Uncertainty Improvements of the NIST Electronic Kilogram

This paper summarizes the latest uncertainty improvements and data for a new result from the National Institute of Standards and Technology (NIST) electronic kilogram experiment. The result of 8 $\pm$ 36 nW/W for the ratio $ \{{W}_{90} /{W}- 1\}$, as well as the Planck's constant, is comparable to the NIST 2005 value but with a lower uncertainty. Of the eight largest Type B uncertainty contributions, gravity acceleration, wheel surface, and reference mass magnetic susceptibility are considerably reduced. Humidity dependence in the recent data and limitations on the volt/velocity signal analysis are now the main limitations on the uncertainty.