  Permanent magnets with yokes are widely used in the watt and joule balances
to measure the Planck constant for the forthcoming redefinition of the unit of
mass, the kilogram. Recently, a permanent magnet system has been in
consideration for a further practice of NIM-2, the generalized joule balance.
In this paper, an analytical model to design the permanent magnet system is
presented. The presented model can be solved to obtain the preliminary
parameters and then is used as guidance for FEA software to optimize the
parameters of such magnetic system. As an instance for the application of the
designing model, the design of the permanent magnet system for NIM-2 is
described and the special design of opening shape makes the misalignment of the
top and middle yokes has little influence on the vertical component of the
magnetic field.
