Towards an electronic kilogram: an improved measurement of the Planck constant and electron mass

The electronic kilogram project of NIST has improved the watt balance method to obtain a new determination of the Planck constant h by measuring the ratio of the SI unit of power W to the electrical realization unit W-90, based on the conventional values for the Josephson constant KJ-90 and von Klitzing constant RK-90. The value h = 6.62606901(34) x 10(-34) J s verifies the NIST result from 1998 with a lower combined relative standard uncertainty of 52 nW/W. A value for the electron mass m(e) = 9.10938214(47) x 10(-31) kg can also be obtained from this result. With uncertainties approaching the limit of those commercially applicable to mass calibrations at the level of 1 kg, an electronically-derived standard for the mass unit kilogram is closer to fruition.