Researchers at the National Institute of Standards and Technology(NIST) estimate the value of the Planck constant as h = 6.62606994(10) x 10^-34 Js, hence with relative standard uncertainty 15.3 x 10^-9. This measurement result is based on over 17,000 weighings of masses with nominal values ranging from 0.5 kg to 2 kg , with the Kibble balance NIST-4. The uncertainty associated with this estimate of h has been reduced by more than twofold relative to a previous determination. This improvement is attributable to three factors: (1) a much larger data set than was previously available allows for a more realistic, and smaller, Type A evaluation of volatile ("random") sources of uncertainty; (2) a more accurate measurement of the back action of the weighing current on the magnet, made possible by weighing masses up to 2 kg , reduces the uncertainty associated with this effect; (3) a rigorous investigation of the dependence of the geometric factor in velocity mode on the coil velocity reduces the uncertainty caused by time dependent leakage of current in the coil.
Planck constant, Kibble balance, watt balance