Nist researchers complete uncertainty analysis for linking ECCS to SI farad. (General Developments).

NIST researchers have completed an uncertainty analysis for the comparison of a cryogenic vacuum-gap capacitor against the NIST calculable capacitor. This work is a critical step in the development of the Electron-Counting Capacitance Standard (ECCS).

The essential idea of the ECCS is to place an accurately known electric charge, by counting individually each of about [10.sup.8] electrons, onto the plate of a cryogenic vacuum-gap capacitor and then to measure the charging voltage with high accuracy. The capacitance of the device is determined directly from the charge-to-voltage ratio. Three valuable uses have been identified for the ECCS. It will serve as a "turnkey" primary representation for capacitance, provide an additional route for the measurement of the fine-structure constant and provide a means to close the "quantum metrology triangle" formed by the Josephson voltage, the quantum Hall resistance, and the fundamental electron charge.

A recently reported advance for the capacitor was to increase its capacitance to a value of approximately 10 pF (within a few percent), thus allowing a direct comparison of the vacuum-gap capacitor to the NIST calculable capacitor using existing high-precision instrumentation. The calculable capacitor is a large mechanical capacitor which, through a fundamental theorem of electrostatics, provides the SI realization of the Farad. The comparison of the two capacitors is crucial for the ECCS, both for demonstrating its usefulness as a representation of capacitance, and for determination of the fine-structure constant or closure of the quantum metrology triangle.

The advance noted here is the completion of the detailed uncertainty analysis of this comparison, including statistical and systematic uncertainties. This analysis shows that the total relative uncertainty of the comparison is about 4 X 10 (-8). This compares favorably with the total relative uncertainty of 4X 10 (-8), which is achieved for the highly optimized comparisons against the calculable capacitor that are performed as part of the regular maintenance of NIST's primary bank of capacitance standards. Indeed, this measurement of the cryogenic capacitor was designed to utilize, as closely as feasible, the same instrumentation and measurement procedures as those routinely used for measurements of the room-temperature transfer standard. The close similarity both simplified this uncertainty analysis and contributed to the ultimate quality of the result. Future improvements in the measurement system could reduce the uncertainty to a value even closer to that for the highly optimized measurements.

With the completion of the uncertainty analysis, the new 10 pF cryogenic capacitor is now fully ready to be used in the ECCS, with a route to tie the ECCS to the SI unit of capacitance with an uncertainty of about 4X[10.sup.-8].

A paper describing this work has been submitted for publication in the IEEE Transactions on Instrumentation and Measurement.

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