Construction, Measurement, Shimming, and Performance of the NIST-4   Magnet System
  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. 
Frank Seifert, Alireza Panna, Shisong Li, Bing Han, Leon Chao, Austin   Cao, Darine Haddad, Heeju Choi, Lori Haley, Stephan ...
physics.ins-det physics.class-ph