THE MOST ACCURATE MEASUREMENT YET OF THE PLANCK CONSTANT, the number which describes the bundle-like nature of matter and energy at the atomic and subatomic levels, has been carried out by NIST physicists, instantly improving the accuracy of related fundamental constants (such as electron mass, proton mass, and Avogadro's number) and paving the way for a quantum-based definition of mass. Carrying out an experiment first proposed by Brian Kibble of the National Physical Laboratory in England (011-44-171-594-7845), a NIST group (Edwin Williams, 301-975-4206) determined Planck's constant, otherwise known as h, by using a "moving-coil watt balance," an apparatus with a kilogram mass connected to a metal coil in a magnetic field. Injecting a current through the coil created an upward magnetic force which exactly balanced the downward force of gravity on the mass. In a second step, the group allowed the coil to move downward, measuring its velocity and the voltage it generated. In both steps, the electrical power associated with the mechanical motions of the system contained quantities proportional to Planck's constant, allowing the researchers to extract the value of h while cancelling out factors such as the geometry of the setup. The team calculated a value for h of 6.62606891 x 10^-34 Joule-seconds, with an uncertainty of 89 parts per billion, two times better than previously published measurements. Their watt-balance 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. (Williams et al., Physical Review Letters, 21 September 1998; figure at Physics News Graphics.)