A nonlinearity in permanent-magnet systems used in watt balances

TL;DR

This paper investigates a nonlinear effect in permanent magnet systems used in watt balances, showing how the weighing current alters the magnetic flux density through changes in yoke reluctance, impacting precision in fundamental constant measurements.

Contribution

A simple model is developed to explain the nonlinear dependence of magnetic flux density on coil current in watt balance magnet systems, highlighting factors influencing this effect.

Findings

The magnetic flux density depends quadratically on coil current.

The effect varies with air gap width and yoke material properties.

Suggestions are provided to mitigate the nonlinear bias.

Abstract

Watt balances are used to measure the Planck constant and will be used in the future to realize mass at the kilogram level. They increasingly rely on permanent magnet systems to generate the magnetic flux. It has been known that the weighing current might effect the magnetization state of the permanent magnetic system used in these systems causing a systematic bias that can lead to an error in the result if not accounted for. In this article a simple model explaining the effect of the weighing current on the yoke of the magnet is developed. This model leads to a nonlinear dependence of the magnetic flux density in the gap that is proportional to the squared value of the coil current. The effect arises from changing the reluctance of the yoke by the additional field produced by the coil. Our analysis shows that the effect depends on the width of the air gap, the magnetic flux density in the air gap, and the $BH$ curve of the yoke material. Suggestions to reduce the nonlinear effect are discussed.