Larger value and SI measurement of the improved cryogenic capacitor for the electron-counting capacitance standard

We report on several advances in the development of a cryogenic vacuum-gap capacitor C_cryo, for use with the electron-counting capacitance standard (ECCS). First, we have increased the value by about a factor of ten, to 10 pF; this will both make the ECCS more useful as a commercial standard and also allows a substantial reduction in the relative uncertainty of the calibration of C_cryo. Second, the capacitor´s stability is excellent, with a relative drift less than 10^-9/hour. This stability is required for the third advance: we have succeeded in tuning the calculable capacitor, which allows us to make a measurement of SI units, without requiring us to fabricate the capacitor to have a precise value of C_cryo. We demonstrate such a measurement, with an uncertainty of about 4×10^-8.