Terry Quinn

Redefinition of the kilogram, ampere, kelvin and mole: a proposed approach to implementing CIPM recommendation 1 (CI-2005)

The International System of Units (SI) is founded on seven base units, the metre, kilogram, second, ampere, kelvin, mole and candela corresponding to the seven base quantities of length, mass, time, electric current, thermodynamic temperature, amount of substance and luminous intensity. At its 94th meeting in October 2005, the International Committee for Weights and Measures (CIPM) adopted a recommendation on preparative steps towards redefining the kilogram, ampere, kelvin and mole so that these units are linked to exactly known values of fundamental constants. We propose here that these four base units should be given new definitions linking them to exactly defined values of the Planck constant h, elementary charge e, Boltzmann constant k and Avogadro constant NA, respectively. This would mean that six of the seven base units of the SI would be defined in terms of true invariants of nature. In addition, not only would these four fundamental constants have exactly defined values but also the uncertainties of many of the other fundamental constants of physics would be either eliminated or appreciably reduced. In this paper we present the background and discuss the merits of these proposed changes, and we also present possible wordings for the four new definitions. We also suggest a novel way to define the entire SI explicitly using such definitions without making any distinction between base units and derived units. We list a number of key points that should be addressed when the new definitions are adopted by the General Conference on Weights and Measures (CGPM), possibly by the 24th CGPM in 2011, and we discuss the implications of these changes for other aspects of metrology.

Redefinition of the kilogram: a decision whose time has come

The kilogram, the base unit of mass in the International System of Units (SI), is defined as the mass of the international prototype of the kilogram. Clearly, this definition has the effect of fixing the value of to be one kilogram exactly. In this paper, we review the benefits that would accrue if the kilogram were redefined so as to fix the value of either the Planck constant h or the Avogadro constant NA instead of , without waiting for the experiments to determine h or NA currently underway to reach their desired relative standard uncertainty of about 10−8. A significant reduction in the uncertainties of the SI values of many other fundamental constants would result from either of these new definitions, at the expense of making the mass of the international prototype a quantity whose value would have to be determined by experiment. However, by assigning a conventional value to , the present highly precise worldwide uniformity of mass standards could still be retained. The advantages of redefining the kilogram immediately outweigh any apparent disadvantages, and we review the alternative forms that a new definition might take.

Primary methods for the measurement of amount of substance

Primary methods of measurement play an important role in metrology because they provide the essential first link in the chain of traceability from the abstract definition of a unit of the International System of Units (SI) to its practical use in measurement. A definition of a primary method of measurement has been developed by the Consultative Committee for Amount of Substance (CCQM) that distinguishes between those methods that measure a quantity directly and those that measure the ratio of two quantities. Methods that do not fulfil the definition of being primary also play an important role in measuring amount of substance. In some cases, it is appropriate to refer to these as secondary methods.

Practical realization of the definition of the metre (1997)

In 1983, at the time of the adoption of the present definition of the metre by the 17th Conférence Générale des Poids et Mesures, the Comité International des Poids et Mesures (CIPM) drew up recommendations for the practical realization of the definition. These have formerly been referred to as the mise en pratique of the definition. It was understood that the practical realization would, from time to time, be updated to take account of new measurements and improvements in techniques of laser stabilization. In 1992 the CIPM, acting on the advice of the then Comité Consultatif pour la Définition du Mètre (CCDM), adopted a revision of the mise en pratique in its Recommendation 3 (CI-1992). The text was published in Metrologia (1993/94, 30, 523-541). In 1997, the CIPM again adopted a revised version of the practical realization of the definition of the metre in its Recommendation 1 (CI-1997). The text of this Recommendation is given here.* In addition, a revised list of recommended radiations, an appendix containing the source data used in estimating the wavelengths, frequencies and uncertainties of the recommended radiations, and an annotated bibliography are given. In adopting the revised practical realization, the CIPM acknowledged the considerable effort put into its preparation by the CCDM through its Working Group on the mise en pratique that included representatives of national metrology institutes and the BIPM. A significant part of the work was related to the estimation of the uncertainties of the values given in the list of

Primary methods of measurement and primary standards

The definition and concept of primary method are explained, taking examples from the fields of thermometry and measurements of amount of substance. It is shown that although few methods can be considered strictly primary, small departures from the strict definition can be quantified and useful practical primary methods are thus available. The links between primary methods of measurement, primary standards of measurement and primary reference materials are discussed.

The Comit? Consultatif pour la Quantit? de Mati?re: a brief review of its origin and present activities

International traceability to the SI is increasingly demanded for measurements in chemistry. These concern measurements made not only for industrial chemical manufacturing but also for checking the safety of a wide range of food and agricultural products, for environmental protection, for medicine, as well as for a multitude of regulatory purposes. This article outlines the role that the Comite Consultatif pour la Quantite de Matiere (CCQM) can play in establishing traceability at the highest level for such measurements. A brief account is given of the actions taken so far by the Committee. These include the precise description of primary methods, the meaning of traceability for measurements in chemistry and the organization of some key international comparisons designed to test the procedures drawn up to guide them. Plans for future work are described.

Experiment and theory in anelasticity

We briefly review the results of experimental and theoretical studies of anelasticity at low frequencies and we discuss the essential features of models that explain consistently the key aspects of this behaviour. In our original paper on this topic we reported that damping in a compound pendulum at frequencies in the range 10-3-10-2 Hz was consistent with an imaginary component of Youngs modulus for the Cu-Be suspension that was independent of frequency. Damping with this characteristic frequency dependence can also be described in terms of viscous damping whose magnitude varies as the inverse of frequency. With the hindsight provided by our further work we now realize that stick-slip processes can give rise to such losses, not only in the suspensions but also in the structures and clamping mechanism of the suspensions of long-period pendulums.

The use and abuse of the terms percent, parts per million and parts in 10n

We discuss two different ways in which terms such as percent (%) and parts per million (ppm) are used, both in the scientific literature and in everyday language. We give examples of the way in which these terms may be used without ambiguity, and we also describe common misuse of the terms. We suggest the adoption of the special name uno (symbol U) for the unit one, for use with the International System of Units (SI) prefixes in place of terms such as percent and parts per million in the technical literature.

Further Measurements of Thermodynamic Temperature Using a Total Radiation Thermometer: the Range - 130°C to +60°C

Brief descriptions are given of the design and operating principles of the NPL total radiation thermometer. Further measurements of T-T68 are reported for the temperature range - 130 to +60°C and tabulated with previous measurements made in the range -40 to +100°C. A graph is presented of all the published measurements of T-T68 from various independent primary thermometers in the range 50 to 400 K, and their relevance to the construction of a reference function for the proposed ITS-90 is discussed.

A 1 kg Mass Comparator Using Flexure-Strip Suspensions: Preliminary Results

This paper describes the design and construction of a novel form of equal-arm balance. The balance has been designed to study the performance of flexure strips for use as pivots in a 1 kg mass comparator working at the highest accuracy. The beam of the balance is servo controlled using optical detection of angular position and electromagnetic control. Small mass differences are measured in terms of the differences in the servo currents required to reproduce the same position of the beam. Preliminary results using this prototype balance indicate that an accuracy in mass comparison of about 5 parts in 1010 can be achieved.