Methods for aligning the NIST watt-balance

Abstract

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 is kept as an artifact. The watt-balance uses a movable coil in a radial magnetic field to compare the mechanical energy required to lift a kilogram mass in earth's gravity with the electrical energy required to move the coil the same distance in a magnetic field. The electrical energy is monitored in terms of quantized Hall resistance and Josephson junction voltage standards. The accuracy of this experiment depends on a large number of factors. Among them are the ability to align the apparatus so that the movable coil and magnet are coaxial and aligned to the local vertical. Misalignments of the coil and magnet result in forces and torques on the coil. The coil is suspended like a pendulum and responds easily to these torques and horizontal forces. This paper describes a computer program that was written to calculate the shape of the magnetic field and the torques and forces on the movable coil that result from any misalignments. This information is being used to develop an alignment procedure that minimizes misalignments and the errors they cause. This program has enhanced our understanding of the cause of torques about the vertical axis on the coil and the dependence of this torque on the magnetic field gradient.