Nick Fletcher

Electrical Units in the New SI: Saying Goodbye to the 1990 Values

Abstract: The proposed redefinition of several International System (SI) base units is a topic that has been on the metrology agenda for the last decade. Recent progress on several determinations of the fundamental constants means that we now have a good idea of the defined numerical values that will be given in the new system to the Planck constant, h, and the elementary charge, e. This is especially relevant to electrical metrology as new numerical values for the von Klitzing and Josephson constants, given by the relations RK = h/e2 and KJ = 2e/h, will replace the existing 1990 ‘conventional’ values, RK-90 and KJ-90. The implementation of the new system cannot be done without introducing small step changes into sizes of the electrical units that are disseminated using Josephson and quantum Hall intrinsic standards. At the time of writing it looks likely that the relative change from KJ-90 to KJ will be of the order 1 × 10−7, and that from RK-90 to RK will be approximately 2 × 10−8. This paper discusses the practical impact of these changes on electrical metrology and highlights the long term benefits that will come from the updated system. The CCEM (Consultative Committee for Electricity and Magnetism) of the International Committee for Weights and Measures is now taking the first steps to ensure a smooth implementation, most probably in 2018.

Progress towards a determination of RK at the BIPM

This paper describes the progress on the implementation of a calculable cross capacitor for the measurement of RK at the BIPM. The main electrodes are installed and the alignment process has been demonstrated. The optical interferometer required to measure the guard displacement is operational. The chain of bridges for the transfer from RK to 1 pF exists, and first measurements can be expected during 2012.

Behavior of 1 Ω resistors at frequencies below 1 Hz and the problem of assigning a dc value

This paper presents a systematic study of the effects of reversals of current polarity on 1 Ω standard resistors. A selection of 1 Ω standards were measured on a dc cryogenic current comparator bridge and also on an ac current comparator bridge operating at 1 Hz. We find that the apparent 4 terminal resistance depends strongly on the dc reversal rate, in one case showing a variation of more than 0.1 μΩ/Ω for cycle times between 10 s and 1000 s. Even in the best cases, effects of at least 0.01 μΩ/Ω are present. Furthermore, the assumption that the apparent resistance value converges to a limiting value for long waiting times after reversal is not always upheld, making it difficult to quote a simple dc value. Existing models of Peltier heating at wire junctions do not completely explain our observations. The problems presented have been the limiting factor for 1 Ω measurements as part of a recent on-site comparison of quantum Hall effect resistance standards.

Testing the new generation of low-frequency current comparators

Improved magnetic materials open the perspective of operating room-temperature current comparators at lower frequency without compromising metrological accuracy. Two prototypes of low-frequency current comparator (LFCCs) have been successfully tested in precision resistance ratio measurements at drive frequencies of 0.5 Hz and 1 Hz using the BIPMs bridge setup with a recently realized frequency extension. These measurements have been performed using standard resistors of 10 kΩ, 1 kΩ and 100 Ω nominal values as well as the quantized Hall resistance (QHR) of GaAs and graphene devices.

A low-frequency current comparator for precision resistance measurements

Two prototypes of a current comparator for operation at low frequencies in the 0.1 Hz to 1 Hz range and at room temperature have been realized. Experimental tests include the determination of the frequency-dependent sensitivity and a comparative measurement of the ratio of two standard resistors with nominal values of 1 kΩ and 100 Ω. The realized current comparator is the main ratio standard in a precision ac resistance bridge operated without need of liquid helium which will be one of the main measurement instruments for a future graphene-based quantum Hall resistance standard.

Standard deviation of the mean and other time series properties of voltages measured with a digital lock-in amplifier

We investigate some statistical properties of ac voltages from a white noise source measured with a digital lock-in amplifier equipped with finite impulse response output filters which introduce correlations between successive voltage values. The main goal of this work is to propose simple solutions to account for correlations when calculating the standard deviation of the mean (SDM) for a sequence of measurement data acquired using such an instrument. The problem is treated by time series analysis based on a moving average model of the filtering process. Theoretical expressions are derived for the power spectral density (PSD), the autocorrelation function, the equivalent noise bandwidth and the Allan variance; all are related to the SDM. At most three parameters suffice to specify any of the above quantities: the filter time constant, the time between successive measurements (both set by the lock-in operator) and the PSD of the white noise input, h0. Our white noise source is a resistor so that the PSD is easily calculated; there are no free parameters. Theoretical expressions are checked against their respective sample estimates and, with the exception of two of the bandwidth estimates, agreement to within 11% or better is found.

A new CCEM-K4 comparison of capacitance at 10 pF

The CCEM-K4 key comparison of capacitance at 10 pF was last carried out between 1996 and 1999. The Consultative Committee for Electricity and Magnetism of the International Committee for Weights and Measures decided that it should be repeated in 2016. We present here the main technical choices adopted for the next comparison.

Experimental study of carrier density tuning in epitaxial graphene QHR devices

Adjustment of the charge carrier density in quantized Hall resistance (QHR) devices based on epitaxial SiC graphene films can be performed using photochemical and chemical gating. Our experiments show that the carrier concentration in fabricated graphene devices can be reversibly varied by either treatment with UV light or using molecules from aqua ammonia. Treatment of SiC graphene QHR devices in such a way provides tuning of carrier concentration to optimum values, in the range (1-3) × 1011 cm-2, at which the dissipation in QHR regime is reduced and the full quantization of Hall resistance is expected.

Progress report on measuring the von Klitzing constant at the BIPM

A project aiming at measuring the von Klitzing constant with an uncertainty of the order of one part in 108 is currently ongoing at the BIPM. This measurement will be performed by linking a calculable cross-capacitor to a quantum Hall resistor through a quadrature transfer. The main parts of the overall measuring setup as well as the current status of the project are presented.

A measurement chain for the determination of RK using a calculable capacitor

The BIPM has an ongoing project to establish a Thompson Lampard calculable capacitor for use in the determination of the von Klitzing constant, RK. This paper describes key parts of the measurement chain used to provide the link between the dc quantum Hall effect and the calculable capacitor. The bridge used to calibrate a conventional 1 pF standard against a known 0.4 pF capacitance change is presented. The target relative uncertainty for the overall measurement is 1 part in 108.