NIST researchers two steps closer to electron-counting capacitance standards. (News Briefs).

A team of NIST scientists in collaboration with an intern from the University of Maryland, has made two advances in the development of cryogenic capacitors for use with the Electron-Counting Capacitance Standard (ECCS). The ECCS is being developed as a quantum-based representation of capacitance. It relies on pumping a precisely counted number of electrons onto the plates of a cryogenic capacitor and measuring the resultant charging voltage. About 100 million electrons are required to reach the desired voltage. Besides providing a "turnkey" primary capacitance representation, the ECCS will also provide an alternate route for experimental determination of the fine-structure constant.

Two recent advances have been implemented for the cryogenic capacitor. First, physical design changes have improved its stability and allowed an increase of its nominal value by an order of magnitude to 10 pF. This larger value makes the ECCS more attractive as a turnkey representation because it should support lower uncertainties for typical commercial capacitance calibrations. This increase in value was made possible by improved machining tolerances, which allowed fabrication of the device with a gap between the two capacitor plates of about 0.05 mm over a distance of about 10 mm. An additional benefit of the improved machining tolerances is that the capacitance value is within 1 % of nominal, which is important for use with much of our precision measurement instrumentation. The new device exhibits excellent stability; after equilibration at low temperatures, its capacitance drift is less than about [10.sup.-9] per hour.

The second advance was the performance of a preliminary measurement of the cryogenic capacitor directly against the NIST calculable capacitor. The calculable capacitor is a large mechanical capacitor that, through a fundamental theorem of electrostatics, provides the SI realization of the Farad through a measurement of displacement. Measurement against the calculable capacitor is crucial for the ECCS, both in terms of demonstrating its usefulness as a capacitance representation and for the measurement of the fine-structure constant. The stability of the cryogenic capacitor was an essential requirement for this measurement. The measurement noise in the comparison of the capacitor at low temperature against the calculable capacitor was roughly the same as observed in the routine measurements involving the calculable capacitor.

Some further refinements are needed. In addition to a full characterization of its performance and error budget, a more accurate means is needed for the course determination of the capacitance value; routine measurement against the calculable capacitor only yields capacitance values modulo 1.5 x [10.sup.-6].

CONTACT: Michael Kelley, [phone]; [e-mail].