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SI - International system of units
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International system of units</span> </li> <li class="active" class="breadcrumb-text"> <span>SI - International system of units</span> </li> </ol> </div> </div> </div> </div> <section class="col-sm-12"> <div class="highlighted"> <div class="region region-highlighted"> <div data-drupal-messages-fallback class="hidden"></div> </div> </div> <a id="main-content"></a> <div class="region region-content"> <article about="/si" class="page full clearfix"> <div class="content"> <div class="dynamic-layouts-container custom-12" > <div class="dynamic-layout-row iec-row "> <div data-region="r5c1c5969093f7c5c1c5969093fa" class="dynamic-layouts-container-column layout-builder--layout__region col-12 iec-col "> <section class="block block-layout-builder block-inline-blockmodule-overview-header clearfix iec-overview-block iec-full-width with-margin" style="background-image:url(/system/files/styles/original_image/private/2019-10/content/media/image/sibg.jpg?itok=mh8Vt18T);"> <div class="iec-overview-block-content"> <div class="container"> <div class="iec-overview-block-text"> <h1 class="text-block-title field--name-field-title condensed">SI - International System of Units</h1> <div class="field field--name-field-description field--type-string-long field--label-hidden field--item">The International System of Units, or SI (from its French name, “Système international d'unités”), is a system of measurement units, based on the International System of Quantities (ISQ), defined in the ISO and IEC 80000 series of standards</div> </div> </div> </div> </section> </div> </div> </div> <div class="dynamic-layouts-container custom-12" > <div class="dynamic-layout-row iec-row "> <div data-region="r5c1c5a740a6d1c5c1c5a740a6d4" class="dynamic-layouts-container-column layout-builder--layout__region col-8 iec-col "> <section class="block block-iec-section-custom block-layout-builder block-inline-blockmodule-single-element-text clearfix single-element-text" id="single-element-text-1696"> <div class="text-block-header" style="text-align:left;"> <h2 class="text-block-title condensed" style="font-size:35px;" id="article-header-id-1696-units-for-electrotechnical-technology"> Units for electrotechnical technology </h1> <h2 class="text-block-subtitle condensed" id="article-header-id-1696-comparing-what-is-comparable"> Comparing what is comparable </h2> </div> <div class="text-block-content" style="text-align:left;"> <div class="field field--name-body field--type-text-with-summary field--label-hidden field--item"><p><a href="/tc25">IEC Technical Committee 25</a> prepares international standards on quantities and units to be used in electrical and electrotechnical technology. It also reviews the use of quantities and units in IEC Standards. These standards should, whenever possible, be based on the SI. They may be related to definitions, names, letter symbols and their use or to the relations in which these quantities and units appear, and to the signs and symbols used with them.</p> <p>Close co-operation is required with <a href="/tc1">IEC TC 1</a>: Terminology, and technical liaison are maintained with the following organizations:</p> <ul> <li><a href="http://www.bipm.org/" target="_blank">BIPM</a> - International Bureau of Weights and Measures</li> <li><a href="http://www.iso.org/iso/iso_technical_committee?commid=46202" target="_blank">ISO/TC 12</a> - Quantities, units, symbols, conversion factors</li> <li><a href="http://www.itu.int/" target="_blank">ITU</a> - International Telecommunication Union</li> <li><a href="http://www.oiml.org/" target="_blank">OIML</a> - International Organization of Legal Metrology</li> </ul> <p> </p> </div> </div> </section> </div> <div data-region="r5c1c5a740a6d1c5c1c5a7c20239" class="dynamic-layouts-container-column layout-builder--layout__region col-4 iec-col "> <section class="block block-iec-section-custom block-layout-builder block-inline-blockmodule-single-element-text clearfix single-element-text" id="single-element-text-2154"> <div class="text-block-header" style="text-align:left;"> <h2 class="text-block-title condensed" style="font-size:35px;" id="article-header-id-2154-base-units"> Base units </h1> <h2 class="text-block-subtitle condensed" id="article-header-id-2154-seven-base-units-form-the-core-of-the-si"> Seven base units form the core of the SI: </h2> </div> <div class="text-block-content" style="text-align:left;"> <div class="field field--name-body field--type-text-with-summary field--label-hidden field--item"><ul type="disc"> <li>the second (s) for duration (i.e., extension of time)</li> <li> <p>the metre (m) for length (i.e., extension of space)</p> </li> <li> <p>the kilogram (kg) for mass</p> </li> <li> <p>the ampere (A) for electric current</p> </li> <li> <p>the kelvin (K) for thermodynamic temperature</p> </li> <li> <p>the mole (mol) for amount of substance</p> </li> <li> <p>the candela (cd) for luminous intensity</p> </li> </ul> </div> </div> </section> </div> </div> </div> <div class="dynamic-layouts-container custom-12" > <div class="iec-full-width iec-bg-grey"> <div class="container"> <div class="dynamic-layout-row iec-row iec-fixed-row-bg "> <div data-region="r5c627bcf91f79c5c627bcf91f7d" class="dynamic-layouts-container-column layout-builder--layout__region col-12 iec-col "> <section class="block block-layout-builder block-inline-block-module-white-gray-space clearfix"> <div class="module-white-gray-space-content iec-bg-color iec-bg-grey"> </div> </section> <section class="block block-iec-section-custom block-layout-builder block-inline-blockmodule-single-element-text clearfix single-element-text" id="single-element-text-2155"> <div class="text-block-header" style="text-align:left;"> <h2 class="text-block-title condensed" style="font-size:35px;" id="article-header-id-2155-the-revised-si"> The revised SI </h1> </div> <div class="text-block-content" style="text-align:left;"> <div class="field field--name-body field--type-text-with-summary field--label-hidden field--item"><p>The changes to the SI were unanimously approved during the 26th General Conference on Weights and Measures (CGPM) in November 2018 and became operational in May 2019.</p> <p>The definitions of all base units are now explicitly based on constant quantities. The kilogram, the ampere, the kelvin, and the mole, in particular, are defined by setting exact numerical values for the Planck constant (h), the elementary electric charge (e), the Boltzmann constant (k), and the Avogadro constant (NA) respectively.</p> <p>The new definitions aim to improve the universality and the stability of the SI, while ensuring continuity of existing instrument calibrations and measurement results. The numerical values of the defining constants have been carefully chosen so as to guarantee that the new definitions identify the same quantities as before.</p> <p>In particular, the “1948 ampere” – which was when the previous definition of the ampere was agreed – and the “2019 ampere”, although defined in a completely different way, is substantially the same intensity of the electric current.</p> </div> </div> <div class="text-link-container" style="text-align:left;"> <div class="item-link iec-read-more"> <a href="/si/latest-developments" class="hvr-icon-forward"> The latest developments: the 2019 revision of the SI <i class="ion ion-ios-arrow-round-forward hvr-icon"></i></a> </div> </div> </section> </div> </div> </div> </div> </div> <div class="dynamic-layouts-container custom-12" > <div class="dynamic-layout-row iec-row "> <div data-region="r5c1c5969093f7c5c1c5969093fa" class="dynamic-layouts-container-column layout-builder--layout__region col-12 iec-col "> <section class="block 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