Direct determination of capacitance standards using a quadrature bridge and a pair of quantized Hall resistors

We have used a quadrature bridge to compare directly two quantum Hall resistors with two 10-nF capacitors at 1233 Hz. This approach eliminates uncertainties due to thermal and structural instabilities and to AC/DC differences of the wire resistors normally used in an intermediate step. We have measured the capacitative interaction between the devices, and have developed a shielded header to suppress this effect. The value for the National Research Council primary standard 10-pF capacitor obtained by this method agrees with the expected value to within (0.1 +- 3.5)aF, or (0.01 +- 0.35) * 10^-6.