 1 V dc voltage standard

“…The latest application of the 1 V programmable JVS is the watt balance experiment [5] which requires a stable, reversible voltage reference with high noise immunity. In the first mode of this experiment, a servo system controls the velocity of an induction coil moving in a magnetic field such that the voltage across the coil is equal to the 1 V reference from the JVS.…”

A result from the NIST watt balance and an analysis of uncertainties

“…The goal is to determine the ratio of power measured in terms of the Josephson and quantum Hall effects versus power measured in terms of the meter, kilogram, and second. For brevity, we must refer to earlier papers containing greater details of this experiment [2] and recent improvements [3]. Another paper presents just the watt results reported here, calculating several fundamental physical constants obtained from this determination [4].…”