Coils used in the Joule Balance at the NIM

In the Joule Balance at the National Institute of Metrology (NIM), the dynamic measurement during the coil movement is avoided. But for this method a rather large current in the primary coil is needed to generate the necessary magnetic field in the force weighing phase. Thus the coil heating becomes an obvious uncertainty contribution. To solve this problem, a new coil system, in which the ferromagnetic material is used to increase the magnetic field and then the coil heating is reduced obviously, is designed and tested recently. But the nonlinear characteristic of the ferromagnetic material brings the difficulty for the measurement. These problems and a solution are discussed in this paper.