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A simple improvement for permanent magnet systems for Kibble balances: More flat field at almost no cost



Stephan Schlamminger, Shisong Li


Permanent magnets together with yokes to concentrate the magnetic flux into a cylindrical air- gap are widely employed in Kibble balances. These experiments require a uniform magnetic flux density along a vertical path, typically a substantial fraction of the length of the air-gap. Fringe fields that are present at both ends of the air-gap limit the region where the flux density does not change more than a certain relative fraction (here: 5E-4) of the flux density in the center of the magnet system. By just adding an iron ring with a rectangular cross- section to the inner yoke at each end of the air gap, the effects of the fringe fields can mostly be counteracted, and, hence, increases the length of the region, where the flux density remains within a given tolerance band. Compared to the alternative, employing a taller magnet, the proposed method yields a magnet system with an extended region of a uniform field without significantly increasing the mass of the magnet system. Potential applications include compact and table-top Kibble balances. In this article, we investigate possible adverse effects caused by the added rings on the performance of the magnet system: magnetic field strength, coil-current effect, a dependence of the radial field on the radius. No substantial disadvantage was found. Instead, the method presented here outperformed previously suggested ways to improve the radial dependence of the radial field, e.g., shorter outer yoke. In summary, adding rings to the inner yoke improves the uniformity of the field without detrimental effect to function, cost, and form factor of the magnet system.
IEEE Transactions on Instrumentation and Measurement


Kibble balance, magnet system, revised SI, mass


Schlamminger, S. and Li, S. (2020), A simple improvement for permanent magnet systems for Kibble balances: More flat field at almost no cost, IEEE Transactions on Instrumentation and Measurement, [online], (Accessed July 22, 2024)


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Created March 18, 2020, Updated March 1, 2021