A magnet system for the MSL watt balance

摘要

A novel design for the magnet system of a watt balance is presented. While primarily intended for the watt balance being developed by MSL, this design could be used for other watt balances. It has a radial magnetic field in an annular gap that is screened from external fields. The magnetic field is generated by a single ring-shaped permanent magnet and the geometry is such that the field induced in the permanent magnet by the current in the coil in the weighing mode is zero on average. Hence the magnetic field in the gap is expected to be the same for both the weighing and dynamic modes of a watt balance. From finite element modelling, the magnetic field strength can be uniform to within 40 parts in 10~6 over more than 50 mm (50% of the gap height) and the usable working range for the coil is about 40 mm. Further, the diameter of the annular gap can be relatively small, allowing the coil and its support structure to be kept relatively stiff and light.