  New technology, the ultrastable 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.
