International System of Units | Wikipedia audio article
Channel: wikipedia tts
Date: 2018-12-08

This is an audio version of the Wikipedia Article:
International System of Units


00:03:06 1 Units and prefixes
00:03:43 1.1 Base units
00:04:46 1.2 Derived units
00:05:45 1.3 Prefixes
00:06:36 1.4 Non-SI units accepted for use with SI
00:07:29 1.5 Common notions of the metric units
00:08:28 2 Lexicographic conventions
00:12:10 2.1 Unit names
00:14:26 2.2 Unit symbols and the values of quantities
00:14:36 2.2.1 General rules
00:15:31 2.2.2 Printing SI symbols
00:16:08 2.2.3 Examples of the variety of symbols in use around the world for kilometres per hour
00:20:13 3 International System of Quantities
00:20:36 4 Realisation of units
00:21:04 5 Evolution of the SI
00:21:54 5.1 Changes to the SI
00:23:42 5.2 2019 redefinitions
00:23:51 6 History
00:25:52 6.1 The improvisation of units
00:27:44 6.2 Metre Convention
00:27:53 6.3 The cgs and MKS systems
00:32:03 6.4 The iPractical system of units/i
00:33:20 6.5 Birth of the SI
00:36:21 7 See also
00:37:37 8 Notes
00:38:03 9 References



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SUMMARY
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The International System of Units (SI, abbreviated from the French Système international (d'unités)) is the modern form of the metric system, and is the most widely used system of measurement. It comprises a coherent system of units of measurement built on seven base units, which are the ampere, kelvin, second, metre, kilogram, candela, mole, and a set of twenty prefixes to the unit names and unit symbols that may be used when specifying multiples and fractions of the units. The system also specifies names for 22 derived units, such as lumen and watt, for other common physical quantities.
The base units are derived from invariant constants of nature, such as the speed of light in vacuum and the triple point of water, which can be observed and measured with great accuracy, and one physical artefact. The artefact is the international prototype kilogram, certified in 1889, and consisting of a cylinder of platinum-iridium, which nominally has the same mass as one litre of water at the freezing point. Its stability has been a matter of significant concern, culminating in a revision of the definition of the base units entirely in terms of constants of nature, scheduled to be put into effect on 20 May 2019.Derived units may be defined in terms of base units or other derived units. They are adopted to facilitate measurement of diverse quantities. The SI is intended to be an evolving system; units and prefixes are created and unit definitions are modified through international agreement as the technology of measurement progresses and the precision of measurements improves. The most recent derived unit, the katal, was defined in 1999.
The reliability of the SI depends not only on the precise measurement of standards for the base units in terms of various physical constants of nature, but also on precise definition of those constants. The set of underlying constants is modified as more stable constants are found, or may be more precisely measured. For example, in 1983 the metre was redefined as the distance that light propagates in vacuum in a given fraction of a second, thus making the value of the speed of light in terms of the defined units exact.
The motivation for the development of the SI was the diversity of units that had sprung up within the centimetre–gram–second (CGS) systems (specifically the inconsistency between the systems of electrostatic units and electromagnetic units) and the lack of coordination between the various disciplines that used them. The General Conference on Weights and Measures (French: Conférence générale des poids et mesures – CGPM), which was established by the Metre Convention of 1875, brought together many international organisations to establish the definitions and standards of a new system and standardise the rules for writing and presenting measurements. The system was published in 1960 as a result of an initiative that began in 1948. It is based on the metre–kilogram–second system of units (MKS) rather tha ...