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This illustration shows the device built by NIST researchers to perform a high-precision measurement of Planck's constant, the number which describes the bundle-like nature of matter and energy at the atomic and subatomic scales. The electrical power associated with the mechanical motions of the system contains quantities proportional to Planck's constant.
The device contains a platform for holding a kilogram mass. A superconducting magnet produces a magnetic field that is perpendicular to the direction of gravity and two metal coils (labelled as "movable induction coil" and "fixed induction coil"). This enabled the researchers to measure all forces and velocities in the vertical direction. A 31-cm-radius wheel (the "balance wheel") operates like a pulley, with the induction coils, kilogram mass, and countermass (on the left-hand side) hanging from flat bands of 50 strands of wire rolling on the wheel. This setup allows the movable induction coil to make strictly vertical displacements of 100 mm when the wheel rotates by ten degrees.
In the first step of the measurement, no masses are on the pans, and the researchers apply a small force from the countermass side, by using the auxiliary magnet and running a current through the auxiliary coil to produce a magnetic force. This causes the moveable induction coil to descend downwards with a velocity of about 2 mm/s. Three laser interferometers record the motion of the coil with utmost precision. Since the coil is immersed in a magnetic field, its movement generates a voltage. This voltage is proportional to Planck's constant.
In the second step of the measurement, a tare weight of 500 g is placed on the countermass pan. The researchers send an electric current through the induction coil. Then a one-kilogram mass is placed on and off the mass pan on the right, as the current running through the coil produces an upward magnetic force that exactly balances the weight of the kilogram. The resistance used to measure the current running through the coil is proportional to Planck's constant as well.
With these measurements, the researchers calculated a value for Planck's constant of 6.62606891 x 10-34 Joule-seconds, with an uncertainty of 89 parts per billion. They plan to immerse their watt balance in a vacuum enclosure (shown in yellow) which will further increase the precision of their experiment. In addition, this setup ultimately promises to lead to a definition of the kilogram based on quantum units, rather than one based on the stalwart physical artifact currently stored in France.
(Figure courtesy NIST. Text adapted from Williams et al, Physical Review Letters, 21 September 1998.)
This research is reported by Edwin R. Williams, Richard L. Steiner, David B. Newell, and Paul T. Olsen in Physical Review Letters, v.81, p. 2404, 21 September 1998.