Table of contents

Techniques related to the establishment and the comparison of primary standards of resistance and capacitance are described and discussed. In the field of resistance, the characteristics of primary standard resistors of 1 Ω and 10 000 Ω are presented and dc resistance-ratio measurement techniques reviewed, including the series-parallel method, the Warshawsky bridge and the cryogenic current comparator bridge. A review of techniques and results regarding very low-frequency ac measurements of dc standard resistors is also given. In the field of capacitance, the realization of the farad using calculable standards is discussed as well as the techniques for accurate measurements of four terminal-pair impedances used in particular for linking the farad and the ohm.

The paper discusses the recent measurements of the fundamental physical constants whose various combinations serve to provide the natural units of measurement in physics and the evaluation of their recommended values by the CODATA Task Group on Fundamental Constants. It emphasizes the strong involvement of the SI units with the present evaluations and shows how the fundamental physical constants play an increasing role in defining and maintaining many of the SI units of today and in sustaining the integrity of our measurement system. The present situation is reviewed and the evolving role of the fundamental physical constants in science and technology is discussed.

The second is the only unit of the SI system which is defined in terms of an internal property of an atom. The definition is realized using a thermal beam of caesium atoms. In classical caesium beam frequency standards, the necessary state selection is accomplished by deflecting atoms in magnet state selectors. In devices under development, this state selection is performed through atom-light interaction. The physical bases underlying the caesium beam frequency standards are given. The main design characteristics of caesium beam tubes are described. The limitations to the properties of metrological interest, frequency stability and accuracy are analysed. In particular, the principal sources of frequency offsets are described. The potentialities of classical and of optically pumped caesium beam frequency standards are compared. Manufactured devices designed for field use are considered as well as laboratory primary frequency and time standards.

This paper summarizes the principles, historical background and present status of three primary areas of rf and microwave measurements and standards: circuit parameter measurement, power measurement, and noise generation and measurement. Both the reference standards and the techniques of measurement and of transfer to secondary standards are addressed. An extensive bibliography is provided to enable the interested reader to pursue areas to greater depth, and brief discussions to indicate likely directions of current and future work are included.

Josephson voltage standards with series arrays of up to 20 000 tunnel junctions can generate stable reference voltages at arbitrary values between - 15 V and + 15 V. These standards, which are now installed in many national and industrial laboratories, are used to calibrate Weston cells or Zener reference standards, to measure the linearity of digital voltmeters, and as precise potentiometers for the measurement of voltage and resistance ratios. The fundamental principles, the historical development, the design, the fabrication and the operation of these voltage standards are reviewed.

Following the discovery of the quantum Hall effect in 1980 metrologists were eager to develop the effect as a resistance standard in a similar manner to the development, a decade earlier, of the Josephson effects in superconductors as a voltage standard. This article reviews the progress made by many national laboratories to assess and compare the quality and limitations of semiconductor devices, mainly silicon MOSFETs and GaAs/AlGaAs heterostructures, as quantized Hall resistance standards with values R_H (i) = R_K/i = h/ie², where R_K = 25812,807 Ω is the von Klitzing constant and i is an integer. Potentiometric and cryogenic current comparator bridge techniques for the measurement of R_K in terms of conventional resistance standards with 1 σ relative accuracies better than 1 part in 10⁸ are compared, and determinations of R_K in ohms with accuracies as high as 2,4 parts in 10⁸ using calculable capacitors are described. The comparison of direct determinations of R_K with alternative derivations of h/e² leading to the CCE recommendations and subsequent international adoption, from 1 January 1990, of the value 25812,807 ± 0,005 Ω for R_K are also described.

Ac-dc transfer standards provide an essential link for the determination of ac electric quantities in terms of the SI units. Thermal converters are by far the most widely used form of primary ac-dc transfer standards. The basic principles of ac-dc transfer measurements are presented together with background to the performance and basic limitations of thermal converters as transfer standards. Developments in basic converters, voltage transfer standards and current transfer standards are reviewed, with emphasis on developments since 1985. Current activities and possible future directions are discussed.

Potential frequency standards based on laser cooled atoms are evaluated. We estimate that a standard based on an atomic fountain of cesium atoms can have an accuracy of Δν/ν = 10^-16. Frequency standards based on ion traps are also evaluated.