Research on the effect of the external magnetic field in the joule balance at NIM

The first determination of the Planck constant with the second generation of the joule balance, NIM-2, was completed in 2017 with an uncertainty of 2.4 x 10(-7). Due to the movement of the magnet during the measurement process, the effect of the external field is a critical problem in NIM-2. At present, the electromagnet system is used in NIM-2. By taking the average of the results with positive and negative exciting currents, the uncertainty from the external field is reduced to 1.7 x 10(-7), which is still the largest source in the uncertainty budget as all the other items are less than 1 x 10(-7). In the near future, a permanent magnet system will be applied in NIM-2 and the main field cannot be reversed. Although the coupling of the external magnetic field in the permanent magnet system is about 40 times less than that in the electromagnet system, further reduction of this effect is still required in the permanent magnet system. In this paper, the effect of the external field is analyzed in both an electromagnet system and a permanent magnet system based on simulations and experiments. Then, the methods of magnetic shielding and compensation coils are proposed and simulated in the permanent magnet system. The results show that it may be possible to reduce the uncertainty of the external field to less than 2 x 10(-8) in the permanent magnet system by employing the two methods.