Rl. Steiner et al., THE NIST WATT BALANCE - PROGRESS TOWARD MONITORING THE KILOGRAM, IEEE transactions on instrumentation and measurement, 46(2), 1997, pp. 601-604
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.