Measurement of the Planck constant at the National Institute of Standards and Technology from 2015 to 2017

Researchers at the National Institute of Standards and Technology(NIST) have measured the value of the Planck constant to be lighl/ig=6.626 069 934(89) ×10lsupg-34l/supg J s (relative standard uncertainty 13 ×10lsupg-9l/supg). The result is based on over 10 000 weighings of masses with nominal values ranging from 0.5 kg to 2 kg with the Kibble balance NIST-4. The uncertainty has been reduced by more than twofold relative to a previous determination because of three factors: (1) a much larger data set than previously available, allowing a more realistic, and smaller, Type A evaluation; (2) a more comprehensive measurement of the back action of the weighing current on the magnet by weighing masses up to 2 kg, decreasing the uncertainty associated with magnet non-linearity; (3) a rigorous investigation of the dependence of the geometric factor on the coil velocity reducing the uncertainty assigned to time-dependent leakage of current in the coil.