Ronald F. Dziuba

Low temperature direct current comparators

This paper presents an improved version of the low temperature direct current comparator described originally by Harvey. The comparator described herein exhibits a resolution of 1.5 nA ·turn and provides ratios which have uncertainties of less than 4 parts in 1010. An alternate approach to the shielding of the windings is discussed and a preliminary comparator based on this concept is shown to have equally good performance. The latter comparator is probably better suited to the attainment of large current ratios.

NBS determination of the fine-structure constant, and of the quantized Hall resistance and Josephson frequency-to-voltage quotient in SI units

Results of US National Bureau of Standards (NBS) experiments to realize the ohm and the watt, to determine the proton gyromagnetic ratio by the low-field method, to determine the time dependence of the NBS representation of the ohm using the quantum Hall effect, and to maintain the NBS representation of the volt using the Josephson effect, are appropriately combined to obtain an accurate value of the fine-structure constant and of the quantized Hall resistance in SI units, and values in SI units of the Josephson frequency-to-voltage quotient, Planck constant and elementary charge. >

Determination of the von Klitzing constant and the fine-structure constant through a comparison of the quantized Hall resistance and the ohm derived from the NIST calculable capacitor

This paper describes a recent determination of the von Klitzing constant and the fine-structure constant by comparisons of values of the ohm as defined in the International System of Units (SI), derived from the National Institute of Standards and Technology (NIST) calculable cross-capacitor, and values of the international practical unit of resistance derived from the integral quantum Hall effect. In this determination, the comparisons were made in a series of measurements lasting three years. A small difference is observed between this determination and an earlier comparison carried out in this laboratory and reported in 1988. The most recent value of the fine-structure constant based on the experimental value and theoretical expression for the magnetic moment anomaly of the electron, which has the smallest uncertainty of any value currently available, is consistent with both of these results. The new value exceeds the 1990 conventional value of the von Klitzing constant RK-90 by slightly more than twice the relative standard uncertainty of the present measurement, which is 2.4 × 10-8.

NIST comparison of the quantized Hall resistance and the realization of the SI ohm through the calculable capacitor

The latest NIST result from the comparison of the quantized Hall resistance (QHR) with the realization of the SI ohm obtained from the NIST calculable capacitor is reported. A small difference between the 1988 result and the present result has led to a re-evaluation of the sources and magnitudes of possible systematic errors.

NIST Measurement Service for DC Standard Resistors

At the National Institute of Standards and Technology (NIST), the U.S. representation of the ohm is based on the quantum Hall effect, and it is maintained and disseminated at various resistance levels by working reference groups of standards. This document describes the measurement systems and procedures used to calibrate standard resistors and current shunts of nominal decade values in the resistance range from 10 Ω to 10 Ω. Resistance scaling techniques used to assign values to the working standards are discussed. Also included is an assessment of the calibration uncertainties at each resistance level.

A Low-Temperature Direct-Current Comparator Bridge

The application of superconducting direct-current comparators to the measurement of resistance ratios is described. One comparator consists of a binary set of ratios between 1:1 and 160:1 providing for self-calibration by a buildup procedure. A second comparator exhibiting discrete ratios of 1:1, 10:1, and 100:1 is also described. Ratio uncertainty of less than 1 part in 109 is achieved by enclosing the ratio windings in overlapping toroidal superconducting shields. Superconducting quantum interference devices (SQUIDs) serve as flux sensors for the comparators. One of these current comparators is used to calibrate a 100-?:1-? resistive divider, which at a current of 10 mA exhibits a self-heating error of 0.0023 ppm.

Fabrication of high-value standard resistors

The National Institute of Standards and Technology (NIST) has fabricated stable, transportable 10 M/spl Omega/ and 1 G/spl Omega/ standard resistors for use in an international comparison of high resistances. This fabrication process is being applied to the construction of standard resistors of values up to 10 T/spl Omega/, with initial results indicating significant improvements in stability and fewer adverse effects induced by mechanical shock and vibration.

Final report on key comparison CCEM-K2 of resistance standards at 10 M? and 1 G?

An international comparison of dc resistance at 10 M? and 1 G? was organized under the auspices of the Consultative Committee for Electricity and Magnetism (CCEM) and piloted by the National Institute of Standards and Technology (NIST, USA). This CCEM comparison began in August 1996 and was completed in March 2000 with the participation of fourteen other national metrology institutes (NMIs). The travelling package included three wirewound 10 M? standards and three film-type 1 G? standards designed by the NIST. Results indicate that the differences at 10 M? and 1 G? between each laboratorys values and the respective reference value, are all within each laboratorys expanded relative uncertainty at a coverage factor k = 2.

New realization of the ohm and farad using the NBS calculable capacitor

Results of a realization of the ohm and farad using the US National Bureau of Standards (NBS) calculable capacitor and associated apparatus are reported. The results show that both the NBS representation of the ohm and the NBS representation of the farad are changing with time, Omega /sub NBS/ at the rate of -0.054 p.p.m./year and F/sub NBS/ at the rate of 0.010 p.p.m./year. The realization of the ohm is of particular significance because of its role in assigning an SI value to the quantized Hall resistance. The estimated uncertainty of the ohm realization is 0.022 p.p.m. (1 sigma ), while the estimated uncertainty of the farad realization is 0.014 p.p.m. (1 sigma ). >

Cryogenic direct current comparators and their applications

Cryogenic direct current comparators, utilizing superconducting shields and Superconducting Quantum Interference Devices (SQUIDs), provide current ratios of up to 100/1 or higher with accuracies of \lsim 1 \times 10^{-9} and current resolutions of \lsim 6 \times 10^{-11} A. Two types of comparators differing in the shielding arrangement of the ratio windings are described. One type consists of unit windings inside a seamless Pb tube; the other consists of multi-turn ratio windings within an overlapping toroidal superconducting shield. For both shielding configurations, SQUIDs serve as flux sensors for the comparators. The application of these comparators to the measurment of resistance ratios is described.