ABSTRACT
H. L. Curtis and R. W. Curtis published an paper, An Absolute Determination of the Ampere, in 1934. This measurement used a Rayleigh current balance, in which the electromagnetic force between concentric coils is balanced by the gravitational force on a mass. By 1944, three absolute-ampere and three absolute-ohm experiments had been completed at the Bureau, and similarly accurate absolute determinations of the ampere and ohm were available from Britain. Absolute experimental determinations of units are known as SI realizations, and the uncertainty of the SI values of the electrical units are limited by the uncertainty of their realizations in terms of the kilogram, meter, and second. The largest uncertainties in the National Institute of Standards and Technology watt experiment of the 1990s arose from operating in air, which required that the changing air buoyancy and refractive index be calculated from many readings of pressure, temperature, and humidity sensors.
