METHODS FOR ALIGNING THE NIST WATT-BALANCE

The NIST watt-balance has been developed to explore the possibility of monitoring the stability of the mass standard by means of electrical quantum standards, The mass standard is the last basic standard that i s kept as an artifact, The watt-balance uses a movable coil in a radia l magnetic field to compare the mechanical energy required to lift a k ilogram mass in earth's gravity with the electrical energy required to move the coil the same distance in a magnetic field, The electrical e nergy is monitored in terms of quantized Hall resistance and Josephson junction voltage standards, The accuracy of this experiment depends o n a large number of factors, Among them are the ability to align the a pparatus so that the movable coil and magnet are coaxial and aligned t o the local vertical, Misalignments of the coil and magnet result in f orces and torques on the coil, The coil is suspended like a pendulum a nd responds easily to these torques and horizontal forces, This paper describes a computer program that was written to calculate the shape o f the magnetic field and the torques and forces on the movable coil th at result from any misalignments, This information is being used to de velop an alignment procedure that minimizes misalignments and the erro rs they cause, This program has enhanced our understanding of the caus e of torques about the vertical axis on the coil and the dependence of this torque on the magnetic field gradient.