From Counting Electrons to Calibrating Ammeters: Improved Methodologies for Traceable Measurements of Small Electric Currents

New technology, the ultra stable low-noise current amplifier and the electron pump, provide new methods for making traceable measurements of small DC electric currents. We review four traceability routes for small current measurements and discuss the merits of each one. We present three case studies of small current calibrations, highlighting the role of noise and drifting instrument offsets. We show how the Allan deviation is used as a statistical tool for designing a calibration cycle to correctly eliminate drifting instrument offsets from calibration data. We also present a simplified noise model for a low-current ammeter which predicts a lower limit to the achievable statistical uncertainty in a calibration.