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The International System of Units (SI)

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The International System of Units (SI) A111D3 b331bl I United States Department of Commerce In # I National Institute of Standards and Technology NIST PUBLICATIONS NIST Special Publication 330 1991 Edition The International System of Units (SI) 4 &//# Oo^ w * <r 4^ A 5 vO r <9 ^ ^ </ QC— V^' ^ .<£ ^ 100 w 4> ** 0 S U57 ^ M m & o9 //330 1991 NIST SPECIAL PUBLICATION 330 1991 EDITION THE INTERNATIONAL SYSTEM OF UNITS (SI) United States of America Editor: Barry N. Taylor National Institute of Standards and Technology Gaithersburg, MD 20899 Approved translation of the sixth edition (1991) of the International Bureau of Weights and Measures publication Le Systeme International d'Unites (SI) (Supersedes NBS Special Publication 330 1986 Edition) Issued August 1991 U.S. DEPARTMENT OF COMMERCE, Robert A. Mosbacher, Secretary NATIONAL INSTITUTE OF STANDARDS AND TECHNOLOGY, John W. Lyons, Director National Institute of Standards and Technology Special Publication 330, 1991 Edition Natl. Inst. Stand. Technol. Spec. Publ. 330, 1991 Edition, 62 pages (Aug. 1991) CODEN: NSPUE2 U.S. GOVERNMENT PRINTING OFFICE WASHINGTON: 1991 For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, DC 20402-9325 Foreword This booklet is the United States of America edition of the English-language translation of the sixth edition of Le Systeme International d'Unites (SI), the definitive reference on the SI published in 1991 by the International Bureau of Weights and Measures (BIPM) in the French language. This USA edition conforms in substance with the English-language translation that follows the French- language text in the BIPM publication. That translation was a joint effort of the BIPM, the National Institute of Standards and Technology (NIST) in the United States, and the National Physical Laboratory (NPL) in the United Kingdom. However, to make this booklet helpful to the broadest community of users in the USA, it was necessary to follow current Federal policy, to recognize present USA practices as they are found in the literature of our domestic voluntary standards organizations such as ASTM and IEEE, and to use American spelling of certain words. Thus, this USA edition differs from the English-language version in the BIPM publication in the following details: (1) the dot is used instead of the comma as the decimal marker; (2) the American spellings "meter," "liter," and "deka" are used instead of "metre," "litre," and "deca"; (3) a small number of footnotes are added for explanatory purposes and to identify USA practices that differ from those suggested in the BIPM publication; (4) in a few instances, American rather than British spelling or usage is followed for a few common words; and (5) the index has been moderately expanded. This English-language edition, the one prepared by U. K. Editor R. J. Bell at the National Physical Laboratory, and the one included in the BIPM publication, represent the best efforts to render an accurate translation of the French-language text. Nevertheless, in case of disagreement it is always the French text that is authoritative. The SI or modernized metric system, long the language universally used in science, is rapidly becoming the language of international commerce and trade. In recognition of this fact and the increasing global nature of the market place, the Omnibus Trade and Competitiveness Act of 1988, which changed the name of the National Bureau of Standards (NBS) to the National Institute of Standards and Technology (NIST) and assigned to NIST new responsibilities for assisting industry in the development of technology, designates "the metric system of measurement as the preferred system of weights and measures for United States trade and commerce." Further, the Act requires "that each Federal agency, by a date certain and to the extent economically feasible by the end of the fiscal year 1992, use the metric system of measurement in its procurements, grants, and other business related activities." I am therefore extremely pleased to present this new USA edition of the definitive publication on the foundation and fundamental principles of the International System of Units to the current USA users of the SI, but especially to the many anticipated future users within the United States. It is my sincere hope that this booklet will contribute to a better understanding within our country of the weights and measures language that is rapidly becoming universal. July 1991 John W. Lyons, Director, National Institute of Standards and Technology iii Preface to the 6th Edition Since 1970, the Bureau International des Poids et Mesures (BIPM), has regularly published this document containing Resolutions and Recommendations of the Conference Generate des Poids et Mesures (CGPM) and the Comite" International des Poids et Mesures (CIPM) on the International System of Units. Explanations have been added as well as relevant extracts from the International Standards of the International Organization for Standardization (ISO) for the practical use of the system. The Comite Consultatif des Unit6s (CCU) of the CIPM helped to draft the document and has approved the final text. Appendix I reproduces in chronological order the decisions (Resolutions, Recommendations, Declarations, etc.) promulgated since 1889 by the CGPM and the CIPM on units of measurement and on the International System of Units. Appendix II outlines the measurements, consistent with the theoretical definitions given here, which metrological laboratories can make to realize the units and to calibrate highest-quality material standards. Unless otherwise specified, uncertainties are given at the level of one standard deviation. The 6th edition is a revision of the 5th edition (1985); it takes into consideration the decisions of the 18th CGPM (1987) and the CIPM (1988, 1989, 1990), and the amendments made by the CCU (1990). The early editions of this document have been used as a work of reference in numerous countries. In order to make the contents more readily accessible for a greater number of readers, the CIPM has decided to include an English-language translation. The BIPM has endeavored to publish the most faithful translation possible through collaboration with the National Physical Laboratory (Teddington, United Kingdom) and the National Institute of Standards and Technology (Gaithers- burg, USA). A particular difficulty arises from the slight spelling variations that occur in the scien- tific language of the English-speaking countries (for instance, "metre" and "meter", "litre" and "liter"). In general, translation follows the Recommendations of ISO (1982) as far as the vocabulary and the spelling of the names of quantities and units are concerned, as well as the writing of numbers. This English translation is not to be considered as an official text. In case of dispute, it is always the French text which is authoritative. February 1991 T. J. Quinn J. de Boer Director, BIPM President, CCU iv The International System of Units Contents Page Foreword iii Preface to the 6th Edition iv I. Introduction 1 1.1 Historical note 1 1.2 The three classes of SI units 1 1.3 The SI prefixes 2 1.4 System of quantities 2 1.5 Legislation on units 2 II. SI Units 3 II. 1 SI base units 3 II. 1.1 Definitions 3 II.1.2 Symbols 5 11.2 SI derived units 6 11.3 SI supplementary units 8 11.4 Rules for writing and using SI unit symbols 9 III. Decimal Multiples and Submultiples of SI Units 10 IH.1 SI prefixes 10 111.2 Rules for using SI prefixes 10 111.3 The kilogram 11 IV. Units Outside the International System 12 IV. 1 Units used with the International System 12 IV.2 Units in use temporarily 13 IV.3 CGS units 14 IV.4 Other units 15 Appendix I. Decisions of the CGPM and the CIPM 16 Appendix II. Practical Realization of the Definitions of Some Important Units 41 Appendix III. The BIPM and the Meter Convention 52 Index 55 v I. INTRODUCTION 1.1 Historical note f In 1948 the 9th General Conference on Weights and Measures (CGPM ), by its Resolution 6, instructed the International Committee for Weights and Measures (CIPM*): "to study the establishment of a complete set of rules for units of measurement"; "to find out for this purpose, by official inquiry, the opinion prevailing in scientific, technical, and educational circles in all countries"; and "to make recommendations on the establishment of a practical system of units of measurement suitable for adoption by all signatories to the Meter Convention."* The same General Conference also laid down, by its Resolution 7, general princi- ples for unit symbols and also gave a list of units with special names. The 10th CGPM (1954), by its Resolution 6, and the 14th CGPM (1971), by its Resolution 3, adopted as base units of this "practical system of units," the units of the following seven quantities: length, mass, time, electric current, thermodynamic temperature, amount of substance, and luminous intensity. The 11th CGPM (1960), by its Resolution 12, adopted the name International Sys- tem of Units, with the international abbreviation SI, for this practical system of units of measurement, and laid down rules for the prefixes, the derived and supple- mentary units, and other matters, thus establishing a comprehensive specification for units of measurement. 1.2 The three classes of SI units SI units are divided into three classes: base units, derived units, and supplementary units. From the scientific point of view, division of SI units into these three classes is to a certain extent arbitrary, because it is not essential to the physics of the subject. Nevertheless, the General Conference, considering the advantages of a single, practical, worldwide system of units for international relations, for teaching, and for scientific work, decided to base the International System on a choice of seven well-defined units which by convention are regarded as dimensionally indepen- dent: the meter, the kilogram, the second, the ampere, the kelvin, the mole, and the candela (see II.
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