Closing in on the Planck constant
Sep 25, 1998
Physicists in the US have made the most accurate measurements ever of the Planck constant, h. Edwin Williams and colleagues at the National Institute of Standards and Technology in Gaithersburg, Maryland, measured h by comparing the voltage needed to control the velocity of a coil moving vertically in an magnetic field, with the current that has to be passed through the coil to balance gravity in the same magnetic field. The measurement could lead to a new reference stand for the kilogram (Phys. Rev. Lett. 81 2404).
The kilogram is currently defined by a platinum-iridium alloy maintained at the Bureau International des Poids et Mesures (BIPM) in Paris and six official copies. However, the official mass of the standard kilogram has been known to vary with time, hence the interest in defining the kilogram is terms of fundamental constants like h.
The highly stable magnetic field needed for the experiment is generated by a superconducting magnet that has been cooled to 4 Kelvin. The experiment also uses two induction coils: the lower coil is fixed to the support structure of the experiment, while the upper coil can move. This upper coil is also attached to a wheel balance above the experiment. In the first stage of the measurement, the mass balance is empty and a small force is applied to the upper coil, forcing it to move at 2mm/s. The researchers found that this generated a voltage of 1.018 ± 0.001 V across the moving coil. In the second stage of the experiment, a 500 g countermass is balanced by a - 10.18 mA current in the induction coil. Both stages were repeated over many months to obtain a value of 6.62606891(58) x 10-34 Joule seconds for the Planck constant.
This result - which corresponds to an accuracy of 9 parts in 108 - is a factor of 15 better than previous measurements. The team hope to improve on this result by another factor of 10 by modifying their experiment.