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].