Origin
The principle of the watt balance was proposed by B. P. Kibble of the UK 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 watt balance originating from the British National Physical Laboratory has been transferred to the National Research Council of Canada and is currently being recommissioned. Other watt balance experiments are being undertaken in the US National Institute of Standards and Technology (NIST), the Swiss Federal Office of Metrology (METAS) in Berne, the International Bureau of Weights and Measures (BIPM) near Paris and Laboratoire national de métrologie et d’essais (LNE) in Trappes, France. The present accuracy record is held by the U.S. NIST, with a relative uncertainty of 3.6×10−8, and experiments are continuing towards a goal of 1×10−8.
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