A permanent magnet system for Kibble balances

Li, Shisong; Bielsa, Franck; Stöck, M; Kiss, A.; Fang, Hao

doi:10.1088/1681-7575/aa71db

Cited by 22 publications

(48 citation statements)

References 25 publications

(46 reference statements)

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“…As described in Fig. 1 d, experiments at the BIPM prove that this additional magnet field does, in fact, exist 46 , 47 . Hence, the parameter introduced in Eq.…”

Section: Introductionmentioning

confidence: 55%

Resolution of the paradox of the diamagnetic effect on the Kibble coil

Schlamminger

Marangoni

et al. 2021

Sci Rep

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Employing very simple electro-mechanical principles known from classical physics, the Kibble balance establishes a very precise and absolute link between quantum electrical standards and macroscopic mass or force measurements. The success of the Kibble balance, in both determining fundamental constants (h, $$N_A$$ N A , e) and realizing a quasi-quantum mass in the 2019 newly revised International System of Units, relies on the perfection of Maxwell’s equations and the symmetry they describe between Lorentz’s force and Faraday’s induction, a principle and a symmetry stunningly demonstrated in the weighing and velocity modes of Kibble balances to within $$1\times 10^{-8}$$ 1 × 10 - 8 , with nothing but imperfect wires and magnets. However, recent advances in the understanding of the current effect in Kibble balances reveal a troubling paradox. A diamagnetic effect, a force that does not cancel between mass-on and mass-off measurement, is challenging balance maker’s assumptions of symmetry at levels that are almost two orders of magnitude larger than the reported uncertainties. The diamagnetic effect, if it exists, shows up in weighing mode without a readily apparent reciprocal effect in the velocity mode, begging questions about systematic errors at the very foundation of the new measurement system. The hypothetical force is caused by the coil current changing the magnetic field, producing an unaccounted force that is systematically modulated with the weighing current. Here we show that this diamagnetic force exists, but the additional force does not change the equivalence between weighing and velocity measurements. We reveal the unexpected way that symmetry is preserved and show that for typical materials and geometries the total relative effect on the measurement is $$\approx 1\times 10^{-9}$$ ≈ 1 × 10 - 9 .

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“…As described in Fig. 1 d, experiments at the BIPM prove that this additional magnet field does, in fact, exist 46 , 47 . Hence, the parameter introduced in Eq.…”

Section: Introductionmentioning

confidence: 55%

Resolution of the paradox of the diamagnetic effect on the Kibble coil

Schlamminger

Marangoni

et al. 2021

Sci Rep

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“…Comparison of the profile obtained by simulation and measurement. The results in (a) are performed on the BIPM magnet system [18]. The theoretical Br curve is the magnetic flux density distribution along the vertical direction, with a mean radius 126 mm.…”

Section: Discussionmentioning

confidence: 99%

“…Br is the magnetic profile averaged over the coil, for the BIPM system a height of 20 mm. The experimental result is taken from a voltage-velocity ratio measurement, presented in [18]. (b) is based on the NIST-4 magnet system [20].…”

Section: Discussionmentioning

confidence: 99%

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A Simple Improvement for Permanent Magnet Systems for Kibble Balances: More Flat Field at Almost No Cost

Schlamminger

Wang

2020

IEEE Trans. Instrum. Meas.

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Permanent magnets together with yokes to concentrate the magnetic flux into a cylindrical airgap 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: 5 × 10 −4 ) of the flux density in the center of the magnet system. By simply 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 be counteracted, and, hence, the length of the region, where the flux density remains within a given tolerance band is increased. 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. We investigate possible adverse effects on the performance of the magnet system caused by the additional rings: magnetic field strength, coil-current effect, and a dependence of the radial field on the radial position in the field. No substantial disadvantage was found. Instead, the method presented here outperformed previously suggested methods 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 a detrimental effect to function, cost, and form factor of the magnet system.

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“…In fact, the present experimental results, despite their conspicuous five-time uncertainty lowering, still do not support the firm continuity of the units' magnitudes. On the other hand, the CODATA 2017 analysis has been unable to take into account some more recent findings, e.g., on the BIPM Kibble balance [7][8][9], about possible systematic effects.…”

Section: "Exactness" Of the Stipulated Datamentioning

confidence: 99%