ACCURATE MEASUREMENT OF THE PLANCK CONSTANT

Using a moving coil watt balance, electric power measured in terms of the Josephson and quantum Hall effects is compared with mechanical pow er measured in terms of the meter, kilogram, and second. We find the P lanck constant h = 6.62606891(58) x 10(-34) Js. The quoted standard un certainty (1 standard deviation estimate) corresponds to (8.7 x 10(-8) )h. Comparing this measurement to an earlier measurement places an upp er limit of 2 x 10(-8)/yr on the drift rate of the SI unit of mass, th e kilogram. [S0031-9007(98)07164-6].