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

TL;DR

This paper investigates the diamagnetic effect in Kibble balances, demonstrating that despite the presence of a diamagnetic force, the fundamental symmetry between weighing and velocity modes remains intact, ensuring measurement accuracy.

Contribution

The study reveals that the diamagnetic effect does not compromise the symmetry in Kibble balances, resolving a paradox and confirming the robustness of the measurement system.

Findings

Diamagnetic force exists but does not affect measurement symmetry.

The relative effect of the diamagnetic force is approximately 1×10⁻⁹.

Symmetry between weighing and velocity modes is preserved despite the diamagnetic effect.

Abstract

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 ($h$, $N_A$, $e$) 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 $1\times10^{-8}$, 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 $\approx 1\times10^{-9}$.