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The principle of modified active compensatory electrode
The project of Research of the Key Measurement Technique for Fine Structure Constant and Capacitance Standard is one of the key program Research of Modern Metrological Standards on Basis of Quantum Physics in National Science & Technology Pillar Program during the Eleventh Five-year Plan Period. Its aim is to build a new set of calculable capacitor and to measure constants for fine-structure with the aid of quantum hall resistance standard at NIM. Hopefully, it is to contribute to redefinition of international basic units (SI units).
In collaboration with National Metrology Institute of Australia (NMIA), a new calculable capacitor at NIM is realized with four vertically arranged stainless electrodes manufactured by NMIA, whose lengths can be accurately measured by a Fabry-Perot interferometer. The total measurement uncertainty of the new calculable capacitor system is expected to be within 1E-8.
In order to reduce errors introduced by end-effect which causes an overwhelming amount of uncertainty in the new calculable capacitor, a modified active compensatory electrode with a transition zone is considered to replace the normally used mechanical nail head. The method of using a compensatory electrode was first proposed by NIM in 1980s and now it is developed and modified to perform better. In this method, an appropriate voltage is applied to an active nail head fitted on the end face of the movable guard electrode. Its static electrical field distribution will be similar to the nail head. The primary experiment results obtained using the primitive model device with same dimensions as an actual new calculable capacitor show that the modified active compensatory electrode performs no poorer than an equivalent geometrical compensating nail head.