Uncertainty budget for the NIST electron counting capacitance standard, ECCS-1

We measure a cryogenic, vacuum-gap capacitor by two methods: (1) charging it with a known number of electrons and measuring the resulting voltage, and (2) using a capacitance bridge traceable to the SI farad. We report a detailed uncertainty budget for the comparison of the two methods and find that they agree within a relative standard uncertainty of 9.2 × 10−7. This comparison closes the quantum metrology triangle with the same uncertainty.