In the joule balance developed at National Institute of Metrology (NIM), thedynamic phase of a watt balance is replaced by the mutual inductancemeasurement in an attempt to provide an alternative method for the kgredefinition. But for this method a rather large current in the exciting coil,is needed to offer the necessary magnetic field in the force weighing phase,and the coil heating becomes an important uncertainty source. To reduce coilheating, a new coil system, in which a ferromagnetic material is used toincrease the magnetic field was designed recently. But adopting theferromagnetic material brings the difficulty from the nonlinear characteristicof material. This problem can be removed by measuring the magnetic flux linkagedifference of the suspended coil at two vertical positions directly to replacethe mutual inductance parameter. Some systematic effects of this magnet arediscussed.
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