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 6.626 068 91͑58͒ 3 10 234 J s. The quoted standard uncertainty (1 standard deviation estimate) corresponds to ͑8.7 3 10 28 ͒h. Comparing this measurement to an earlier measurement places an upper limit of 2 3 10 28 ͞yr on the drift rate of the SI unit of mass, the kilogram. [S0031-9007(98)

The effects of alignment uncertainties of the NIST watt balance with respect to local gravity and the magnetic flux density of the balance have been analyzed. Techniques for measuring all quantities relevant to misalignment have been developed. The components of the relative combined standard uncertainty of the measured value of the watt due to alignment uncertainties have been reduced to 20 nW/W, and potential improvements in the balance design have been identified which could ultimately lead to a reduction of that uncertainty to below 10 nW/W.

We measure the proton gyromagnetic ratio in H 2 0 by the low field method, $,(low). The result $,(low) = 2.67 513 376 loss-' 7 & (0.11 ppm), leads to a value of the fine structure constant of a-' = 137.0 359 840 (0.037 ppm) and a value for the quantized Hall resistance in SI units of RH = 25 812.80460 Q (0.037 ppm). To achieve this result, we measured the dimensions of a 2.1-m solenoid with an accuracy of 0.04 pm, and then measured the NMR frequency of a water sample in the field of the solenoid.

A new value for the fine-structure constant has been obtained from a new measurement of the gyromagnetic ratio of the proton. The result, jp (IOW)NBS^ 2.675 132 29(57) xio^ s" ^ TNBS"^ (0.21 ppm), is used to derive a" 1 = 137.035963(15) (0.11 ppm). This value of a"i is (0.33±0.14) ppm less than the value oia"^ derived from measurements of the anomalous magnetic moment of the electron, a^, and its current best quantum electrodynamics theoretical estimate.