To avoid such unwanted effects, PTB developed a special shielding technique.Īccording to the recommendation of the CIPM, the quantum Hall resistance is referred to R K-90, to ensure the best possible agreement with the SI farad. It is important to make sure that the ac value of the quantum Hall resistance agrees with the quantised dc value and in particular does not deviate due to parasitic ac dissipation. Δ is the (usually very small) relative deviation of the 10 nF capacitance standard from nominal and is determined from a bridge balance system which, for the sake of simplicity, is not shown in the figure above. With ω = 2π f the angular frequency and f = 1233,147 Hz the frequency, traced to the frequency standard of PTB ( Department 4.4). Using Ohm’s law I = U/ R H for the left arm of the bridge and I = ωCU for the right arm of the bridge (which by the way is the definition of resistance and capacitance, respectively), the capacitance of the standard under calibration can be expressed in terms of the known quantum Hall resistance R H: Note that the same alternating current I flows through both standards. The following figure shows the scheme of such a quadrature bridge: The capacitance unit is realised at PTB by means of a so-called quadrature bridge which links a 10 nF capacitance standard under calibration to the known quantum Hall resistance. The realisation of the capacitance unit at PTB by coaxial measuring bridges Often used capacitance standards are commercial parallel-plate capacitors made of Invar and thermostated fused-silica standards because they, amongst other features, have a very small dissipation factor.ġ nF capacitor of the type "General Radio 1404 A", for didactic purposes with a cut case to make the stack of parallel capacitor plates visible. Therefore, in the following only the ac capacitance is considered.
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