Watt-balance experiments, which compare mechanical power to electrical power, produce the best value for the Planck constant h. In NIST's two-story-tall watt balance, the gravitational force on a mass standard is balanced by the magnetic force on a current-carrying coil in one phase of the experiment.
(a) Richard Steiner positions the mass standard at the top of the watt balance.
(b) Edwin Williams checks the superconducting magnet used to generate the radial magnetic field at the lower level of the watt-balance experiment.

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© 2001 American Institute of Physics