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The NIST’s watt balance is a project of the U.S. Government to develop an “electronic kilogram.” The vacuum chamber dome, which lowers over the entire apparatus, is visible at top.

The watt balance is an experimental electromechanical apparatus that may one day provide a definition of the kilogram based on electronics.

The watt balance is a more accurate version of the ampere balance, in which the force between two current-carrying coils is measured and then used to calculate the magnitude of the current. The principle of the watt balance was proposed by B. P. Kibble of the UK's National Physical Laboratory (NPL) in 1975.

The main weakness of the ampere balance method is that the result depends on the accuracy with which the dimensions of the coils are measured. The watt balance method has an extra calibration step in which the effect of the geometry of the coils is eliminated, removing the main source of uncertainty. This extra step involves moving the force coil through a known magnetic flux at a known speed.

The present accuracy record is held by NIST, the standards agency of the USA, with an error of 5.2 × 10^-8, and experiments are continuing towards a goal of 1 × 10^-8. The long-term goal of these experiments is to produce a new definition of the kilogram based on fundamental SI units, to replace the present definition based on the International Prototype Kilogram, a cylinder of platinum/iridium owned by the International Bureau of Weights and Measures.

External links

• R. Steiner, E.R. Williams, D.B. Newell and R. Liu. "Towards an electronic kilogram: an improved measurement of the Planck constant and electron mass." Metrologia. (October 2005)
• Joshua P. Schwarz, Ruimin Liu, David B. Newell, Richard L. Steiner, Edwin R. Williams, Douglas Smith "Hysteresis and Related Error Mechanisms in the NIST Watt Balance Experiment" (August 2001)
• Bureau International des Poids et Mesures
• Swiss Federal Office of Metrology

Wikipedia content modification information:

• This page was last modified on 23 July 2008, at 12:04.

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