  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.
