The calculable capacitor allows the realization of the SI unit of capacitance, the farad, and by use of a quadrature bridge, the SI unit of resistance, the ohm.
The calculable capacitor is based on a theorem in electrostatics (described by A. Thompson and D. Lampard of CSIRO-NML in 1956) which allows the calculation of the capacitance of a special type of capacitor directly from a single dimensional measurement which can be made traceable to the SI unit of length.
The CSIRO of Australia developed and built the first calculable capacitor about 40 years ago. This instrument is routinely operated and enables a capacitance of about 0.25 pF to be realized with an uncertainty of 5 parts in 108. The accuracy is limited by geometrical imperfections of the capacitor's structure.
Since the 1960s, fabrication techniques have improved considerably and the Australian National Measurement Institute (NMIA) and the BIPM are therefore constructing two new and improved calculable capacitors based on the previous design. At the conclusion of the project, each institute will have, and maintain, one instrument. Although the basic mechanical design will be similar, the measurement systems associated with them will differ, so helping to provide independent realizations. The design goal is an uncertainty of the order of 1 part in 108. Since the start of the project the National Research Council Canada (NRC) and the National Institute of Metrology (NIM), China, have joined. Each will build their own instrument using the same precision electrodes although some elements of the overall design will be different.
Final assembly of the calculable capacitor at the BIPM commenced in 2010. The original timeline of the project was to deliver a new measured value of the von Klitzing constant, RK, in time for 2010 CODATA fundamental constants adjustment. Although this is no longer possible, a result can be expected in 2011, which will be highly relevant for the continuing discussions on unit redefinitions.