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