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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. [ABSTRACT FROM AUTHOR]

The four-terminal-pair (4TP) definition of impedance standards allows reaching ultimate accuracy in impedance metrology. In general, the 4TP definition requires dedicated circuitry to be included in the bridge network and the attainment of auxiliary bridge balances during the measurement. A careful choice of the network topology allows the bridge to be self-compensating: the 4TP definition is in large part achieved by the behavior of the network itself without any adjustment. The additional circuitry required for impedance definition can therefore be simpler, the balancing procedure becomes easier, and a more robust 4TP can be achieved. This paper analyzes a current comparator bridge network with a topology featuring self-compensation. A digitally assisted test bridge has been implemented; test measurements on ac resistance comparison in the 25–100- $\Omega $ range at kilohertz frequencies are reported. [ABSTRACT FROM AUTHOR]

We present the latest improvements in the traceable measurement and generation of small electric currents. A central tool in our traceability chain for small direct currents is a new binary cryogenic current comparator (CCC) with a total of 18 276 turns. This 14-bit CCC is well suited for the calibration of high-value resistors and current amplifiers, but also for the direct amplification of small currents. A noise level of 5 fA/ \surd Hz at 0.05 Hz is routinely achieved. The systematic uncertainty due to noise rectification was exemplarily investigated in a ratio-error test configuration, showing that a total uncertainty of about one part in 10^{6} can be achieved at 100 pA. For further improvement, a new instrument was developed, the ultrastable low-noise current amplifier (ULCA). Its transfer coefficient is highly stable versus time, temperature, and current amplitude within a full dynamic range of ±5 nA. The ULCA is calibrated with the 14-bit CCC at high current amplitude, and allows the measurement or generation of 100-pA direct current with an uncertainty of one part in 10^{7}$ . The novel setup was successfully used to investigate the uncertainty of the established capacitor charging method. A quantum metrology triangle experiment based on the presented instruments is proposed. [ABSTRACT FROM PUBLISHER]

A new current-comparator-based dual-channel calibration system for AC currents and AC voltages in the millivolt range has been developed. The range of generated currents is 1 mA–5 A, and the range of generated voltages is 1–500 mV. The calibration system in-phase and quadrature expanded measurement uncertainties ( $k=2$ ) are within 25\cdot 10^{\mathrm {\mathbf {-6}}} . Although the system has been developed primarily for operation at power frequencies, it is also capable of operating at frequencies up to 1000 Hz with slightly increased uncertainties. The traceability to SI units is established through the calibration of AC voltage by means of AC/DC transfer and DC voltage reference, standard resistor, capacitor, and current comparator ratios. [ABSTRACT FROM PUBLISHER]

We present here the concept of three-arm current comparator impedance bridge, which allows one to perform comparisons among three unlike impedances. Its purpose is the calibration of impedances having arbitrary phase angles against calibrated nearly pure impedances. An analysis of the bridge optimal setting and proper operation is presented. To test the concept, a two-terminal-pair digitally assisted bridge has been realized; preliminary measurements of an air-core inductor and of an $RC$ network versus decadic resistance and capacitance standards, at kilohertz frequency, have been performed. The bridge measurements are compatible with previous knowledge of the standard values with relative deviations of parts in 10^{-5} . [ABSTRACT FROM PUBLISHER]