Construction, Measurement, Shimming, and Performance of the NIST-4 Magnet System

Abstract

The magnet system is one of the key elements of a watt balance. For the new watt balance currently under construction at the National Institute of Standards and Technology, a permanent magnet system was chosen. We describe the detailed construction of the magnet system, first measurements of the field profile, and shimming techniques that were used to achieve a flat field profile. The relative change of the radial magnetic flux density is less than $10^{-4}$ over a range of 5 cm. We further characterize the most important aspects of the magnet and give order of magnitude estimates for several systematic effects that originate from the magnet system.

v2: changed layout; 13 pages, 15 figures. published in IEEE Trans. Instrum. Meas. 63, 3027-3028 (2014), http://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=6827217&isnumber=6940066