Determination of the time-dependence of Omega /sub NBS/ using the quantized Hall resistance

The quantum Hall effect is being used to monitor the US legal representation of the ohm, or as-maintained ohm, Omega /sub NBS/. Measurements have been made on a regular basis since August 1983. Individual transfers between the quantized Hall resistance R/sub H/ and the five 1- Omega resistors which comprise Omega /sub NBS/ can be made with a total of one standard deviation (1 sigma ) uncertainty of +or-0.014 p.p.m. This uncertainty is the root-sum-square of 32 individual components. The time-dependent expression for R/sub H/ in terms of Omega /sub NBS/ is: R/sub H/=25812.8(1+(1.842+or-0.012)*10/sup -6/=(0.0529+or-0.0040)(t-0.7785)*10/sup -6//year) Omega /sub NBS/, where t is measured in years from January 1, 1987. The value of Omega /sub NBS/ is, therefore, decreasing at the rate of (0.0529+or-0.0040) p.p.m./year.<
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