부산대학교 도서관

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 $2 \times 10^{-7}$ . 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- $\text{G}\Omega $ resistors with relative uncertainties at the $10^{-7}$ level.