Construction and performance of the new magnet system for Joule balance at NIM

For the new Joule balance currently under construction at the National Institute of Metrology, called NIM-2, a coaxial coils system with soft iron yokes was selected as main magnet system. We present the detailed construction of the magnet system and first measurements of the field profile that were made on the real magnet system. The initial experiments showed that variation of the field approached 8.9 ppm/mm, which is about a factor 5 larger than intended.