Interlaboratory Nanoamp Current Comparison With Subpart-Per-Million Uncertainty

An ultrastable low-noise current amplifier (ULCA) has been transferred between two laboratories, National Physical Laboratory (NPL) and Physikalisch-Technische Bundesanstalt, four times, with three of the transfers yielding a relative change in the transresistance gain of less than 2 × 10
^-7
. This is a new benchmark for interlaboratory transfer of small current. We describe in detail the use of a cryogenic current comparator to calibrate the ULCA at NPL, and the use of the ULCA to measure 1-GQ resistors with relative uncertainties at the 10
^-7
level.