  We report on new calibration methods for picoammeters and low-current
sources. The "Ultrastable Low-noise Current Amplifier" (ULCA) was used for the
exemplary calibration of commercial state-of-the-art ammeter and current source
instruments in the current range between 1 femtoampere and 1 microampere. The
uncertainties achieved are compared with results obtained from two other
calibration methods for small direct currents. It is shown that using the ULCA
as calibrator allows achieving lower uncertainties together with the benefit of
easier handling compared to alternative techniques. Also, the ULCA allows
performing current meter and source calibrations between 1 femtoampere and 1
microampere with a single method.
