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 power measured in terms of the meter, kilogram, and second. We find the Planck constant $h=6.626\mathrm{}06891\left(58\right)\times {10}^{-34\mathrm{}}\mathrm{J}\mathrm{s}$. The quoted standard uncertainty (1 standard deviation estimate) corresponds to $(8.7\times {10}^{-8\mathrm{}})h$. Comparing this measurement to an earlier measurement places an upper limit of $2\times {10}^{-8\mathrm{}}/\mathrm{yr}$ on the drift rate of the SI unit of mass, the kilogram.