The Improvements of the NIM-2 Joule Balance

Since 2013, the NIM-2 joule balance apparatus has been designed and constructed at the National Institute of Metrology for the precise measurement of the Planck constant and the redefinition of the kilogram. By the end of 2016, the NIM-2 was ready for measurement in vacuum. Before the deadline of July 1, 2017, the first determination of the Planck constant obtained with the NIM-2 in vacuum was published. The measured Planck constant is 6.626 069 2(16) × 10^-34 Js, which differs in relative terms by -1.27 × 10^-7 from the Committee on Data of the International Council for Science (CODATA) 2014 value, with a relative standard uncertainty of 2.4 × 10^-7. However, this result is not used for the CODATA 2017 special adjustment because its uncertainty is bigger than 6 × 10^-8. The main uncertainty contributions are from the external field, alignment, and mass calibration. At present, the consistent research on the experiments related to the redefinition of the kilogram is still encouraged and expected by the Consultative Committee for Units. Therefore, great efforts are still put forward to improve the NIM-2 joule balance to get an uncertainty of 10^-8 level. A series of improvements in progress is presented here.