  In early 2014, construction of a new watt balance, named NIST-4, has started
at the National Institute of Standards and Technology (NIST). In a watt
balance, the gravitational force of an unknown mass is compensated by an
electromagnetic force produced by a coil in a magnet system. The
electromagnetic force depends on the current in the coil and the magnetic flux
integral. Most watt balances feature an additional calibration mode, referred
to as velocity mode, which allows one to measure the magnetic flux integral to
high precision. In this article we describe first measurements of the flux
integral in the new watt balance. We introduce measurement and data analysis
techniques to assess the quality of the measurements and the adverse effects of
vibrations on the instrument.
