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Metric system

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 Title: Metric system Author: World Heritage Encyclopedia Language: English Subject: Collection: Publisher: World Heritage Encyclopedia Publication Date:

Metric system

For a topical guide to this subject, see Outline of the metric system.
Countries which have officially adopted the metric system
Countries which have not officially adopted the metric system (US, Myanmar (formerly Burma) and Liberia)
"The metric system is for all people for all time." (Condorcet, 1791). Four everyday measuring devices that have metric calibrations: a tape measure calibrated in centimetres, a thermometer calibrated in degrees Celsius, a kilogram weight, and an electrical multimeter that measures volts, amperes and ohms.

The metric system is an internationally agreed decimal system of measurement that was originally based on the mètre des Archives and the kilogramme des Archives introduced by the French First Republic in 1799. Over the years, the definitions of the metre and the kilogram have been refined, and the metric system has been extended to incorporate many more units. Although a number of variants of the metric system emerged in the late nineteenth and early twentieth centuries, the term is now often used as a synonym for "SI"[Note 1] or the "International System of Units"—the official system of measurement in almost every country in the world.

The metric system has been officially sanctioned for use in the United States since 1866, but it remains the only industrialised country that has not adopted the metric system as its official system of measurement. Many sources also cite Liberia and Burma as the only other countries not to have done so. Although the United Kingdom uses the metric system for most official purposes, the use of the imperial system of measure, particularly for use at home, is widespread and is permitted by the law.

Although the originators intended to devise a system that was equally accessible to all, it proved necessary to use General Conference on Weights and Measures (CGPM).[Note 1] It is now hoped that the last of these prototypes can be retired by 2014.

From its beginning, the main features of the metric system were the standard set of inter-related base units and a standard set of prefixes in powers of ten. These base units are used to derive larger and smaller units that could replace a huge number of other units of measure in existence. Although the system was first developed for commercial use, the development of coherent units of measure made it particularly suitable for science and engineering.

The uncoordinated use of the metric system by different scientific and engineering disciplines, particularly in the late 19th century, resulted in different choices of fundamental units, even though all were based on the same definitions of the metre and the kilogram. During the 20th century, efforts were made to rationalise these units, and in 1960 the CGPM published the International System of Units which, since then, has been the internationally recognised standard metric system.

Contents

• Features 1
• Universality 1.1
• Decimal multiples 1.2
• Realisability and replicable prototypes 1.3
• Metre and kilogram 1.3.1
• Other base units 1.3.2
• Coherence 1.4
• History 2
• Original metric system 2.1
• International standards 2.3
• Variants 3
• Centimetre-gram-second systems 3.1
• Metre-kilogram-second systems 3.2
• Metre-tonne-second systems 3.3
• Gravitational systems 3.4
• International System of Units 3.5
• Relating SI to the real world 4
• Usage around the world 5
• Variations in spelling 5.1
• Conversion and calculation incidents 5.2
• Conversion between SI and legacy units 6
• Future developments 7
• Notes 9
• References 10

Features

Although the metric system has changed and developed since its inception, its basic concepts have hardly changed. Designed for transnational use, it consisted of a basic set of units of measurement, now known as base units. Derived units were built up from the base units using logical rather than empirical relationships while multiples and submultiples of both base and derived units were decimal-based and identified by a standard set of prefixes.

Universality

Chinese road sign listing distances on an expressway in eastern Beijing. Although the primary text is in Chinese, the distances use internationally recognised characters.
At the outbreak of the French Revolution in 1789, most countries and even some cities had their own system of measurement. Although different countries might have used units of measure with the same name, such as the foot, or local language equivalents such as pied, fuß and voet, there was no consistency in the magnitude of those units, nor in the relationships with their multiples and sub-multiples,[1] much like the modern-day differences between the US and the UK pints and gallons.[2]

The metric system was designed to be universal—in the words of the French philosopher Marquis de Condorcet it was to be "for all people for all time".[3]:1 It was designed for ordinary people, for engineers who worked in human-related measurements and for astronomers and physicists who worked with numbers both small and large, hence the huge range of prefixes that have now been defined in SI.[4]

When the French Government first investigated the idea of overhauling their system of measurement, the concept of universality was put into practice when, in 1789,