Construction and performance of the NIST-4 magnet system

A watt balance is a promising instrument to realize the unit of mass at the kilogram level. The magnet system is one of the key elements of such an instrument. 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 construction of the magnet system, first measurements of the field profile, and techniques that were used to achieve an ideal field profile.