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 The measurement chain for linking the calculable capacitor to the quantum Hall resistance is the same as that used for regular maintenance of capacitance, and is thus in good working order. One extra bridge is required for the first step from the calculable capacitor to 1 pF, and this has been tested using fixed 0.2 pF standards and 0.6 pF standards. The tests were limited by the stability of these standards, but served to show that there are no major problems with the bridge. A full evaluation of the bridge can only be done with the calculable capacitor itself, and this will form part of the measurements planned for early 2011. The new resistance bridge dedicated to measuring a pair of 51.6 kΩ resistors (which forms part of the BIPM quadrature bridge) directly against the quantized Hall resistance is also under construction. It will shorten the measurement chain linking the QHR and the BIPM reference capacitors, with the aim of reducing the overall uncertainties, and will form part of the final measurement chain.
The measurement task will be to relate the capacitance implemented by the new instrument (around 0.2 pF) to the standards maintained by the BIPM in terms of the quantum Hall effect (a quantized resistance of around 13 kΩ). The result can be interpreted in two ways: as a direct SI measurement of the von Klitzing constant, RK, or as a purely electrical determination of the fine structure constant, α.
Regardless of the interpretation, the aim is the same: to contribute new experimental data on the fundamental constants as part of the effort to ensure any future changes to the SI are based on solid metrology. The completed calculable capacitor should also serve as a practical capacitance standard for BIPM's calibration services and comparisons work.
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Summary
