  An interlaboratory comparison of small-current generation and measurement
capability is presented with the ultrastable low-noise current amplifier (ULCA)
acting as travelling standard. Various measurements at direct currents between
0.16 nA and 13 nA were performed to verify the degree of agreement between the
three national metrology institutes involved in the study. Consistency well
within one part per million (ppm) was found. Due to harsh environmental
conditions during shipment, the ULCA's transfer accuracy had been limited to
about +/-0.4 ppm. Supplemental measurements performed at PTB indicate that
further improvements in accuracy are possible. Relative uncertainties of 0.1
ppm are achieved by applying on-site calibration of the ULCA with a suitable
cryogenic current comparator.
