Fabrication and Measurement of the Main Electrodes of the NMIA-BIPM Calculable Cross Capacitors

Two new calculable cross capacitors are being constructed by the National Measurement Institute of Australia and the Bureau International des Poids et Mesures, with the aim of limiting sources of uncertainty to 1 nF/F, where achievable. In particular, the main electrodes, which are four stainless steel bars 470 mm long and 50 mm in diameter, must be geometrically accurate to 100 nm. An automated system based on capacitance probes has been constructed for the purpose of measuring the geometry of the bars with an uncertainty of around 20 nm. Two sets of electrodes have been finished to the required accuracy, although further improvement may be possible.