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 Australian National Measurement Institute (NMIA) and the BIPM are constructing two new and improved calculable capacitors based on a previous design by the CSIRO of Australia which was developed and built about 40 years ago. 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.
The following pages give an introduction to the project, an update on its current status and details of the next steps.