Design of a simple thermal control for gas-dielectric standard capacitors

3 p. : il.A two terminal-pair coaxial capacitance bridge was built recently at Inmetro. It operates mainly at 1 kHz and 1.592 kHz and compares decadic capacitors in the range from 10 pF to 1 nF at the ratios 1:1 and 10:1 and 1:10, with an uncertainty of about one part in 108. The 1 nF standard capacitors commercially available are made from invar with gas dielectric. Such capacitors have a temperature coefficient of (0 ± 2)μF/F per K. This is too high if one plans to use them in a traceability chain to derive the capacitance unit from the quantum Hall resistance. The design of a simple thermal control for gas-dielectric capacitors to reduce by two orders of magnitude their temperature variation is presented here