The first determination of the Planck constant with the joule balance NIM-2

The National Institute of Metrology (NIM, China) proposed a joule balance method to measure the Planck constant in 2006, and built the first prototype NIM-1 to verify its principle with a relative uncertainty of 8.9 × 10-6 by 2013. Since 2013, a new joule balance NIM-2 has been designed, with a series of improvements to reduce the measurement uncertainty. By April 2017, NIM-2 has been constructed and can be employed to measure the Planck constant in vacuum. A first measurement on NIM-2 yields a determination of the Planck constant is 6.626 069 2(16) × 10-34 Js with a relative uncertainty of 2.4 × 10-7. The determination differs in relative terms by −1.27 × 10-7 from the CODATA 2014 value. Further improvement of NIM-2 is still in progress towards 10-8 level uncertainty in the future.