A closed magnet circuit is employed. The circuit design includes two discs of Sm2Co17 magnets, magnetized in opposite directions, as the flux source. The yoke is made of a high permeability FeNi alloy which eliminates the non-uniformity of the magnetic flux density in the air gap, which would otherwise result from the small magnetization asymmetry of the two permanent magnet discs. The mean diameter of the circular air gap is 250 mm and the width is 13 mm. The magnetic field has radial symmetry and a magnetic flux density of about 0.5 T.
Due to the geometry of the system, the magnets and the air gap are completely screened by the high permeability iron yoke. This considerably reduces the level of external electromagnetic perturbations detected by the coil. The symmetry of the circuit with respect to the horizontal plane helps to improve the uniformity of the flux density in the air gap. The uniformity of the vertical profile of the flux density in the air gap was measured to be below 1 × 10-4 within the working length of 40 mm.
A theoretical and experimental study was conducted to investigate the influence of the coil-current on the magnetic field. The magnetic flux generated by the coil goes through the magnet and produces an additional contribution to the magnetic field at the coil position. It has been found that this mechanism leads to a significant slope of the vertical magnetic field profile. It has also been demonstrated that the effect on the magnetic field in the velocity measurement is twice that deduced from the weighing measurement. The exact size of the effect depends on the specific geometry and dimensions of each magnetic circuit, but in general has to be taken into account at the present level of uncertainty of Kibble balances.
For further details see:
- Li S., Bielsa F., Stock M., Kiss A., Fang H., A permanent magnet system for Kibble balances, Metrologia, 2017, 54, 775-783.
- Li S., Bielsa F., Stock M., Kiss A., Fang H., Coil-current effect in Kibble balances: analysis, measurement, and optimization, Metrologia, 2018, 55, 75-83.