  In the joule balance developed at National Institute of Metrology (NIM), the
dynamic phase of a watt balance is replaced by the mutual inductance
measurement in an attempt to provide an alternative method for the kg
redefinition. 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 coil
heating, a new coil system, in which a ferromagnetic material is used to
increase the magnetic field was designed recently. But adopting the
ferromagnetic material brings the difficulty from the nonlinear characteristic
of material. This problem can be removed by measuring the magnetic flux linkage
difference of the suspended coil at two vertical positions directly to replace
the mutual inductance parameter. Some systematic effects of this magnet are
discussed.
