Coils and the Electromagnet Used in the Joule Balance at the NIM

TL;DR

This paper discusses the design and analysis of coils and electromagnets used in the NIM's joule balance, focusing on reducing coil heating and addressing nonlinear magnetic properties for improved kilogram redefinition methods.

Contribution

It introduces a new coil system using ferromagnetic material and a flux linkage measurement method to mitigate heating and nonlinear effects in the joule balance.

Findings

Reduced coil heating with ferromagnetic core design

Effective flux linkage measurement to replace mutual inductance

Analysis of systematic effects in the magnet system

Abstract

In the joule balance developed at National Institute of Metrology (NIM), the dynamic phase of a watt balance is replaced by the mutual inductance measurement in an attempt to provide an alternative method for the kg redefinition. But for this method a rather large current in the exciting coil, is needed to offer the necessary magnetic field in the force weighing phase, and the coil heating becomes an important uncertainty source. To reduce coil heating, a new coil system, in which a ferromagnetic material is used to increase the magnetic field was designed recently. But adopting the ferromagnetic material brings the difficulty from the nonlinear characteristic of material. This problem can be removed by measuring the magnetic flux linkage difference of the suspended coil at two vertical positions directly to replace the mutual inductance parameter. Some systematic effects of this magnet are discussed.