the international system of units the international system of... · the international system of...

The International System of UnitsAnd its base units

Contents

1 Overview 11.1 International System of Units . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

1.1.1 History . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.1.2 SI Brochure and conversion factors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51.1.3 Units and prefixes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51.1.4 Writing unit symbols and the values of quantities . . . . . . . . . . . . . . . . . . . . . . 71.1.5 Realisation of units . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91.1.6 Post-1960 changes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101.1.7 Global adoption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111.1.8 Redefinition of units . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 151.1.9 See also . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 151.1.10 Notes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 161.1.11 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 161.1.12 Further reading . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 201.1.13 External links . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

2 Organisations 222.1 Metre Convention . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

2.1.1 Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 222.1.2 The 1875 Conference . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 232.1.3 Post 1875 developments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 252.1.4 Membership . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 272.1.5 Notes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 282.1.6 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

2.2 General Conference on Weights and Measures . . . . . . . . . . . . . . . . . . . . . . . . . . . . 302.2.1 Establishment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 302.2.2 CGPM meetings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 312.2.3 See also . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 312.2.4 Notes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 322.2.5 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

2.3 International Bureau of Weights and Measures . . . . . . . . . . . . . . . . . . . . . . . . . . . . 322.3.1 History . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 322.3.2 Function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

i

ii CONTENTS

2.3.3 Directors of the BIPM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 332.3.4 See also . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 332.3.5 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 332.3.6 External links . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

2.4 International Committee for Weights and Measures . . . . . . . . . . . . . . . . . . . . . . . . . . 332.4.1 Mission . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 332.4.2 Consultative Committees . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 342.4.3 Major reports . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 342.4.4 Notes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 342.4.5 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 352.4.6 See also . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 352.4.7 External links . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

3 Base units 363.1 SI base unit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

3.1.1 The seven SI base units . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 363.1.2 Proposed redefinitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 363.1.3 See also . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 373.1.4 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 373.1.5 External links . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37

3.2 Metre . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 373.2.1 Spelling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 383.2.2 Etymology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 383.2.3 History of definition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 383.2.4 SI prefixed forms of metre . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 413.2.5 Equivalents in other units . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 413.2.6 See also . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 413.2.7 Notes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 423.2.8 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 433.2.9 Further reading . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44

3.3 Kilogram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 443.3.1 Name and terminology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 443.3.2 Nature of mass . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 453.3.3 Kilogramme des Archives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 453.3.4 International prototype kilogram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 463.3.5 Proposed future definitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 503.3.6 SI multiples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 553.3.7 Glossary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 553.3.8 See also . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 563.3.9 Notes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 563.3.10 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 583.3.11 External links . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61

CONTENTS iii

3.4 Second . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 613.4.1 International second . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 613.4.2 Equivalence to other units . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 623.4.3 History of definition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 623.4.4 SI multiples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 643.4.5 Other current definitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 653.4.6 See also . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 653.4.7 Notes and references . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 653.4.8 External links . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66

3.5 Ampere . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 663.5.1 Definition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 663.5.2 History . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 673.5.3 Realization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 673.5.4 Proposed future definition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 673.5.5 Everyday examples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 673.5.6 See also . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 683.5.7 Notes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 683.5.8 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 683.5.9 External links . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68

3.6 Kelvin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 683.6.1 History . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 693.6.2 Usage conventions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 693.6.3 Proposed redefinition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 703.6.4 Practical uses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 703.6.5 Unicode character . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 713.6.6 See also . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 713.6.7 Notes and references . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 713.6.8 External links . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71

3.7 Mole . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 713.7.1 Definition and related concepts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 723.7.2 History . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 723.7.3 Unit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 733.7.4 Other units calledmole . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 733.7.5 Proposed future definition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 743.7.6 Related units . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 743.7.7 The unit's holiday . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 743.7.8 See also . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 743.7.9 Notes and references . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 743.7.10 External links . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75

3.8 Candela . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 753.8.1 Definition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75

iv CONTENTS

3.8.2 Explanation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 763.8.3 Origin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 763.8.4 SI photometric light units . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 763.8.5 See also . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 773.8.6 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77

4 Apprendix 784.1 SI derived unit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78

4.1.1 Derived units with special names . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 784.1.2 Examples of derived quantities and units . . . . . . . . . . . . . . . . . . . . . . . . . . . 784.1.3 Other units used with SI . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 784.1.4 Supplementary units . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 784.1.5 See also . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 784.1.6 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 784.1.7 Bibliography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78

4.2 Units accepted for use with SI . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 784.2.1 Units officially accepted for use with the SI . . . . . . . . . . . . . . . . . . . . . . . . . 794.2.2 Common units not officially sanctioned . . . . . . . . . . . . . . . . . . . . . . . . . . . 794.2.3 See also . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 794.2.4 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79

5 Text and image sources, contributors, and licenses 805.1 Text . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 805.2 Images . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 865.3 Content license . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93

Chapter 1

Overview

1.1 International System of Units

SIredirects here. For other uses, see SI (disambigua-tion).For a topical guide to this subject, see Outline of the met-ric system.The International System of Units (French: Sys-

KA s

cd kg

mmol

The seven base units in the International System of Units

tme international d'units, SI) is the modern form of themetric system, and is themost widely used system ofmea-surement. It comprises a coherent system of units ofmea-surement built on seven base units. It defines twenty-twonamed units, and includes many more unnamed coherentderived units. The system also establishes a set of twentyprefixes to the unit names and unit symbols that may beused when specifying multiples and fractions of the units.The system was published in 1960 as the result of aninitiative that began in 1948. It is based on the metre-kilogram-second system of units (MKS) rather than anyvariant of the centimetre-gram-second system (CGS). SIis intended to be an evolving system, so prefixes and unitsare created and unit definitions are modified throughinternational agreement as the technology of measure-ment progresses and the precision of measurements im-

proves. The 24th and 25th General Conferences onWeights and Measures (CGPM) in 2011 and 2014, forexample, discussed a proposal to change the definitionof the kilogram, linking it to an invariant of nature ratherthan to the mass of a material artefact, thereby ensuringlong-term stability.*[1]The motivation for the development of the SI was the di-versity of units that had sprung up within the CGS sys-tems and the lack of coordination between the variousdisciplines that used them. The CGPM, which was estab-lished by theMetre Convention of 1875, brought togethermany international organisations to not only agree on thedefinitions and standards of the new system but also agreeon the rules for writing and presenting measurements ina standardised manner around the world.The International System of Units has been adopted bymost developed countries; however, the adoption has notbeen universal in all English-speaking countries. Whilemetrication in the United States is consistent in science,medicine, government, and various fields of technologyand engineering, common measurements are mostly per-formed in United States customary units, although thesehave officially been defined in terms of SI units. TheUnited Kingdom has officially adopted a policy of partialmetrication. Canada has adopted the SI for most gov-ernmental, medical and scientific purposes and for suchvaried uses as grocery weights, weather reports, trafficsigns and gasoline sales, but imperial units are still legallypermitted and remain in common use throughout manysectors of Canadian society, particularly in the buildingtrade and the railway sector.

1.1.1 History

Main article: History of the metric system

The metric system was first implemented during theFrench Revolution (1790s) with just the metre andkilogram as standards of length and mass*[Note 1] re-spectively. In the 1830s Carl Friedrich Gauss laid thefoundations for a coherent system based on length, mass,and time. In the 1860s a group working under the aus-pices of the British Association for the Advancement of

1

https://en.wikipedia.org/wiki/SI_(disambiguation)https://en.wikipedia.org/wiki/SI_(disambiguation)https://en.wikipedia.org/wiki/Outline_of_the_metric_systemhttps://en.wikipedia.org/wiki/Outline_of_the_metric_systemhttps://en.wikipedia.org/wiki/French_languagehttps://en.wikipedia.org/wiki/Metric_systemhttps://en.wikipedia.org/wiki/System_of_measurementhttps://en.wikipedia.org/wiki/System_of_measurementhttps://en.wikipedia.org/wiki/Coherence_(units_of_measurement)https://en.wikipedia.org/wiki/Units_of_measurementhttps://en.wikipedia.org/wiki/Units_of_measurementhttps://en.wikipedia.org/wiki/SI_base_unithttps://en.wikipedia.org/wiki/SI_derived_unithttps://en.wikipedia.org/wiki/Metric_prefixhttps://en.wikipedia.org/wiki/MKS_system_of_unitshttps://en.wikipedia.org/wiki/MKS_system_of_unitshttps://en.wikipedia.org/wiki/Centimetre%E2%80%93gram%E2%80%93second_system_of_unitshttps://en.wikipedia.org/wiki/General_Conference_on_Weights_and_Measureshttps://en.wikipedia.org/wiki/General_Conference_on_Weights_and_Measureshttps://en.wikipedia.org/wiki/Kilogramhttps://en.wikipedia.org/wiki/Discipline_(academia)https://en.wikipedia.org/wiki/Metre_Conventionhttps://en.wikipedia.org/wiki/Metricationhttps://en.wikipedia.org/wiki/Developed_countryhttps://en.wikipedia.org/wiki/Metrication_in_the_United_Stateshttps://en.wikipedia.org/wiki/United_States_customary_unitshttps://en.wikipedia.org/wiki/Metrication_in_the_United_Kingdomhttps://en.wikipedia.org/wiki/Metrication_in_the_United_Kingdomhttps://en.wikipedia.org/wiki/Metrication_in_Canadahttps://en.wikipedia.org/wiki/History_of_the_metric_systemhttps://en.wikipedia.org/wiki/Metric_systemhttps://en.wikipedia.org/wiki/French_Revolutionhttps://en.wikipedia.org/wiki/Metrehttps://en.wikipedia.org/wiki/Kilogramhttps://en.wikipedia.org/wiki/Standard_(metrology)https://en.wikipedia.org/wiki/Lengthhttps://en.wikipedia.org/wiki/Masshttps://en.wikipedia.org/wiki/Carl_Friedrich_Gausshttps://en.wikipedia.org/wiki/Coherence_(units_of_measurement)https://en.wikipedia.org/wiki/Timehttps://en.wikipedia.org/wiki/British_Association_for_the_Advancement_of_Science

2 CHAPTER 1. OVERVIEW

Stone marking the Austro-Hungarian/Italian border at Pontebbadisplaying myriametres, a unit of 10 km used in Central Europein the 19th century (but since deprecated).*[2]*[3]

Science formulated the requirement for a coherent systemof units with base units and derived units. The inclusionof electrical units into the system was hampered by thecustomary use of more than one set of units, until 1900when Giovanni Giorgi identified the need to define onesingle electrical quantity as a fourth base quantity along-side the original three base quantities.Meanwhile, in 1875, the Treaty of the Metre passed re-sponsibility for verification of the kilogram and metreagainst agreed prototypes from French to internationalcontrol. In 1921, the Treaty was extended to include allphysical quantities including electrical units originally de-fined in 1893.In 1948, an overhaul of the metric system was set in mo-tion which resulted in the development of thePracticalsystem of unitswhich, on its publication in 1960, wasgiven the nameThe International System of Units. In1954, the 10th General Conference onWeights and Mea-sures (CGPM) identified electric current as the fourthbase quantity in the practical system of units and addedtwo more base quantitiestemperature and luminous in-tensitymaking six base quantities in all. The units as-sociated with these quantities were the metre, kilogram,second, ampere, kelvin and candela. In 1971, a seventhbase quantity, amount of substance represented by the

mole, was added to the definition of SI.

Early development

The metric system was developed from 1791 onwardsby a committee of the French Academy of Sciences,commissioned by the National Assembly and LouisXVI to create a unified and rational system of mea-sures.*[4] The group, which included Antoine Lavoisier(thefather of modern chemistry) and the mathemati-cians Pierre-Simon Laplace and Adrien-Marie Legen-dre,*[5]*:89 used the same principles for relating length,volume, and mass that had been proposed by the Englishclergyman John Wilkins in 1668*[6]*[7] and the conceptof using the Earth's meridian as the basis of the defini-tion of length, originally proposed in 1670 by the Frenchabbot Mouton.*[8]*[9]

Carl Friedrich Gauss

On 30 March 1791, the Assembly adopted the commit-tee's proposed principles for the new decimal system ofmeasure and authorised a survey between Dunkirk andBarcelona to establish the length of the meridian. On11 July 1792, the committee proposed the names metre,are, litre and grave for the units of length, area, capac-ity, and mass, respectively. The committee also proposedthat multiples and submultiples of these units were to bedenoted by decimal-based prefixes such as centi for a hun-dredth and kilo for a thousand.*[10]*:82

https://en.wikipedia.org/wiki/British_Association_for_the_Advancement_of_Sciencehttps://en.wikipedia.org/wiki/Pontebbahttps://en.wikipedia.org/wiki/Metric_prefix#Obsolete_metric_prefixeshttps://en.wikipedia.org/wiki/Central_Europehttps://en.wikipedia.org/wiki/Deprecationhttps://en.wikipedia.org/wiki/SI_base_unithttps://en.wikipedia.org/wiki/SI_derived_unithttps://en.wikipedia.org/wiki/Electricityhttps://en.wikipedia.org/wiki/Giovanni_Giorgihttps://en.wikipedia.org/wiki/Treaty_of_the_Metrehttps://en.wikipedia.org/wiki/Physical_quantityhttps://en.wikipedia.org/wiki/General_Conference_on_Weights_and_Measureshttps://en.wikipedia.org/wiki/General_Conference_on_Weights_and_Measureshttps://en.wikipedia.org/wiki/Electric_currenthttps://en.wikipedia.org/wiki/Temperaturehttps://en.wikipedia.org/wiki/Luminous_intensityhttps://en.wikipedia.org/wiki/Luminous_intensityhttps://en.wikipedia.org/wiki/Metrehttps://en.wikipedia.org/wiki/Kilogramhttps://en.wikipedia.org/wiki/Secondhttps://en.wikipedia.org/wiki/Amperehttps://en.wikipedia.org/wiki/Kelvinhttps://en.wikipedia.org/wiki/Candelahttps://en.wikipedia.org/wiki/Amount_of_substancehttps://en.wikipedia.org/wiki/Mole_(unit)https://en.wikipedia.org/wiki/French_Academy_of_Scienceshttps://en.wikipedia.org/wiki/National_Assembly_(France)https://en.wikipedia.org/wiki/Louis_XVIhttps://en.wikipedia.org/wiki/Louis_XVIhttps://en.wikipedia.org/wiki/Antoine_Lavoisierhttps://en.wikipedia.org/wiki/Pierre-Simon_Laplacehttps://en.wikipedia.org/wiki/Adrien-Marie_Legendrehttps://en.wikipedia.org/wiki/Adrien-Marie_Legendrehttps://en.wikipedia.org/wiki/An_Essay_towards_a_Real_Character_and_a_Philosophical_Languagehttps://en.wikipedia.org/wiki/John_Wilkinshttps://en.wikipedia.org/wiki/Meridian_(geography)https://en.wikipedia.org/wiki/Gabriel_Moutonhttps://en.wikipedia.org/wiki/Carl_Friedrich_Gausshttps://en.wikipedia.org/wiki/Dunkirkhttps://en.wikipedia.org/wiki/Barcelonahttps://en.wikipedia.org/wiki/Metrehttps://en.wikipedia.org/wiki/Hectarehttps://en.wikipedia.org/wiki/Litrehttps://en.wikipedia.org/wiki/Grave_(unit)

1.1. INTERNATIONAL SYSTEM OF UNITS 3

Thomson

Maxwell

William Thomson (Lord Kelvin) and James ClerkMaxwell played a prominent role in the development ofthe principle of coherence and in the naming of manyunits of measure.*[11]*[12]*[13]*[14]*[15]

The law of 7 April 1795 (loi du 18 germinal) defined theterms gramme and kilogramme, which replaced the for-mer terms gravet (correctly milligrave) and grave, and on22 June 1799 (after Pierre Mchain and Jean-BaptisteDelambre had completed the meridian survey) the defini-tive standard mtre des Archives and kilogramme desArchiveswere deposited in theArchives nationales. On 10December 1799 (a month after Napoleon's coup d'tat),the law by which the metric system was to be defini-tively adopted in France (loi du 19 frimaire*[16]) waspassed.*[17]During the first half of the nineteenth century there waslittle consistency in the choice of preferred multiples ofthe base units typically the myriametre (10000 metres)was in widespread use in both France and parts of Ger-many, while the kilogram (1000 grams) rather than themyriagram was used for mass.*[2]In 1832, the German mathematician Carl FriedrichGauss, assisted by WilhelmWeber, implicitly defined thesecond as a base unit when he quoted the earth's magneticfield in terms of millimetres, grams, and seconds.*[11]Prior to this, the strength of the earths magnetic field

had only been described in relative terms. The techniqueused by Gauss was to equate the torque induced on a sus-pended magnet of known mass by the earths magneticfield with the torque induced on an equivalent system un-der gravity. The resultant calculations enabled him to as-sign dimensions based on mass, length and time to themagnetic field.*[18]In the 1860s, James Clerk Maxwell, William Thomson(later Lord Kelvin) and others working under the aus-pices of the British Association for the Advancement ofScience, built on Gauss' work and formalised the con-cept of a coherent system of units with base units andderived units. The principle of coherence was success-fully used to define a number of units of measure basedon the centimetregramsecond (CGS) system of units(CGS), including the erg for energy, the dyne for force,the barye for pressure, the poise for dynamic viscosityand the stokes for kinematic viscosity.*[14]

Metre Convention

Main article: Metre Convention

A French-inspired initiative for international cooperationin metrology led to the signing in 1875 of the MetreConvention.*[5]*:353354 Initially the convention onlycovered standards for the metre and the kilogram. Aset of 30 prototypes of the metre and 40 prototypes ofthe kilogram,*[Note 3] in each case made of a 90%platinum10% iridium alloy, were manufactured by theBritish firm Johnson, Matthey & Co and accepted bythe CGPM in 1889. One of each was selected at ran-dom to become the International prototype metre andInternational prototype kilogram that replaced the mtredes Archives and kilogramme des Archives respectively.Each member state was entitled to one of each of the re-maining prototypes to serve as the national prototype forthat country.*[20]

Closeup of the National Prototype Metre, serial number 27, allo-cated to the United States

https://en.wikipedia.org/wiki/William_Thomson,_1st_Baron_Kelvinhttps://en.wikipedia.org/wiki/James_Clerk_Maxwellhttps://en.wikipedia.org/wiki/Gramhttps://en.wikipedia.org/wiki/Kilogramhttps://en.wikipedia.org/wiki/Pierre_M%C3%A9chainhttps://en.wikipedia.org/wiki/Jean-Baptiste_Delambrehttps://en.wikipedia.org/wiki/Jean-Baptiste_Delambrehttps://en.wikipedia.org/wiki/History_of_the_metre#M%C3%A8tre_des_Archiveshttps://en.wikipedia.org/wiki/Kilogram#Kilogramme_des_Archiveshttps://en.wikipedia.org/wiki/Kilogram#Kilogramme_des_Archiveshttps://en.wikipedia.org/wiki/Archives_nationales_(France)https://en.wikipedia.org/wiki/Coup_of_18_Brumairehttps://en.wikipedia.org/wiki/Mathematicianhttps://en.wikipedia.org/wiki/Carl_Friedrich_Gausshttps://en.wikipedia.org/wiki/Carl_Friedrich_Gausshttps://en.wikipedia.org/wiki/Wilhelm_Eduard_Weberhttps://en.wikipedia.org/wiki/Relative_change_and_differencehttps://en.wikipedia.org/wiki/Torquehttps://en.wikipedia.org/wiki/James_Clerk_Maxwellhttps://en.wikipedia.org/wiki/William_Thomson,_1st_Baron_Kelvinhttps://en.wikipedia.org/wiki/British_Association_for_the_Advancement_of_Sciencehttps://en.wikipedia.org/wiki/British_Association_for_the_Advancement_of_Sciencehttps://en.wikipedia.org/wiki/Centimetre%E2%80%93gram%E2%80%93second_system_of_unitshttps://en.wikipedia.org/wiki/Erghttps://en.wikipedia.org/wiki/Energyhttps://en.wikipedia.org/wiki/Dynehttps://en.wikipedia.org/wiki/Forcehttps://en.wikipedia.org/wiki/Baryehttps://en.wikipedia.org/wiki/Pressurehttps://en.wikipedia.org/wiki/Poisehttps://en.wikipedia.org/wiki/Viscosity#Dynamic_(shear)_viscosityhttps://en.wikipedia.org/wiki/Stokes_(unit)https://en.wikipedia.org/wiki/Viscosity#Kinematic_viscosityhttps://en.wikipedia.org/wiki/Metre_Conventionhttps://en.wikipedia.org/wiki/Metrologyhttps://en.wikipedia.org/wiki/Metre_Conventionhttps://en.wikipedia.org/wiki/Metre_Conventionhttps://en.wikipedia.org/wiki/Platinumhttps://en.wikipedia.org/wiki/Iridiumhttps://en.wikipedia.org/wiki/Johnson_Mattheyhttps://en.wikipedia.org/wiki/International_prototype_metrehttps://en.wikipedia.org/wiki/International_prototype_kilogramhttps://en.wikipedia.org/wiki/History_of_the_metre#M%C3%A8tre_des_Archiveshttps://en.wikipedia.org/wiki/History_of_the_metre#M%C3%A8tre_des_Archiveshttps://en.wikipedia.org/wiki/Kilogram#Kilogramme_des_Archives


The treaty established three international organisations tooversee the keeping of international standards of mea-surement:*[21]

General Conference onWeights andMeasures (Con-frence gnrale des poids et mesures or CGPM) ameeting every four to six years of delegates from allmember states that receives and discusses a reportfrom the CIPM and that endorses new developmentsin the SI on the advice of the CIPM.

International Committee for Weights and Measures(Comit international des poids et mesures or CIPM) a committee that meets annually at the BIPM andis made up of eighteen individuals of high scien-tific standing, nominated by the CGPM to advise theCGPM on administrative and technical matters

International Bureau of Weights and Measures (Bu-reau international des poids et mesures or BIPM) an international metrology centre at Svres in Francethat has custody of the International prototype kilo-gram, provides metrology services for the CGPMand CIPM, houses the secretariat for these organi-sations and hosts their formal meetings. Initially itsprime metrological purpose was a periodic recali-bration of national prototype metres and kilogramsagainst the international prototype.

In 1921, the Metre Convention was extended to includeall physical units, including the ampere and others definedby the Fourth International Conference of Electricians inChicago in 1893, thereby enabling the CGPM to addressinconsistencies in the way that the metric system had beenused.*[12]*[22]*:96The official language of the Metre Convention isFrench*[23] and the definitive version of all official doc-uments published by or on behalf of the CGPM is theFrench-language version.*[22]*:94

Towards the SI

Main article: MetricationAt the close of the 19th century three different systemsof units of measure existed for electrical measurements:a CGS-based system for electrostatic units, also knownas the Gaussian or ESU system, a CGS-based system forelectromechanical units (EMU) and an MKS-based sys-tem (international system)*[24] for electrical distri-bution systems. Attempts to resolve the electrical units interms of length, mass, and time using dimensional analy-sis was beset with difficultiesthe dimensions dependedon whether one used the ESU or EMU systems.*[15] Thisanomaly was resolved in 1900whenGiovanni Giorgi pub-lished a paper in which he advocated using a fourth baseunit alongside the existing three base units. The fourthunit could be chosen to be electric current , voltage, orelectrical resistance.*[25]

World map showing metrication, colour-coded by year of con-version: from ca 1800 (green) to 1980 (red). Black indicates thenations that have not yet adopted the SI-system: Burma, Liberia,and the United States. Canada and the United Kingdom bothhave fairly common use of previously used units, e.g., speed lim-its in the UK and human height reporting in Canada.

In the late 19th and early 20th centuries, a number of non-coherent units of measure based on the gram/kilogram,the centimetre/metre, and the second, such as thePferdestrke (metric horsepower) for power,*[26]*[Note4] the darcy for permeability*[27] and the use of"millimetres of mercury" for the measurement of bothbarometric and blood pressure were developed or prop-agated, some of which incorporated standard gravity intheir definitions.At the end of the Second World War, a number of dif-ferent systems of measurement were in use throughoutthe world. Some of these systems were metric systemvariations, whereas others were based on customary sys-tems of measure. In 1948, after representations by theInternational Union of Pure and Applied Physics (IU-PAP) and by the French Government, the 9th GeneralConference on Weights and Measures (CGPM) askedthe International Committee for Weights and Measures(CIPM) to conduct an international study of the measure-ment needs of the scientific, technical, and educationalcommunities andto make recommendations for a sin-gle practical system of units of measurement, suitable foradoption by all countries adhering to the Metre Conven-tion.*[28]On the basis of the findings of this study, the 10th CGPMin 1954 decided that an international system should be de-rived from six base units to provide for the measurementof temperature and optical radiation in addition to me-chanical and electromagnetic quantities. Six base unitswere recommended: the metre, kilogram, second, am-pere, degree Kelvin (later renamed kelvin), and candela.In 1960, the 11th CGPM named the system the Interna-tional System of Units, abbreviated SI from the Frenchname, Le Systme International d'Units.*[22]*:110*[29]The BIPM has also described SI asthe modern metricsystem.*[22]*:95 The seventh base unit, the mole, wasadded in 1971 by the 14th CGPM.*[30]

https://en.wikipedia.org/wiki/General_Conference_on_Weights_and_Measureshttps://en.wikipedia.org/wiki/International_Committee_for_Weights_and_Measureshttps://en.wikipedia.org/wiki/International_Bureau_of_Weights_and_Measureshttps://en.wikipedia.org/wiki/S%C3%A8vreshttps://en.wikipedia.org/wiki/Metrologyhttps://en.wikipedia.org/wiki/Metricationhttps://en.wikipedia.org/wiki/Centimetre%E2%80%93gram%E2%80%93second_system_of_units#Electrostatic_units_(ESU)https://en.wikipedia.org/wiki/Centimetre%E2%80%93gram%E2%80%93second_system_of_units#Electromagnetic_units_(EMU)https://en.wikipedia.org/wiki/Centimetre%E2%80%93gram%E2%80%93second_system_of_units#Electromagnetic_units_(EMU)https://en.wikipedia.org/wiki/MKS_system_of_unitshttps://en.wikipedia.org/wiki/MKS_system_of_unitshttps://en.wikipedia.org/wiki/Dimensional_analysishttps://en.wikipedia.org/wiki/Dimensional_analysishttps://en.wikipedia.org/wiki/Giovanni_Giorgihttps://en.wikipedia.org/wiki/Electric_currenthttps://en.wikipedia.org/wiki/Voltagehttps://en.wikipedia.org/wiki/Electrical_resistance_and_conductancehttps://en.wikipedia.org/wiki/Pferdest%C3%A4rkehttps://en.wikipedia.org/wiki/Power_(physics)https://en.wikipedia.org/wiki/Darcy_(unit)https://en.wikipedia.org/wiki/Permeability_(earth_sciences)https://en.wikipedia.org/wiki/Torr#Manometric_units_of_pressurehttps://en.wikipedia.org/wiki/Atmospheric_pressurehttps://en.wikipedia.org/wiki/Blood_pressurehttps://en.wikipedia.org/wiki/Standard_gravityhttps://en.wikipedia.org/wiki/Second_World_Warhttps://en.wikipedia.org/wiki/Systems_of_measurementhttps://en.wikipedia.org/wiki/Systems_of_measurementhttps://en.wikipedia.org/wiki/International_Union_of_Pure_and_Applied_Physicshttps://en.wikipedia.org/wiki/CGPMhttps://en.wikipedia.org/wiki/CIPMhttps://en.wikipedia.org/wiki/SI_electromagnetism_units


International System of Quantities

Main article: International System of Quantities

The International System of Quantities (ISQ) is a sys-tem based on seven base quantities: length, mass, time,electric current, thermodynamic temperature, amount ofsubstance, and luminous intensity. Other quantities suchas area, pressure, and electrical resistance are derivedfrom these base quantities by clear non-contradictoryequations. The ISQ defines the quantities that are mea-sured with the SI units.*[31] The ISQ is defined in theinternational standard ISO/IEC 80000, and was finalisedin 2009 with the publication of ISO 80000-1.*[32]

1.1.2 SI Brochure and conversion factors

Cover of brochure The International System of Units

The CGPM publishes a brochure which defines andpresents SI.*[22] Its official version is in French, inline with the Metre Convention.*[22]*:102 It leavessome scope for local interpretation, particularly regard-ing names and terms in different languages, so for exam-ple the United States' National Institute of Standards andTechnology (NIST) has produced a version of the CGPMdocument (NIST SP 330) which clarifies local interpreta-tion for English-language publications that use AmericanEnglish*[33] and another document (NIST SP 811) thatgives general guidance for the use of SI in the UnitedStates and conversion factors between SI and customary

units.*[34]The writing and maintenance of the CGPM brochure iscarried out by one of the committees of the InternationalCommittee for Weights and Measures (CIPM), the Con-sultative Committee for Units (CCU). The CIPM nom-inates the chairman of this committee, but the com-mittee includes representatives of various other inter-national bodies rather than CIPM or CGPM nomi-nees.*[35]*[Note 5] This committee thus provides a fo-rum for the bodies concerned to provide input to theCIPM in respect of ongoing enhancements to SI.The definitions of the termsquantity,unit,di-mensionetc. that are used in the SI Brochure are thosegiven in the International vocabulary of metrology, a pub-lication produced by the Joint Committee for Guides inMetrology (JCGM), a working group consisting of eightinternational standards organisations under the chairman-ship of the director of the BIPM.*[36] The quantities andequations that define the SI units are now referred to asthe International System of Quantities (ISQ), and are setout in the International Standard ISO/IEC 80000 Quanti-ties and Units.

1.1.3 Units and prefixes

The International System of Units consists of a set of baseunits, a set of derived units with special names, and aset of decimal-based multipliers that are used as prefixes.The term SI Units covers all three categories, but the termcoherent SI units includes only base units and coherent de-rived units.*[22]*:103106

Base units

Main article: SI base units

The SI base units are the building blocks of the sys-tem and all other units are derived from them. WhenMaxwell first introduced the concept of a coherent sys-tem, he identified three quantities that could be used asbase units: mass, length and time. Giorgi later identi-fied the need for an electrical base unit. Theoreticallyany one of electric current, potential difference, electricalresistance, electrical charge or a number of other quanti-ties could have provided the base unit, with the remain-ing units then being defined by the laws of physics. Inthe event, the unit of electric current was chosen for SI.Another three base units (for temperature, substance andluminous intensity) were added later.

Derived units

Main article: Derived units

https://en.wikipedia.org/wiki/International_System_of_Quantitieshttps://en.wikipedia.org/wiki/Quantityhttps://en.wikipedia.org/wiki/Length_dimensionhttps://en.wikipedia.org/wiki/Masshttps://en.wikipedia.org/wiki/Timehttps://en.wikipedia.org/wiki/Electric_currenthttps://en.wikipedia.org/wiki/Thermodynamic_temperaturehttps://en.wikipedia.org/wiki/Amount_of_substancehttps://en.wikipedia.org/wiki/Amount_of_substancehttps://en.wikipedia.org/wiki/Luminous_intensityhttps://en.wikipedia.org/wiki/Areahttps://en.wikipedia.org/wiki/Pressurehttps://en.wikipedia.org/wiki/Electrical_resistancehttps://en.wikipedia.org/wiki/ISO/IEC_80000https://en.wikipedia.org/wiki/ISO_80000-1http://www.bipm.org/en/publications/si-brochure/https://en.wikipedia.org/wiki/Metre_Conventionhttps://en.wikipedia.org/wiki/National_Institute_of_Standards_and_Technologyhttps://en.wikipedia.org/wiki/National_Institute_of_Standards_and_Technologyhttps://en.wikipedia.org/wiki/American_Englishhttps://en.wikipedia.org/wiki/American_Englishhttps://en.wikipedia.org/wiki/Conversion_of_unitshttps://en.wikipedia.org/wiki/International_Committee_for_Weights_and_Measureshttps://en.wikipedia.org/wiki/International_Committee_for_Weights_and_Measureshttps://en.wikipedia.org/wiki/International_vocabulary_of_metrologyhttps://en.wikipedia.org/wiki/Joint_Committee_for_Guides_in_Metrologyhttps://en.wikipedia.org/wiki/Joint_Committee_for_Guides_in_Metrologyhttps://en.wikipedia.org/wiki/ISO/IEC_80000https://en.wikipedia.org/wiki/ISO/IEC_80000https://en.wikipedia.org/wiki/SI_base_unithttps://en.wikipedia.org/wiki/SI_base_unithttps://en.wikipedia.org/wiki/SI_derived_unithttps://en.wikipedia.org/wiki/SI_prefixhttps://en.wikipedia.org/wiki/SI_base_unitshttps://en.wikipedia.org/wiki/Electric_currenthttps://en.wikipedia.org/wiki/Potential_differencehttps://en.wikipedia.org/wiki/Electrical_resistancehttps://en.wikipedia.org/wiki/Electrical_resistancehttps://en.wikipedia.org/wiki/Electrical_chargehttps://en.wikipedia.org/wiki/Physical_lawhttps://en.wikipedia.org/wiki/Derived_units


The derived units in the SI are formed by powers, prod-ucts or quotients of the base units and are unlimited innumber.*[22]*:103*[33]*:3 Derived units are associatedwith derived quantities, for example velocity is a quantitythat is derived from the base quantities of time and length,so in SI the derived unit is metres per second (symbolm/s). The dimensions of derived units can be expressedin terms of the dimensions of the base units.Coherent units are derived units that contain no numeri-cal factor other than 1quantities such as standard grav-ity and density of water are absent from their definitions.In the example above, one newton is the force required toaccelerate a mass of one kilogram by one metre per sec-ond squared. Since the SI units of mass and accelerationare kg and ms*2 respectively and F m a, the unitsof force (and hence of newtons) is formed by multipli-cation to give kgms*2. Since the newton is part of acoherent set of units, the constant of proportionality is 1.For the sake of convenience, some derived units have spe-cial names and symbols.*[13] Such units may themselvesbe used in combination with the names and symbols forbase units and for other derived units to express the unitsof other derived quantities. For example, the SI unit offorce is the newton (N), the SI unit of pressure is thepascal (Pa)and the pascal can be defined asnewtonsper square metre(N/m2).*[40]

Prefixes

Main article: Metric prefix

Prefixes are added to unit names to produce multiple andsub-multiples of the original unit. All multiples are inte-ger powers of ten, and above a hundred or below a hun-dredth all are integer powers of a thousand. For example,kilo- denotes a multiple of a thousand andmilli- denotes amultiple of a thousandth, so there are one thousand mil-limetres to the metre and one thousand metres to the kilo-metre. The prefixes are never combined, so for examplea millionth of a metre is amicrometre, not a millimillime-tre. Multiples of the kilogram are named as if the gramwere the base unit, so a millionth of a kilogram is a mil-ligram, not a microkilogram.*[22]*:122*[34]*:14

Non-SI units accepted for use with SI

Main article: non-SI units accepted for use with SI

Although, in theory, SI can be used for any physical mea-surement, the CIPM has recognised that some non-SIunits still appear in the scientific, technical, and com-mercial literature, and will continue to be used for manyyears to come. In addition, certain other units are sodeeply embedded in the history and culture of the hu-man race that they will continue to be used for the fore-

seeable future. The CIPM has catalogued several suchunits and published them in the SI Brochure so that theiruse may be consistent around the world. These unitshave been grouped as follows:*[22]*:123129*[34]*:711 *[Note 6]

10 cm

10 cm10 cm

The litre is classed as a non-SI unit accepted for use with the SI.Being one thousandth of a cubic metre, the litre is not a coherentunit of measure with respect to SI.

Non-SI units accepted for use with the SI (Table6):

Certain units of time, angle, and legacy non-SImetric units have a long history of consistentuse. Most of mankind has used the day andits non-decimal subdivisions as a basis of timeand, unlike the foot or the pound, these werethe same regardless of where they were beingmeasured. The radian, being 1/2 of a revo-lution, has mathematical advantages but it iscumbersome for navigation, and, as with time,the units used in navigation have a large degreeof consistency around the world. The tonne,litre, and hectare were adopted by the CGPMin 1879 and have been retained as units thatmay be used alongside SI units, having beengiven unique symbols. The catalogued unitsare

minute, hour, day, degree of arc,minute of arc, second of arc,hectare, litre, tonne, astronomicalunit and [deci]bel

Non-SI units whose values in SI units must beobtained experimentally (Table 7).

Physicists often use units of measure that arebased on natural phenomena, particularly whenthe quantities associated with these phenom-ena are many orders of magnitude greater thanor less than the equivalent SI unit. The most

https://en.wikipedia.org/wiki/Velocityhttps://en.wikipedia.org/wiki/Standard_gravityhttps://en.wikipedia.org/wiki/Standard_gravityhttps://en.wikipedia.org/wiki/Density_of_waterhttps://en.wikipedia.org/wiki/Accelerationhttps://en.wikipedia.org/wiki/Metre_per_second_squaredhttps://en.wikipedia.org/wiki/Metre_per_second_squaredhttps://en.wikipedia.org/wiki/Forcehttps://en.wikipedia.org/wiki/Newton_(unit)https://en.wikipedia.org/wiki/Pressurehttps://en.wikipedia.org/wiki/Pascal_(unit)https://en.wikipedia.org/wiki/Metric_prefixhttps://en.wikipedia.org/wiki/Non-SI_units_accepted_for_use_with_SIhttps://en.wikipedia.org/wiki/Non-SI_units_accepted_for_use_with_SIhttps://en.wikipedia.org/wiki/Non-SI_units_accepted_for_use_with_SIhttps://en.wikipedia.org/wiki/Foot_(unit)https://en.wikipedia.org/wiki/Pound_(mass)https://en.wikipedia.org/wiki/Radianhttps://en.wikipedia.org/wiki/Tonnehttps://en.wikipedia.org/wiki/Litrehttps://en.wikipedia.org/wiki/Hectarehttps://en.wikipedia.org/wiki/Minutehttps://en.wikipedia.org/wiki/Hourhttps://en.wikipedia.org/wiki/Dayhttps://en.wikipedia.org/wiki/Degree_(angle)https://en.wikipedia.org/wiki/Minute_of_archttps://en.wikipedia.org/wiki/Minute_of_archttps://en.wikipedia.org/wiki/Hectarehttps://en.wikipedia.org/wiki/Litrehttps://en.wikipedia.org/wiki/Tonnehttps://en.wikipedia.org/wiki/Astronomical_unithttps://en.wikipedia.org/wiki/Astronomical_unithttps://en.wikipedia.org/wiki/Decibel


common ones have been catalogued in the SIBrochure together with consistent symbols andaccepted values, but with the caveat that theirphysical values need to be measured.*[Note 7]

electronvolt, dalton/unified atomicmass unit, Planck constant, andelectron mass

Other non-SI units (Table 8):

A number of non-SI units that had never beenformally sanctioned by the CGPM have contin-ued to be used across the globe inmany spheresincluding health care and navigation. As withthe units of measure in Tables 6 and 7, thesehave been catalogued by the CIPM in the SIBrochure to ensure consistent usage, but withthe recommendation that authors who use themshould define them wherever they are used.

bar, millimetre of mercury,ngstrm, nautical mile, barn, knotand neper

Non-SI units associated with the CGS and theCGS-Gaussian system of units (Table 9)

The SI manual also catalogues a number oflegacy units of measure that are used in spe-cific fields such as geodesy and geophysicsor are found in the literature, particularly inclassical and relativistic electrodynamics wherethey have certain advantages: The units that arecatalogued are:

erg, dyne, poise, stokes, stilb, phot,gal, maxwell, gauss, and rsted.

1.1.4 Writing unit symbols and the valuesof quantities

Before 1948, the writing of metric quantities was hap-hazard. In 1879, the CIPM published recommendationsfor writing the symbols for length, area, volume andmass,but it was outside its domain to publish recommendationsfor other quantities. Beginning in about 1900, physicistswho had been using the symbol "" formicrometre(or

micron), "" formicrolitre, and "" formicro-gramstarted to use the symbols "m, "Land "g,but it was only in 1935, a decade after the revision of theMetre Convention that the CIPM formally adopted thisproposal and recommended that the symbol "" be useduniversally as a prefix for 10*6.*[41]In 1948, the ninth CGPM approved the first formal rec-ommendation for the writing of symbols in themetric sys-tem when the basis of the rules as they are now known

was laid down.*[42] These rules were subsequently ex-tended by International Organization for Standardization(ISO) and the International Electrotechnical Commis-sion (IEC) and now cover unit symbols and names, pre-fix symbols and names, how quantity symbols shouldbe written and used and how the values of quantitiesshould be expressed.*[22]*:104,130 Both ISO and theIEC have published rules for the presentation of SI unitsthat are generally compatible with those published in theSI Brochure.*[43] As of August 2013 ISO and IEC werein the process of merging their standards for quantitiesand units into a single set of compatible documents iden-tified as the ISO/IEC 80000 Standard. The rules coveringprinting of quantities and units are part of ISO 80000-1:2009.*[44]

Unit names

Names of units follow the grammatical rules associatedwith common nouns: in English and in French they startwith a lowercase letter (e.g., newton, hertz, pascal), evenwhen the symbol for the unit begins with a capital letter.This also applies todegrees Celsius, sincedegreeis the unit.*[45]*[46] In German, however, the names ofunits, as with all German nouns, start with capital let-ters.*[47] The spelling of unit names is a matter for theguardians*[Note 8] of the language concerned the of-ficial British and American spellings for certain SI unitsdiffer British English, as well as Australian, Canadianand New Zealand English, uses the spelling deca-, me-tre, and litre whereas American English uses the spellingdeka-, meter, and liter, respectively.*[48]Likewise, the plural forms of units follow the grammar ofthe language concerned: in English, the normal rules ofEnglish grammar are used, e.g. henriesis the pluralof "henry".*[49]*[34]*:31 However, the units lux, hertz,and siemens have irregular plurals in that they remain thesame in both their singular and plural form.In English, when unit names are combined to denote mul-tiplication of the units concerned, they are separated witha hyphen or a space (e.g. newton-metre or newtonmetre).The plural is formed by converting the last unit name tothe plural form (e.g. ten newton-metres).

Unit names as adjectives

In English, a space is recommended between the numberand the unit symbol when used as an adjective, e.g. a25 kg sphere.The normal rules of English apply to unit names, where ahyphen is incorporated into the adjectival sense, e.g. a25-kilogram sphere.*[50]

https://en.wikipedia.org/wiki/Electronvolthttps://en.wikipedia.org/wiki/Atomic_mass_unithttps://en.wikipedia.org/wiki/Atomic_mass_unithttps://en.wikipedia.org/wiki/Planck_constanthttps://en.wikipedia.org/wiki/Electron_masshttps://en.wikipedia.org/wiki/Health_carehttps://en.wikipedia.org/wiki/Navigationhttps://en.wikipedia.org/wiki/Bar_(unit)https://en.wikipedia.org/wiki/Millimetre_of_mercuryhttps://en.wikipedia.org/wiki/Angstromhttps://en.wikipedia.org/wiki/Nautical_milehttps://en.wikipedia.org/wiki/Barn_(unit)https://en.wikipedia.org/wiki/Knot_(unit)https://en.wikipedia.org/wiki/Neperhttps://en.wikipedia.org/wiki/Geodesyhttps://en.wikipedia.org/wiki/Geophysicshttps://en.wikipedia.org/wiki/Mathematical_descriptions_of_the_electromagnetic_field#Curved_spacetimehttps://en.wikipedia.org/wiki/Erghttps://en.wikipedia.org/wiki/Dynehttps://en.wikipedia.org/wiki/Poisehttps://en.wikipedia.org/wiki/Stokes_(unit)https://en.wikipedia.org/wiki/Stilb_(unit)https://en.wikipedia.org/wiki/Phothttps://en.wikipedia.org/wiki/Gal_(unit)https://en.wikipedia.org/wiki/Maxwell_(unit)https://en.wikipedia.org/wiki/Gauss_(unit)https://en.wikipedia.org/wiki/Oerstedhttps://en.wikipedia.org/wiki/Metre_Conventionhttps://en.wikipedia.org/wiki/International_Organization_for_Standardizationhttps://en.wikipedia.org/wiki/International_Electrotechnical_Commissionhttps://en.wikipedia.org/wiki/International_Electrotechnical_Commissionhttps://en.wikipedia.org/wiki/ISO/IEC_80000https://en.wikipedia.org/wiki/Units_of_measurementhttps://en.wikipedia.org/wiki/Common_nounhttps://en.wikipedia.org/wiki/British_Englishhttps://en.wikipedia.org/wiki/American_Englishhttps://en.wikipedia.org/wiki/Grammarhttps://en.wikipedia.org/wiki/English_grammarhttps://en.wikipedia.org/wiki/Henry_(unit)https://en.wikipedia.org/wiki/Luxhttps://en.wikipedia.org/wiki/Hertzhttps://en.wikipedia.org/wiki/Siemens_(unit)https://en.wikipedia.org/wiki/English_pluralhttps://en.wikipedia.org/wiki/Hyphen


Chinese expressway distances road sign in eastern Beijing. Al-though the primary text is in Chinese, the distances use the inter-nationally recognised numerals and symbols.

Chinese and Japanese

Chinese uses traditional logograms for writing the unitnames, while in Japanese unit names are written in thephonetic katakana script; in both cases, symbols are alsowritten using the internationally recognised Latin andGreek characters.

Chinese

The basic Chinese units are metre ( m ), litre (shng), gram ( k), and second ( mio), while oth-ers include watt ( w). Prefixes include deci- ( fn),centi- ( l), milli- ( ho), micro- ( wi), kilo- (qin), and mega- ( zho). These are combined to formdisyllabic characters, such as lm centimetreor qinw kilowatt.*[51] In the 19th century,various compound characters were also used, similar toJapanese, either imported or formed on the same princi-ples, such as for qinw (kilowatt) or for . These are generally not used today, but are occasion-ally found in older or technical writing.*[52]

Japanese

A set of characters representing various metric units wascreated in Japan in the late 19th century. Characters,same as the Chinese, exist for three base units: the me-tre (), litre () and gram (). These were combinedwith a set of six prefix characters kilo- (), hecto- (),deca- (), deci- (), centi- () andmilli- () to forman additional 18 single-character units. The seven lengthunits (kilometre to millimetre), for example, are,,, , , and . These characters, however, arenot in common use today; instead, units are written inkatakana, the Japanese syllabary used for foreign bor-rowings, such as (kiromtoru) for kilo-metre, but are also written in standard prefixes such as

kmfor kilometre. A few Sino-Japanese words for these

units remain in use in Japanese, most significantly ""(heibei) forsquare metre, but otherwise borrowed pro-nunciations are used.These characters are examples of the rare phenomenon ofsingle-character loan words a foreign word representedby a single Japanese character and form the plurality ofsuch words. Similar characters were also coined for otherunits, such as British units, though these also have fallenout of use; see Single character gairaigo: Metric units andSingle character gairaigo: Other units for a full list.

Unit symbols and the values of quantities

Although the writing of unit names is language-specific,the writing of unit symbols and the values of quan-tities is consistent across all languages and thereforethe SI Brochure has specific rules in respect of writ-ing them.*[22]*:130135 The guideline produced bythe National Institute of Standards and Technology(NIST)*[53] clarifies language-specific areas in respect ofAmerican English that were left open by the SI Brochure,but is otherwise identical to the SI Brochure.*[49]

General rules General rules*[Note 9] for writing SIunits and quantities apply to text that is either handwrittenor produced using an automated process:

The value of a quantity is written as a numberfollowed by a space (representing a multiplicationsign) and a unit symbol; e.g., 2.21 kg, 7.3102 m2,22 K. This rule explicitly includes the percent sign(%)*[22]*:134 and the symbol for degrees of tem-perature (C).*[22]*: 133 Exceptions are the sym-bols for plane angular degrees, minutes, and seconds(, , and ), which are placed immediately after thenumber with no intervening space.

Symbols are mathematical entities, not abbrevia-tions, and as such do not have an appended pe-riod/full stop (.), unless the rules of grammar de-mand one for another reason, such as denoting theend of a sentence.

A prefix is part of the unit, and its symbol isprepended to the unit symbol without a separator(e.g., k in km, M in MPa, G in GHz). Compoundprefixes are not allowed.

Symbols for derived units formed by multiplicationare joined with a centre dot () or a non-breakingspace; e.g., Nm or N m.

Symbols for derived units formed by division arejoined with a solidus (/), or given as a negativeexponent. E.g., themetre per secondcan be writ-ten m/s, m s*1, ms*1, or m/s. Only one solidusshould be used; e.g., kg/(ms2) and kgm*1s*2

https://en.wikipedia.org/wiki/Chinahttps://en.wikipedia.org/wiki/Expressways_of_Beijinghttps://en.wikipedia.org/wiki/Beijinghttps://en.wikipedia.org/wiki/Chinese_languagehttps://en.wikipedia.org/wiki/Numerical_digithttps://en.wikipedia.org/wiki/Symbol_(formal)https://en.wikipedia.org/wiki/Chinese_languagehttps://en.wikipedia.org/wiki/Logogramhttps://en.wikipedia.org/wiki/Japanese_languagehttps://en.wikipedia.org/wiki/Symbolhttps://en.wikipedia.org/wiki/Latin_charactershttps://en.wikipedia.org/wiki/Greek_alphabethttps://en.wikipedia.org/wiki/Kanjihttps://en.wikipedia.org/wiki/Kokujihttps://en.wikipedia.org/wiki/Meiji_periodhttps://en.wikipedia.org/wiki/Katakanahttps://en.wikipedia.org/wiki/Syllabaryhttps://en.wikipedia.org/wiki/Ateji#Single-character_loan_wordshttps://en.wiktionary.org/wiki/Appendix:Single%2520character%2520gairaigo#Metric%2520unitshttps://en.wiktionary.org/wiki/Appendix:Single%2520character%2520gairaigo#Other%2520unitshttps://en.wikipedia.org/wiki/National_Institute_of_Standards_and_Technologyhttps://en.wikipedia.org/wiki/Dot_operatorhttps://en.wikipedia.org/wiki/Solidus_(punctuation)https://en.wikipedia.org/wiki/Exponent


are acceptable, but kg/m/s2 is ambiguous and unac-ceptable.

Acceleration due to gravity.Note the lowercase letters (neithermetresnorsecondswerenamed after people), the space between the value and the units,and the superscript2to denotesquared.

The first letter of symbols for units derived from thename of a person is written in upper case; other-wise, they are written in lower case. E.g., the unit ofpressure is named after Blaise Pascal, so its symbolis written Pa, but the symbol for mole is writ-ten mol. Thus, Tis the symbol for tesla,a measure of magnetic field strength, and tthesymbol for tonne, a measure of mass. Since 1979,the litre may exceptionally be written using eitheran uppercaseLor a lowercasel, a decisionprompted by the similarity of the lowercase letter

lto the numeral 1, especially with certaintypefaces or English-style handwriting. The Ameri-can NIST recommends that within the United States

Lbe used rather thanl.

Symbols of units do not have a plural form; e.g., 25kg, not 25 kgs.

Uppercase and lowercase prefixes are not inter-changeable. E.g., the quantities 1 mW and 1 MWrepresent two different quantities; the former is thetypical power requirement of a hearing aid (1 mil-liwatt or 0.001 watts), and the latter the typicalpower requirement of a suburban train (1 megawattor 1000000 watts).

The 10th resolution of CGPM in 2003 declaredthat the symbol for the decimal marker shallbe either the point on the line or the comma onthe line.In practice, the decimal point is used inEnglish-speaking countries and most of Asia, andthe comma in most of Latin America and in conti-nental European languages.*[54]

Spaces should be used as a thousands separa-tor (1000000) in contrast to commas or periods(1,000,000 or 1.000.000) to reduce confusion re-sulting from the variation between these forms indifferent countries.

Any line-break inside a number, inside a compoundunit, or between number and unit should be avoided.

Where this is not possible, line breaks should coin-cide with thousands separators.

Since the value ofbillionandtrillioncan varyfrom language to language, the dimensionless terms

ppb(parts per billion) andppt(parts per trillion)should be avoided. No alternative is suggested in theSI Brochure.

Printing SI symbols Further rules*[Note 9] are speci-fied in respect of production of text using printing presses,word processors, typewriters and the like.

Symbols are written in upright (Roman) type (m formetres, s for seconds), so as to differentiate fromthe italic type used for quantities (m for mass, s fordisplacement). By consensus of international stan-dards bodies, this rule is applied independent of thefont used for surrounding text.

In Chinese, Japanese, and Korean language com-puting (CJK), some of the commonly used units,prefixunit combinations, or unitexponent combi-nations have been allocated predefined single char-acters taking up a full square. Unicode includesthese in its CJK Compatibility and letter-like sym-bols sub-ranges for back compatibility, without nec-essarily recommending future usage. These aresummarised in Unicode symbols. The cursive , aletter-like symbol, has been used in a number ofcountries in addition to China and Japan as a symbolfor the litre, but this is not currently recommendedby any standards body.

In print, the space used as a thousands separator(commonly called a thin space) is typically narrowerthan that used between words.

1.1.5 Realisation of units

Metrologists carefully distinguish between the definitionof a unit and its realisation. The definition of each baseunit of the SI is drawn up so that it is unique and providesa sound theoretical basis on which the most accurate andreproducible measurements can be made. The realisationof the definition of a unit is the procedure by which thedefinition may be used to establish the value and asso-ciated uncertainty of a quantity of the same kind as theunit. A description of the mise en pratique*[Note 10] ofthe base units is given in an electronic appendix to the SIBrochure.*[56]*[22]*:168169The published mise en pratique is not the only way inwhich a base unit can be determined: the SI Brochurestates that any method consistent with the laws ofphysics could be used to realise any SI unit.*[22]*:111In the current (2016 ) exercise to overhaul the definitionsof the base units, various consultative committees of the

https://en.wikipedia.org/wiki/Upper_casehttps://en.wikipedia.org/wiki/Lower_casehttps://en.wikipedia.org/wiki/Pressurehttps://en.wikipedia.org/wiki/Blaise_Pascalhttps://en.wikipedia.org/wiki/Tesla_(unit)https://en.wikipedia.org/wiki/Magnetic_field_strengthhttps://en.wikipedia.org/wiki/Tonnehttps://en.wikipedia.org/wiki/Masshttps://en.wikipedia.org/wiki/Litrehttps://en.wikipedia.org/wiki/CGPMhttps://en.wikipedia.org/wiki/Decimal_separatorhttps://en.wikipedia.org/wiki/Full_stophttps://en.wikipedia.org/wiki/Commahttps://en.wikipedia.org/wiki/Latin_Americahttps://en.wikipedia.org/wiki/Languages_of_Europehttps://en.wikipedia.org/wiki/Thousands_separatorhttps://en.wikipedia.org/wiki/Thousands_separatorhttps://en.wikipedia.org/wiki/Long_and_short_scaleshttps://en.wikipedia.org/wiki/Long_and_short_scaleshttps://en.wikipedia.org/wiki/Billion_(word)https://en.wikipedia.org/wiki/Long_and_short_scaleshttps://en.wikipedia.org/wiki/Printing_presshttps://en.wikipedia.org/wiki/Word_processorhttps://en.wikipedia.org/wiki/Typewriterhttps://en.wikipedia.org/wiki/Roman_typehttps://en.wikipedia.org/wiki/Italic_typehttps://en.wikipedia.org/wiki/Chinese_languagehttps://en.wikipedia.org/wiki/Japanese_languagehttps://en.wikipedia.org/wiki/Korean_languagehttps://en.wikipedia.org/wiki/CJK_charactershttps://en.wikipedia.org/wiki/Unicodehttp://www.unicode.org/charts/PDF/U3300.pdfhttp://www.unicode.org/charts/PDF/U2100.pdfhttp://www.unicode.org/charts/PDF/U2100.pdfhttps://en.wikipedia.org/wiki/Unicode_symbolshttps://en.wikipedia.org/wiki/Standards_organizationhttps://en.wikipedia.org/wiki/Space_(punctuation)https://en.wikipedia.org/wiki/Thin_spacehttps://en.wikipedia.org/wiki/New_SI_definitionshttps://en.wikipedia.org/wiki/New_SI_definitions


Silicon sphere for the Avogadro project used for measuring theAvogadro constant to a relative uncertainty of 210*8 or less,held by Achim Leistner.*[55]

CIPM have required that more than one mise en pratiqueshall be developed for determining the value of each unit.In particular:

At least three separate experiments be carried outyielding values having a relative standard uncer-tainty in the determination of the kilogram of nomore than 510*8 and at least one of these valuesshould be better than 210*8. Both the Watt bal-ance and the Avogadro project should be included inthe experiments and any differences between thesebe reconciled.*[57]*[58]

When the kelvin is being determined, the relativeuncertainty of the Boltzmann constant derived fromtwo fundamentally different methods such as acous-tic gas thermometry and dielectric constant gas ther-mometry be better than one part in 10*6 andthat these values be corroborated by other measure-ments.*[59]

1.1.6 Post-1960 changes

The preamble to the Metre Convention read "Desiringthe international uniformity and precision in standards ofweight and measure, have resolved to conclude a conven-tion ....*[23] Changing technology has led to an evolu-tion of the definitions and standards that has followed two

principal strands changes to SI itself and clarification ofhow to use units of measure that are not part of SI, butare still nevertheless used on a worldwide basis.

Changes to the SI

Since 1960 the CGPM has made a number of changes toSI. These include:

The 13th CGPM (1967) renamed the degreeKelvin(symbol K) to the kelvin(symbolK)*[22]*:156

The 14th CGPM (1971) added the mole (symbolmol) to the list of base units.*[60]

The 14th GCPM (1971) added the pascal (sym-bol Pa) for pressure and the siemens (symbol S) forelectrical conductance to the list of named derivedunits.*[22]*:156

The 15th CGPM (1975) added the becquerel (sym-bol Bq) for "activity referred to a radionuclide" andthe gray (symbol Gy) for ionizing radiation to thelist of named derived units*[22]*:156

In order to distinguish between "absorbed dose" and"dose equivalent", the 16th CGPM (1979) added thesievert (symbol Sv) to the list of named derived unitsas the unit of dose equivalent.*[22]*:158

The 16th CGPM (1979) clarified that in a breakwithconvention either the letter Lor the letter lmay be used as a symbol for the litre.*[22]*:159

The 21st CGPM (1999) added the katal (symbolkat) for catalytic activity to the list of named derivedunits.*[22]*:165

In its original form (1960), the SI defined pre-fixes for values ranging from pico- (symbol p) hav-ing a value of 10*12 to tera- (symbol T) hav-ing a value of 1012. The list was extended atthe 12th CGPM (1964),*[22]*:152 at the 15thCGPM (1975)*[22]*:158 and at the 19th CGPM(1991)*[22]*:164 to give the current range of pre-fixes.

In addition, advantage was taken of developments in tech-nology to redefine many of the base units enabling the useof higher precision techniques.

Retention of non-SI units

Although, in theory, SI can be used for any physical mea-surement, it is recognised that some non-SI units still ap-pear in the scientific, technical and commercial literature,and will continue to be used for many years to come. In

https://en.wikipedia.org/wiki/Avogadro_projecthttps://en.wikipedia.org/wiki/Achim_Leistnerhttps://en.wikipedia.org/wiki/Kilogramhttps://en.wikipedia.org/wiki/Watt_balancehttps://en.wikipedia.org/wiki/Watt_balancehttps://en.wikipedia.org/wiki/Avogadro_projecthttps://en.wikipedia.org/wiki/Kelvinhttps://en.wikipedia.org/wiki/Boltzmann_constanthttps://en.wikipedia.org/wiki/Thermometryhttps://en.wikipedia.org/wiki/Metre_Conventionhttps://en.wikipedia.org/wiki/Mole_(unit)https://en.wikipedia.org/wiki/Pascal_(unit)https://en.wikipedia.org/wiki/Pressurehttps://en.wikipedia.org/wiki/Siemens_(unit)https://en.wikipedia.org/wiki/Becquerelhttps://en.wikipedia.org/wiki/Radioactive_decayhttps://en.wikipedia.org/wiki/Radionuclidehttps://en.wikipedia.org/wiki/Gray_(unit)https://en.wikipedia.org/wiki/Absorbed_dosehttps://en.wikipedia.org/wiki/Dose_equivalenthttps://en.wikipedia.org/wiki/Sieverthttps://en.wikipedia.org/wiki/Litrehttps://en.wikipedia.org/wiki/Katalhttps://en.wikipedia.org/wiki/Catalytic_activity


Sphygmomanometer the traditional device that measures bloodpressure using mercury in a manometer. Pressures are recordedin "millimetres of mercury" a non-SI unit

addition, certain other units are so deeply embedded inthe history and culture of the human race that they willcontinue to be used for the foreseeable future.*[61] TheCIPM has catalogued such units and included them in theSI Brochure so that they can be used consistently.The first such group comprises the units of time and ofangles and certain legacy non-SI metric units. Most ofmankind has used the day and its subdivisions as a basis oftime with the result that the second, minute, hour and day,unlike the foot or the pound, were the same regardlessof where it was being measured. The second has beencatalogued as an SI unit, its multiples as units of measurethat may be used alongside the SI. The measurement ofangles has likewise had a long history of consistent use the radian, being 1/2 of a revolution, has mathematicalniceties, but it is cumbersome for navigation, hence theretention of the degree, minute and second of arc. Thetonne, litre and hectare were adopted by the CGPM in1879 and have been retained as units that may be usedalongside SI units, having been given unique symbols.Physicists often use units of measure that are based onnatural phenomena such as the speed of light, the massof a proton (approximately one dalton), the charge ofan electron and the like. These too have been cata-logued in the SI Brochure with consistent symbols, butwith the caveat that their physical values need to be mea-sured.*[Note 11]In the interests of standardising health-related units ofmeasure used in the nuclear industry, the 12th CGPM(1964) accepted the continued use of the curie (symbolCi) as a non-SI unit of activity for radionuclides;*[22]*:152 the becquerel, sievert and gray were adopted in

later years. Similarly, the millimetre of mercury (sym-bol mmHg) was retained for measuring blood pres-sure.*[22]*: 127

1.1.7 Global adoption

Top view of the 2 kg weight (witha credit card to show the weight's size)

Bottom view of the 2 kg showingthe lead plug and assayer's stamp.A commercial-quality hexagonal 2 kg weight with a leadplug and assayer's mark and year of manufacture (79representing 1979) that is similar to, but whichpredates OIML recommendation R52.*[62]

SI has become the world's most widely used systemof measurement, used in both everyday commerce andscience.*[63]*[64] The change to SI had little effect oneveryday life in countries that used the metric system the metre, kilogram, litre and second remained un-changed as did the way in which they were used mostof the changes only affected measurements in the work-place.*[65] The CGPM has a role of recommendingchanges, but no formal role in the enforcement of suchchangesanother inter-governmental organisation, theInternational Organization of Legal Metrology (OIML)provides a forum for harmonisation of national standardsand legislation in respect of metrology.Both the degree and rate of adoption of SI varied fromcountry to countrycountries that had not adopted themetric system by 1960 and subsequently adopted SI didso directly as part of their metrication programs whileothers migrated from the CGS system of units to SI. In1960, the world's largest economy was that of the UnitedStates, followed by the United Kingdom, West Germany,France, Japan, China and India.*[66] The United Statesand the United Kingdom were non-metric, France andGermany had been using the metric system for about a

https://en.wikipedia.org/wiki/Sphygmomanometerhttps://en.wikipedia.org/wiki/Mercury_(element)https://en.wikipedia.org/wiki/Pressure_measurement#Liquid_columnhttps://en.wikipedia.org/wiki/Foot_(unit)https://en.wikipedia.org/wiki/Pound_(mass)https://en.wikipedia.org/wiki/Radianhttps://en.wikipedia.org/wiki/Tonnehttps://en.wikipedia.org/wiki/Litrehttps://en.wikipedia.org/wiki/Hectarehttps://en.wikipedia.org/wiki/Dalton_(unit)https://en.wikipedia.org/wiki/Curiehttps://en.wikipedia.org/wiki/Credit_cardhttps://en.wikipedia.org/wiki/International_Organization_of_Legal_Metrologyhttps://en.wikipedia.org/wiki/Systems_of_measurementhttps://en.wikipedia.org/wiki/Systems_of_measurementhttps://en.wikipedia.org/wiki/Commercehttps://en.wikipedia.org/wiki/Sciencehttps://en.wikipedia.org/wiki/General_Conference_on_Weights_and_Measureshttps://en.wikipedia.org/wiki/International_Organization_of_Legal_Metrologyhttps://en.wikipedia.org/wiki/Centimetre%E2%80%93gram%E2%80%93second_system_of_units


century, and China had been using the metric system for35 years, while India and Japan had adopted the metricsystem within the preceding five years. Other non-metriccountries were those where the United Kingdom or theUnited States had considerable influence.*[Note 12]*[67]These differences are brought out in the examples below:

United Kingdom and the former British Empire

See also: Metrication in the United Kingdom,Metrication in Australia, Metrication in New Zealand,and Metrication in Canada

Even though the use ofmetric units was legalised for tradein the UK in 1864, the UK had signed the Metre Con-vention in 1884 and the UK Parliament had defined theyard and the pound in terms of the metre and the kilo-gram in 1897, the UK continued to use the imperial sys-tem of measure*[68] and to export the imperial systemof units to the Empire.*[Note 13] In 1932, the systemof Imperial Preference was set up at the Ottawa Confer-ence. Although Ireland left the Commonwealth in 1948and South Africa in 1961,*[69] both continued their closeeconomic ties with the Commonwealth.*[70]

Metrication logo of the Board of Trade

When the SI standard was published in 1960, the onlyma-jor Commonwealth country to have adopted the metricsystem was India. In 1863, the first reading of a bill thatwould have made the metric system compulsory passedits first reading in the House of Commons by 110 votes to75. The bill, however, failed to make the statute book be-cause of lack of parliamentary time.*[10]*:136 In 1965,after this and similar false starts the then Federation ofBritish Industry informed the British Government thatits members favoured the adoption of the metric sys-tem. The rationale behind the request was that 80% ofBritish exports were to countries that used the metric sys-tem or that were considering changing to the metric sys-tem. The Board of Trade, on behalf of the Government,agreed to support a ten-year metrication programme. Thegovernment agreed to a voluntary policy requiring mini-mal legislation and costs to be borne where they fell. SIwould be used from the outset.*[71] The rest of the Com-monwealth, South Africa and Ireland followed within a

few years; in some countries such as South Africa andAustralia metrication was mandatory rather than volun-tary.*[72]*[73]By 1980 all apart from the United Kingdom, Canadaand Ireland had effectively completed their programs. Inthe United Kingdom the breakdown of voluntary met-rication in the mid-1970s*[74]*:1.8 coincided with theUnited Kingdom's obligations as part of the EEC to adoptthe metric system, resulting in legislation to force met-rication in certain areas and the Eurosceptic movementadopting an anti-metrication stance and the United King-dom seeking a number of derogations from the rele-vant EEC directives. Once the metrication of most con-sumer goods was completed in 2000, aspects of Britishlife, especially in government, commerce and industryused SI.*[74]*:1.6 & 1.10 Although imperial units arewidely encountered in unregulated areas such as the pressand everyday speech, SI or units approved for use along-side SI are used in most areas where units of measureare regulated. High-profile exceptions include the sale ofdraught beer, the sale of milk in returnable containers,and United Kingdom road signs. Irish road signs (roaddistances and speeds) were converted to metric units dur-ing the first decade of the 21st century;*[75] otherwise,the situation in Ireland is similar to that in the UnitedKingdom.*[76]Canada has adopted it for most purposes, but imperialunits are still legally permitted and remain in common usethroughout a few sectors of Canadian society, particularlyin the buildings, trades and railways sectors.*[77]*[78]

European Union

Main article: European units of measurement directives

In 1960, all the largest industrialised nations that had anestablished history of using the metric system were mem-bers of the European Economic Community (EEC).In 1972, in order to harmonise units of measure as part ofa programme to facilitate trade between member states,the EEC issued directive 71/354/EEC.*[79] This direc-tive catalogued units of measure that could be used for

economic, public health, public safety and administra-tive purposesand also provided instructions for a tran-sition from the existing units of measure that were in use.The directive replicated the CGPM SI recommendationsand in addition pre-empted some of the additions whoseuse had been recommended by the CIPM in 1969, buthad not been ratified by the CGPM.*[Note 14] The direc-tive also catalogued units of measure whose status wouldbe reviewed by the end of 1977 (mainly coherent CGSunits of measure) and also catalogued units of measurethat were to be phased out by the end of 1977, includingthe use of obsolete names for the sale of timber such asthe stere, the use of units of force and pressure that madeuse of the acceleration due to gravity,*[Note 15] the use

https://en.wikipedia.org/wiki/Metrication_in_the_United_Kingdomhttps://en.wikipedia.org/wiki/Metrication_in_Australiahttps://en.wikipedia.org/wiki/Metrication_in_New_Zealandhttps://en.wikipedia.org/wiki/Metrication_in_Canadahttps://en.wikipedia.org/wiki/Metre_Conventionhttps://en.wikipedia.org/wiki/Metre_Conventionhttps://en.wikipedia.org/wiki/Parliament_of_the_United_Kingdomhttps://en.wikipedia.org/wiki/Imperial_unitshttps://en.wikipedia.org/wiki/Imperial_unitshttps://en.wikipedia.org/wiki/British_Empirehttps://en.wikipedia.org/wiki/Imperial_Preferencehttps://en.wikipedia.org/wiki/British_Empire_Economic_Conferencehttps://en.wikipedia.org/wiki/British_Empire_Economic_Conferencehttps://en.wikipedia.org/wiki/Republic_of_Irelandhttps://en.wikipedia.org/wiki/Commonwealth_of_Nationshttps://en.wikipedia.org/wiki/House_of_Commons_of_the_United_Kingdomhttps://en.wikipedia.org/wiki/Metrication_in_the_United_Kingdomhttps://en.wikipedia.org/wiki/Federation_of_British_Industryhttps://en.wikipedia.org/wiki/Federation_of_British_Industryhttps://en.wikipedia.org/wiki/Board_of_Tradehttps://en.wikipedia.org/wiki/European_Economic_Communityhttps://en.wikipedia.org/wiki/Eurosceptichttps://en.wikipedia.org/wiki/Anti-metricationhttps://en.wikipedia.org/wiki/Metrication_in_Canadahttps://en.wikipedia.org/wiki/Imperial_unitshttps://en.wikipedia.org/wiki/Imperial_unitshttps://en.wikipedia.org/wiki/European_units_of_measurement_directiveshttps://en.wikipedia.org/wiki/European_Economic_Communityhttps://en.wikipedia.org/wiki/European_units_of_measurement_directiveshttps://en.wikipedia.org/wiki/Stere


of non-coherent units of power such as the Pferdestrke(PS), the use of the calorie as a measure of energy andthe stilb as a measure of luminance. The directive wassilent in respect of units that were specific to one or twocountries including the pond, pfund, livre (Dutch, Ger-man and French synonyms for 500 g), thereby effectivelyprohibiting their use as well.When the directive was revisited during 1977, some ofthe older units that were being reviewed (such as millime-tre of mercury for blood pressure)*[Note 16] were re-tained but others were phased out, thereby broadly align-ing the allowable units with SI. The directive was how-ever overhauled to accommodate British and Irish inter-ests in retaining the imperial system in certain circum-stances.*[80] It was reissued as directive 80/181/EEC.During subsequent revisions, the directive has reflectedchanges in the definition of SI. The directive also for-malised the use of supplementary units, which in 1979were permitted for a period of ten years. The cut-off datefor the use of supplementary units was extended a num-ber of times and in 2009 was extended indefinitely.*[81]

India

Main article: Metrication in India

India was one of the last countries to start a metricationprogramme before the advent of SI. When it became in-dependent in 1947, both imperial and native units of mea-sure were in use. Its metrication programme started in1956 with the passing of the Standards of Weights andMeasures Act. Part of the act fixed the value of the seer(a legacy unit of mass) to 0.9331 kg exactly; elsewherethe Act declared that from 1960 all non-metric units ofmeasure were to be illegal.*[82]Four years after the Indian Government announced itsmetrication programme, SI was published. The resultwas that the initial metrication programme was a con-version to the CGS system of units and the subsequentadoption of SI has been haphazard. Fifty years later,many of the country's schoolbooks still use CGS or im-perial units.*[83] Originally the Indian Government hadplanned to replace all units of measure with metric unitsby 1960. In 1976 a new Weights and Measures Act re-placed the 1956 Act which, amongst other things, re-quired that all weighing devices be approved before be-ing released onto the market place. However, in 2012, itwas reported that traditional units were still encounteredin small manufacturing establishments and in the market-place alongside CGS, SI and imperial measures, particu-larly in the poorer areas.*[84]The use of the Indian numbering system of crores(10000000) and lakhs (100000), which do not map ontothe SI system of prefixes, is widespread and is oftenfound alongside or in place of the western numbering sys-tem.*[86]

Banana market stall in Kerala, India the trader is using hexag-onal weights a shape that is consistent with metric weights, bothin India*[85] and internationally*[62]

United States

Main article: Metrication in the United StatesEven though Congress set up a framework for the use of

Logo of the American National Metrication Council, an organi-sation set up by American National Standards Institute (ANSI) tocoordinate the country's transition to the metric system.

the metric system in the nineteenth century,*[87]*[Note17] the United States continues to use US customaryunits, based on English measure passed by parliament un-der the reign of Queen Anne in 1706, for most purposesapart from science and medicine.*[88] In Puerto Rico,

https://en.wikipedia.org/wiki/Pferdest%C3%A4rkehttps://en.wikipedia.org/wiki/Pferdest%C3%A4rkehttps://en.wikipedia.org/wiki/Caloriehttps://en.wikipedia.org/wiki/Stilb_(unit)https://en.wikipedia.org/wiki/Luminancehttps://en.wikipedia.org/wiki/Blood_pressurehttps://en.wikipedia.org/wiki/European_units_of_measurement_directiveshttps://en.wikipedia.org/wiki/Metrication_in_Indiahttps://en.wikipedia.org/wiki/Seer_(unit)https://en.wikipedia.org/wiki/Government_of_Indiahttps://en.wikipedia.org/wiki/Textbookhttps://en.wikipedia.org/wiki/Weighing_scalehttps://en.wikipedia.org/wiki/Indian_numbering_systemhttps://en.wikipedia.org/wiki/Crorehttps://en.wikipedia.org/wiki/Lakhhttps://en.wikipedia.org/wiki/Western_worldhttps://en.wikipedia.org/wiki/Keralahttps://en.wikipedia.org/wiki/Metrication_in_the_United_Stateshttps://en.wikipedia.org/wiki/United_States_Congresshttps://en.wikipedia.org/wiki/American_National_Standards_Institutehttps://en.wikipedia.org/wiki/United_States_customary_unitshttps://en.wikipedia.org/wiki/United_States_customary_unitshttps://en.wikipedia.org/wiki/Sciencehttps://en.wikipedia.org/wiki/Puerto_Rico


metric units are widely used due to the vast majority ofthe population having Spanish heritage.*[89]On 10 February 1964, the National Bureau of Standards(now the National Institute of Standards and Technol-ogy) issued a statement that it was to use SI except wherethis would have an obvious detrimental effect. In 1968Congress authorised the U.S. Metric Study the empha-sis of which was to examine the feasibility of adoptingSI.*[90] The first volumewas delivered in 1970.*[91] Thestudy recommended that the United States adopt the In-ternational System of units,*[92] and in 1975 Congresspassed the Metric Conversion Act of 1975 which estab-lished a national policy of coordinating and planning forthe increased use of the metric measurement system inthe United States.*[93] Metrication was voluntary andto be coordinated by the United States Metric Board(USMB).*[94]Efforts during the Ford and Carter administrations toforce metrication were seized on by many newspaper ed-itorialists as being dictatorial.*[5]*:365 Public responseincluded resistance, apathy, and sometimes ridicule.*[95]The underlying reasons for this response include a rel-ative uniformity of weights and measures (though, no-tably, US liquid measure differed by about 20% fromBritish Imperial measure, which was adopted throughoutthe British Empire in 1824) inherited from the UnitedKingdom in 1776, a homogeneous economy and the influ-ence of business groups and populists in Congress causedthe country to look at the short-term costs associated withthe change-over, particularly those that would be borneby the consumer rather than long-term benefits of effi-ciency and international trade. The Metrication Boardwas disbanded under President Ronald Reagan's direc-tion in 1982.*[5]*:362365The 1988 Omnibus Foreign Trade and CompetitivenessAct removed international trade barriers and amended theMetric Conversion Act of 1975, designating the metricsystem asthe Preferred system of weights and measuresfor United States trade and commerce. The legislationstated that the federal government has a responsibility toassist industry, especially small business, as it voluntar-ily converts to the metric system of measurement.*[96]Exceptions were made for the highway and constructionindustries; the Department of Transportation planned torequire metric units by 2000, but this plan was cancelledby the 1998 highway bill TEA21.*[97] However, the USmilitary uses the metric system widely, partly becauseof the need to work with armed services from other na-tions.*[98] Although overall responsibility for labellingrequirements of consumer goods lies with Congress andis therefore covered by federal law, details of labellingrequirements for certain commodities are controlled bystate law or by other authorities such as the Food andDrug Administration, Environmental Protection Agencyand Alcohol and Tobacco Tax and Trade

the international system of units the international system of... · the international system of...

Documents