Monitoring the U.S. legal unit of resistance via the quantum Hall effect

The quantum Hall effect is being used to monitor the resistances of the five 1-Ω Thomas-type resistors which define the U.S. legal unit of resistance, the ohm maintained at the National Bureau of Standards (Ω
NBS
). Typically, the total one-standard-deviation (1σ) accuracy for the transfer between three different GaAs quantum Hall devices and the five 1-Ω resistors is ±0.05 ppm. Measurements to date provide the first direct evidence that the value of Ω
NBS
is decreasing by about (0.05 ± 0.02) ppm per year.