The progress of Joule balance at the National Institute of Metrology

The Joule balance method to measure the Planck constant and redefine the Kilogram ultimately has been going on at the National Institute of Metrology of China (NIM) since 2007. The first prototype has been built to check the feasibility of the principle. In 2012, the relative uncertainty of the plank constant measurement result is at 10^-5 level. Self-heating of the coils, swing of the coil, etc., are the main uncertainty contributions. Recently, some improvements have been done to decrease these uncertainties. The uncertainty of the Joule balance is decreased from 7.7×10^-5 obtained at 2012 to 8.9×10^-6 at present. The plank constant measured with Joule balance is h=6.6261041×10^-34Js. The relative difference between it and CODATA2010 recommendation value is 5×10^-6. At the same time, the design and key component test of the 2^nd generation Joule balance are also in progress and presented here briefly.