Improvements of the NIM-2 Joule Balance Since 2020

The Joule balance, which relates mechanical energy and electrical energy, was proposed by the National Institute of Metrology (NIM) to measure the Planck constant and realize the kilogram with the Planck constant based on the new definition of the kilogram. In 2019, the NIM-2 Joule balance participated in the first key comparison of kilogram realizations (CCM.M-K8.2019) with a combined relative standard uncertainty of $4.5\times 10^{-8}$ . After the first key comparison, a series of improvements on the mechanical, electrical, and alignment systems have been carried out. To improve the accuracy of the electrical measurement, the periodical calibration of the digital voltmeter (DVM) with the programmable Josephson voltage system (PJVS) was also realized. The calibration period of the DVM was reduced from five days to 3.5 h. With these improvements, the corresponding uncertainties were decreased significantly.