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mw-first-heading"><span class="mw-page-title-main">Historical definitions of the SI base units</span></h1> <div id="p-lang-btn" class="vector-dropdown mw-portlet mw-portlet-lang" > <input type="checkbox" id="p-lang-btn-checkbox" role="button" aria-haspopup="true" data-event-name="ui.dropdown-p-lang-btn" class="vector-dropdown-checkbox mw-interlanguage-selector" aria-label="Go to an article in another language. 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typeof="mw:File"><a href="/wiki/File:Wiki_letter_w.svg" class="mw-file-description"><img src="//upload.wikimedia.org/wikipedia/en/thumb/6/6c/Wiki_letter_w.svg/40px-Wiki_letter_w.svg.png" decoding="async" width="40" height="40" class="mw-file-element" srcset="//upload.wikimedia.org/wikipedia/en/thumb/6/6c/Wiki_letter_w.svg/60px-Wiki_letter_w.svg.png 1.5x, //upload.wikimedia.org/wikipedia/en/thumb/6/6c/Wiki_letter_w.svg/80px-Wiki_letter_w.svg.png 2x" data-file-width="44" data-file-height="44" /></a></span></div></td><td class="mbox-text"><div class="mbox-text-span">This article's <a href="/wiki/Wikipedia:Manual_of_Style/Lead_section#Length" title="Wikipedia:Manual of Style/Lead section">lead section</a> <b>may be too short to adequately <a href="/wiki/Wikipedia:Summary_style" title="Wikipedia:Summary style">summarize</a> the key points</b>.<span class="hide-when-compact"> Please consider expanding the lead to <a href="/wiki/Wikipedia:Manual_of_Style/Lead_section#Provide_an_accessible_overview" title="Wikipedia:Manual of Style/Lead section">provide an accessible overview</a> of all important aspects of the article. The reason given is: <b>No lede at all, really</b></span> <span class="date-container"><i>(<span class="date">January 2024</span>)</i></span></div></td></tr></tbody></table> <p>Since its introduction in 1960, the base units for the <a href="/wiki/International_system_of_units" class="mw-redirect" title="International system of units">International system of units</a>, known as SI, have changed several times. Tables in this article summarize those changes. </p> <meta property="mw:PageProp/toc" /> <div class="mw-heading mw-heading2"><h2 id="Background_for_the_tables">Background for the tables</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Historical_definitions_of_the_SI_base_units&action=edit&section=1" title="Edit section: Background for the tables"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>When <a href="/wiki/James_Clerk_Maxwell" title="James Clerk Maxwell">Maxwell</a> first introduced the concept of a coherent system, he identified three quantities that could be used as base units: mass, length, and time. <a href="/wiki/Giovanni_Giorgi" title="Giovanni Giorgi">Giorgi</a> later identified the need for an electrical base unit, for which the unit of electric current was chosen for SI. Another three base units (for temperature, amount of substance, and luminous intensity) were added later.<sup id="cite_ref-NIST330_1-0" class="reference"><a href="#cite_note-NIST330-1"><span class="cite-bracket">[</span>1<span class="cite-bracket">]</span></a></sup> </p><p>The early metric systems defined a unit of weight as a base unit, while the SI defines an analogous unit of mass. In everyday use, these are mostly interchangeable, but in scientific contexts the difference matters. Mass, strictly the inertial mass, represents a quantity of matter. It relates the acceleration of a body to the applied force via <a href="/wiki/Newton%27s_laws_of_motion" title="Newton's laws of motion">Newton's law</a>, <span class="nowrap"><i>F</i> = <i>m</i> × <i>a</i></span>: force equals mass times acceleration. A force of 1 N (newton) applied to a mass of 1 kg will accelerate it at 1 m/s<sup>2</sup>. This is true whether the object is floating in space or in a gravity field e.g. at the Earth's surface. Weight is the force exerted on a body by a gravitational field, and hence its weight depends on the strength of the gravitational field. Weight of a 1 kg mass at the Earth's surface is <span class="nowrap"><i>m</i> × <i>g</i></span>; mass times the acceleration due to gravity, which is 9.81 newtons at the Earth's surface and is about 3.5 newtons at the surface of Mars. Since the acceleration due to gravity is local and varies by location and altitude on the Earth, weight is unsuitable for precision measurements of a property of a body, and this makes a unit of weight unsuitable as a base unit.<sup class="noprint Inline-Template Template-Fact" style="white-space:nowrap;">[<i><a href="/wiki/Wikipedia:Citation_needed" title="Wikipedia:Citation needed"><span title="This claim needs references to reliable sources. (January 2024)">citation needed</span></a></i>]</sup> </p><p>Since 1960 the CGPM has made a number of changes to the SI to meet the needs of specific fields, notably chemistry and radiometry. These are mostly additions to the list of named derived units, and include the <i><a href="/wiki/Mole_(unit)" title="Mole (unit)">mole</a></i> (symbol mol) for an amount of substance, the <i><a href="/wiki/Pascal_(unit)" title="Pascal (unit)">pascal</a></i> (symbol Pa) for <a href="/wiki/Pressure" title="Pressure">pressure</a>, the <i><a href="/wiki/Siemens_(unit)" title="Siemens (unit)">siemens</a></i> (symbol S) for electrical conductance, the <i><a href="/wiki/Becquerel" title="Becquerel">becquerel</a></i> (symbol Bq) for "<a href="/wiki/Radioactive_decay" title="Radioactive decay">activity</a> referred to a <a href="/wiki/Radionuclide" title="Radionuclide">radionuclide</a>", the <i><a href="/wiki/Gray_(unit)" title="Gray (unit)">gray</a></i> (symbol Gy) for ionising radiation, the <i><a href="/wiki/Sievert" title="Sievert">sievert</a></i> (symbol Sv) as the unit of dose equivalent radiation, and the <i><a href="/wiki/Katal" title="Katal">katal</a></i> (symbol kat) for <a href="/wiki/Catalytic_activity" class="mw-redirect" title="Catalytic activity">catalytic activity</a>.<sup id="cite_ref-SIBrochure_2-0" class="reference"><a href="#cite_note-SIBrochure-2"><span class="cite-bracket">[</span>2<span class="cite-bracket">]</span></a></sup><sup class="reference nowrap"><span title="Page: 156, 158–159, 165">: 156, 158–159, 165 </span></sup><sup id="cite_ref-3" class="reference"><a href="#cite_note-3"><span class="cite-bracket">[</span>3<span class="cite-bracket">]</span></a></sup><sup class="reference nowrap"><span title="Page / location: 221">: 221 </span></sup> </p><p>The range of defined prefixes pico- (10<sup>−12</sup>) to tera- (10<sup>12</sup>) was extended to quecto- (10<sup>−30</sup>) to quetta- (10<sup>30</sup>).<sup id="cite_ref-SIBrochure_2-1" class="reference"><a href="#cite_note-SIBrochure-2"><span class="cite-bracket">[</span>2<span class="cite-bracket">]</span></a></sup><sup class="reference nowrap"><span title="Page: 152, 158, 164">: 152, 158, 164 </span></sup> </p><p>The 1960 definition of the standard metre in terms of wavelengths of a specific emission of the krypton-86 atom was replaced in 1983 with the distance that light travels in vacuum in exactly <style data-mw-deduplicate="TemplateStyles:r1214402035">.mw-parser-output .sfrac{white-space:nowrap}.mw-parser-output .sfrac.tion,.mw-parser-output .sfrac .tion{display:inline-block;vertical-align:-0.5em;font-size:85%;text-align:center}.mw-parser-output .sfrac .num{display:block;line-height:1em;margin:0.0em 0.1em;border-bottom:1px solid}.mw-parser-output .sfrac .den{display:block;line-height:1em;margin:0.1em 0.1em}.mw-parser-output .sr-only{border:0;clip:rect(0,0,0,0);clip-path:polygon(0px 0px,0px 0px,0px 0px);height:1px;margin:-1px;overflow:hidden;padding:0;position:absolute;width:1px}</style><span class="sfrac">⁠<span class="tion"><span class="num">1</span><span class="sr-only">/</span><span class="den"><span class="nowrap"><span data-sort-value="7008299792458000000♠"></span>299<span style="margin-left:.25em;">792</span><span style="margin-left:.25em;">458</span></span></span></span>⁠</span> second, so that the speed of light is now an exactly specified constant of nature.<sup class="noprint Inline-Template Template-Fact" style="white-space:nowrap;">[<i><a href="/wiki/Wikipedia:Citation_needed" title="Wikipedia:Citation needed"><span title="This claim needs references to reliable sources. (January 2024)">citation needed</span></a></i>]</sup> </p><p>A few changes to notation conventions have also been made to alleviate lexicographic ambiguities. An analysis under the aegis of <a href="/wiki/CSIRO" title="CSIRO">CSIRO</a>, published in 2009 by the <a href="/wiki/Royal_Society" title="Royal Society">Royal Society</a>, has pointed out the opportunities to finish the realisation of that goal, to the point of universal zero-ambiguity machine readability.<sup id="cite_ref-Foster_2009_4-0" class="reference"><a href="#cite_note-Foster_2009-4"><span class="cite-bracket">[</span>4<span class="cite-bracket">]</span></a></sup> </p> <table class="wikitable"> <caption><big>Evolution of the SI base units</big><sup id="cite_ref-NIST330_1-1" class="reference"><a href="#cite_note-NIST330-1"><span class="cite-bracket">[</span>1<span class="cite-bracket">]</span></a></sup><sup class="reference nowrap"><span title="Page: 6">: 6 </span></sup><sup id="cite_ref-BIPM1975_5-0" class="reference"><a href="#cite_note-BIPM1975-5"><span class="cite-bracket">[</span>5<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-archive1975_6-0" class="reference"><a href="#cite_note-archive1975-6"><span class="cite-bracket">[</span>6<span class="cite-bracket">]</span></a></sup> </caption> <tbody><tr> <th scope="col">Unit name </th> <th scope="col">Definition<sup id="cite_ref-7" class="reference"><a href="#cite_note-7"><span class="cite-bracket">[</span>n 1<span class="cite-bracket">]</span></a></sup> </th></tr> <tr> <th scope="row"><a href="/wiki/Second" title="Second">second</a> </th> <td> <ul><li><b>Prior</b>: (1675) <link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1214402035"><span class="sfrac">⁠<span class="tion"><span class="num">1</span><span class="sr-only">/</span><span class="den"><span class="nowrap"><span data-sort-value="7004864000000000000♠"></span>86<span style="margin-left:.25em;">400</span></span></span></span>⁠</span> of a day of 24 hours of 60 minutes of 60 seconds.<sup>TLB</sup></li> <li><b>Interim</b> (1956): <link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1214402035"><span class="sfrac">⁠<span class="tion"><span class="num">1</span><span class="sr-only">/</span><span class="den"><span class="nowrap"><span data-sort-value="7007315569259747000♠"></span>31<span style="margin-left:.25em;">556</span><span style="margin-left:.25em;">925</span>.9747</span></span></span>⁠</span> of the <a href="/wiki/Tropical_year" title="Tropical year">tropical year</a> for 1900 January 0 at 12 hours <a href="/wiki/Ephemeris_time" title="Ephemeris time">ephemeris time</a>.</li> <li><b>Current</b> (1967): The duration of <span class="nowrap"><span data-sort-value="7009919263177000000♠"></span>9<span style="margin-left:.25em;">192</span><span style="margin-left:.25em;">631</span><span style="margin-left:.25em;">770</span></span> periods of the radiation corresponding to the transition between the two <a href="/wiki/Hyperfine_structure" title="Hyperfine structure">hyperfine</a> levels of the <a href="/wiki/Ground_state" title="Ground state">ground state</a> of the <a href="/wiki/Caesium-133" class="mw-redirect" title="Caesium-133">caesium-133</a> atom.</li></ul> </td></tr> <tr> <th scope="row"><a href="/wiki/Metre" title="Metre">metre</a> </th> <td> <ul><li><b>Prior</b> (1793): <link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1214402035"><span class="sfrac">⁠<span class="tion"><span class="num">1</span><span class="sr-only">/</span><span class="den"><span class="nowrap"><span data-sort-value="7007100000000000000♠"></span>10<span style="margin-left:.25em;">000</span><span style="margin-left:.25em;">000</span></span></span></span>⁠</span> of the <a href="/wiki/Meridian_(geography)" title="Meridian (geography)">meridian</a> through Paris between the North Pole and the Equator.<sup>FG</sup></li> <li><b>Interim</b> (1889): The <a href="/wiki/Prototype_of_the_metre" class="mw-redirect" title="Prototype of the metre">prototype of the metre</a> chosen by the CIPM, at the temperature of melting ice, represents the metric unit of length.</li> <li><b>Interim</b> (1960): <span class="nowrap"><span data-sort-value="7006165076373000000♠"></span>1<span style="margin-left:.25em;">650</span><span style="margin-left:.25em;">763</span>.73</span> <a href="/wiki/Wavelength" title="Wavelength">wavelengths</a> in <a href="/wiki/Vacuum" title="Vacuum">vacuum</a> of the <a href="/wiki/Electromagnetic_radiation" title="Electromagnetic radiation">radiation</a> corresponding to the transition between the 2p<sup>10</sup> and 5d<sup>5</sup> quantum levels of the <a href="/wiki/Krypton-86" class="mw-redirect" title="Krypton-86">krypton-86</a> <a href="/wiki/Atom" title="Atom">atom</a>.</li> <li><b>Current</b> (1983): The distance travelled by light in vacuum in <link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1214402035"><span class="sfrac">⁠<span class="tion"><span class="num">1</span><span class="sr-only">/</span><span class="den"><span class="nowrap"><span data-sort-value="7008299792458000000♠"></span>299<span style="margin-left:.25em;">792</span><span style="margin-left:.25em;">458</span></span></span></span>⁠</span> second.</li></ul> </td></tr> <tr> <th scope="row"><a href="/wiki/Kilogram" title="Kilogram">kilogram</a> </th> <td> <ul><li><b>Prior</b> (1793): The <b><a href="/wiki/Grave_(unit)" title="Grave (unit)">grave</a></b> was defined as being the mass (then called <i>weight</i>) of one litre of pure water at its freezing point.<sup>FG</sup></li> <li><b>Interim</b> (1889): The mass of a small squat cylinder of ≈47 cubic centimetres of platinum-iridium alloy kept in the <a href="/wiki/International_Bureau_of_Weights_and_Measures" title="International Bureau of Weights and Measures">International Bureau of Weights and Measures</a> (BIPM), <a href="/wiki/Pavillon_de_Breteuil" title="Pavillon de Breteuil">Pavillon de Breteuil</a>, France.<sup id="cite_ref-8" class="reference"><a href="#cite_note-8"><span class="cite-bracket">[</span>a<span class="cite-bracket">]</span></a></sup> Also, in practice, any of numerous official replicas of it.</li> <li><b>Current</b> (2019): The kilogram is defined by setting the <a href="/wiki/Planck_constant" title="Planck constant">Planck constant</a> <span class="texhtml"><i>h</i></span> exactly to <span class="nowrap"><span data-sort-value="6966662607015000000♠"></span>6.626<span style="margin-left:.25em;">070</span><span style="margin-left:.25em;">15</span><span style="margin-left:0.25em;margin-right:0.15em;">×</span>10<sup>−34</sup> J⋅s</span> (<span class="nowrap">J = kg⋅m<sup>2</sup>⋅s<sup>−2</sup></span>), given the definitions of the metre and the second.<sup id="cite_ref-NIST_2018-11_9-0" class="reference"><a href="#cite_note-NIST_2018-11-9"><span class="cite-bracket">[</span>7<span class="cite-bracket">]</span></a></sup> Then the formula would be <span class="nowrap">kg = <link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1214402035"><span class="sfrac">⁠<span class="tion"><span class="num"><span class="texhtml"><i>h</i></span></span><span class="sr-only">/</span><span class="den"><span class="nowrap"><span data-sort-value="6966662607015000000♠"></span>6.626<span style="margin-left:.25em;">070</span><span style="margin-left:.25em;">15</span><span style="margin-left:0.25em;margin-right:0.15em;">×</span>10<sup>−34</sup></span>⋅m<sup>2</sup>⋅s<sup>−1</sup></span></span>⁠</span></span></li></ul> </td></tr> <tr> <th scope="row"><a href="/wiki/Ampere" title="Ampere">ampere</a> </th> <td> <ul><li><b>Prior</b> (1881): A tenth of the electromagnetic CGS unit of current. The [CGS] electromagnetic unit of current is that current, flowing in an arc 1 cm long of a circle 1 cm in radius, that creates a field of one <a href="/wiki/Oersted" title="Oersted">oersted</a> at the centre.<sup id="cite_ref-10" class="reference"><a href="#cite_note-10"><span class="cite-bracket">[</span>8<span class="cite-bracket">]</span></a></sup> <sup>IEC</sup></li> <li><b>Interim</b> (1946): The constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross-section, and placed 1 m apart in vacuum, would produce between these conductors a <a href="/wiki/Force" title="Force">force</a> equal to <span class="nowrap"><span data-sort-value="6993200000000000000♠"></span>2<span style="margin-left:0.25em;margin-right:0.15em;">×</span>10<sup>−7</sup></span> <a href="/wiki/Newton_(unit)" title="Newton (unit)">newtons</a> per metre of length.</li> <li><b>Current</b> (2019): The flow of <link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1214402035"><span class="sfrac">⁠<span class="tion"><span class="num">1</span><span class="sr-only">/</span><span class="den"><span class="nowrap"><span data-sort-value="6981160217663400000♠"></span>1.602<span style="margin-left:.25em;">176</span><span style="margin-left:.25em;">634</span><span style="margin-left:0.25em;margin-right:0.15em;">×</span>10<sup>−19</sup></span></span></span>⁠</span> times the <a href="/wiki/Elementary_charge" title="Elementary charge">elementary charge</a> <span class="texhtml"><i>e</i></span> per second.</li></ul> </td></tr> <tr> <th scope="row"><a href="/wiki/Kelvin" title="Kelvin">kelvin</a> </th> <td> <ul><li><b>Prior</b> (1743): The <b>centigrade scale</b> is obtained by assigning 0 °C to the <a href="/wiki/Melting_point" title="Melting point">freezing</a> point of water and 100 °C to the <a href="/wiki/Boiling_point" title="Boiling point">boiling point</a> of water.</li> <li><b>Interim</b> (1954): The <a href="/wiki/Triple_point_of_water" class="mw-redirect" title="Triple point of water">triple point of water</a> (0.01 °C) defined to be exactly 273.16 K.<sup id="cite_ref-11" class="reference"><a href="#cite_note-11"><span class="cite-bracket">[</span>n 2<span class="cite-bracket">]</span></a></sup></li> <li><b>Previous</b> (1967): <link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1214402035"><span class="sfrac">⁠<span class="tion"><span class="num">1</span><span class="sr-only">/</span><span class="den">273.16</span></span>⁠</span> of the <a href="/wiki/Thermodynamic_temperature" title="Thermodynamic temperature">thermodynamic temperature</a> of the triple point of water.</li> <li><b>Current</b> (2019): The kelvin is defined by setting the fixed numerical value of the <a href="/wiki/Boltzmann_constant" title="Boltzmann constant">Boltzmann constant</a> <span class="texhtml"><i>k</i></span> to <span class="nowrap"><span data-sort-value="6977138064900000000♠"></span>1.380<span style="margin-left:.25em;">649</span><span style="margin-left:0.25em;margin-right:0.15em;">×</span>10<sup>−23</sup> J⋅K<sup>−1</sup></span>, (J = kg⋅m<sup>2</sup>⋅s<sup>−2</sup>), given the definition of the kilogram, the metre, and the second.</li></ul> </td></tr> <tr> <th scope="row"><a href="/wiki/Mole_(unit)" title="Mole (unit)">mole</a> </th> <td> <ul><li><b>Prior</b> (1900): A stoichiometric quantity which is the equivalent mass in grams of the <a href="/wiki/Avogadro_number" class="mw-redirect" title="Avogadro number">Avogadro number</a> of molecules of a substance.<sup>ICAW</sup></li> <li><b>Interim</b> (1967): The amount of substance of a system which contains as many elementary entities as there are atoms in 0.012 kilogram of <a href="/wiki/Carbon-12" title="Carbon-12">carbon-12</a>.</li> <li><b>Current</b> (2019): The amount of substance of exactly <span class="nowrap"><span data-sort-value="7023602214076000000♠"></span>6.022<span style="margin-left:.25em;">140</span><span style="margin-left:.25em;">76</span><span style="margin-left:0.25em;margin-right:0.15em;">×</span>10<sup>23</sup></span> elementary entities. This number is the fixed numerical value of the <a href="/wiki/Avogadro_constant" title="Avogadro constant">Avogadro constant</a>, <span class="texhtml"><i>N</i></span><sub>A</sub>, when expressed in the unit mol<sup>−1</sup> and is called the Avogadro number.</li></ul> </td></tr> <tr> <th scope="row"><a href="/wiki/Candela" title="Candela">candela</a> </th> <td> <ul><li><b>Prior</b> (1946): The value of the new candle (early name for the candela) is such that the brightness of the full radiator at the temperature of solidification of <a href="/wiki/Platinum" title="Platinum">platinum</a> is 60 new candles per square centimetre.</li> <li><b>Current</b> (1979): The luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency <span class="nowrap"><span data-sort-value="7014540000000000000♠"></span>5.4<span style="margin-left:0.25em;margin-right:0.15em;">×</span>10<sup>14</sup></span> hertz and that has a radiant intensity in that direction of <link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1214402035"><span class="sfrac">⁠<span class="tion"><span class="num">1</span><span class="sr-only">/</span><span class="den">683</span></span>⁠</span> watt per <a href="/wiki/Steradian" title="Steradian">steradian</a>.</li></ul> <dl><dd>Note: both old and new definitions are approximately the luminous intensity of a <a href="/wiki/Spermaceti" title="Spermaceti">spermaceti</a> candle burning modestly bright, in the late 19th century called a "candlepower" or a "candle".</dd></dl> </td></tr> <tr> <td colspan="2"> <dl><dt>Notes</dt></dl> <style data-mw-deduplicate="TemplateStyles:r1239543626">.mw-parser-output .reflist{margin-bottom:0.5em;list-style-type:decimal}@media screen{.mw-parser-output .reflist{font-size:90%}}.mw-parser-output .reflist .references{font-size:100%;margin-bottom:0;list-style-type:inherit}.mw-parser-output .reflist-columns-2{column-width:30em}.mw-parser-output .reflist-columns-3{column-width:25em}.mw-parser-output .reflist-columns{margin-top:0.3em}.mw-parser-output .reflist-columns ol{margin-top:0}.mw-parser-output .reflist-columns li{page-break-inside:avoid;break-inside:avoid-column}.mw-parser-output .reflist-upper-alpha{list-style-type:upper-alpha}.mw-parser-output .reflist-upper-roman{list-style-type:upper-roman}.mw-parser-output .reflist-lower-alpha{list-style-type:lower-alpha}.mw-parser-output .reflist-lower-greek{list-style-type:lower-greek}.mw-parser-output .reflist-lower-roman{list-style-type:lower-roman}</style><div class="reflist"> <div class="mw-references-wrap"><ol class="references"> <li id="cite_note-7"><span class="mw-cite-backlink"><b><a href="#cite_ref-7">^</a></b></span> <span class="reference-text">Interim definitions are given here only when there has been a <i>significant</i> difference in the definition.</span> </li> <li id="cite_note-11"><span class="mw-cite-backlink"><b><a href="#cite_ref-11">^</a></b></span> <span class="reference-text">In 1954 the unit of thermodynamic temperature was known as the "degree Kelvin" (symbol °K; "Kelvin" spelt with an upper-case "K"). It was renamed the "kelvin" (symbol "K"; "kelvin" spelt with a lower case "k") in 1967.</span> </li> </ol></div></div> <p>The <b>Prior</b> definitions of the various base units in the above table were made by the following authors and authorities: </p> <dl><dd><ul><li><b>TLB</b> = <a href="/wiki/Tito_Livio_Burattini" title="Tito Livio Burattini">Tito Livio Burattini</a>, <i>Misura universale</i>, Vilnius, 1675</li> <li><b>FG</b> = French Government</li> <li><b>IEC</b> = <a href="/wiki/International_Electrotechnical_Commission" title="International Electrotechnical Commission">International Electrotechnical Commission</a></li> <li><b>ICAW</b> = <a href="/wiki/International_Committee_on_Atomic_Weights" class="mw-redirect" title="International Committee on Atomic Weights">International Committee on Atomic Weights</a></li></ul></dd></dl> <p>All other definitions result from resolutions by either CGPM or the CIPM and are catalogued in the <i>SI Brochure</i>. </p> </td></tr></tbody></table> <div class="mw-heading mw-heading2"><h2 id="2005">2005</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Historical_definitions_of_the_SI_base_units&action=edit&section=2" title="Edit section: 2005"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>Prior to the <a href="/wiki/2019_revision_of_the_SI" title="2019 revision of the SI">2019 revision of the SI</a>, from 2005 to early 2019, the <a href="/wiki/SI_base_units" class="mw-redirect" title="SI base units">SI base units</a> were defined as follows. </p> <table class="wikitable"> <caption style="font-size:largerfont-weight:bold;">Historical SI base units </caption> <tbody><tr> <th>Name </th> <th style="text-align: center">Symbol </th> <th>Measure </th> <th>Pre-2019 (2005) formal definition<sup id="cite_ref-SIBrochure_2-2" class="reference"><a href="#cite_note-SIBrochure-2"><span class="cite-bracket">[</span>2<span class="cite-bracket">]</span></a></sup> </th> <th>Historical origin / justification </th> <th><a href="/wiki/Dimensional_analysis" title="Dimensional analysis">Dimension<br />symbol</a> </th></tr> <tr> <td><a href="/wiki/Metre" title="Metre">metre</a> </td> <td style="text-align: center">m </td> <td><a href="/wiki/Length" title="Length">length</a> </td> <td>"The metre is the length of the path travelled by light in vacuum during a time interval of 1 / <span class="nowrap"><span data-sort-value="7008299792458000000♠"></span>299<span style="margin-left:.25em;">792</span><span style="margin-left:.25em;">458</span></span> of a second."<br /><i>17th CGPM (1983, Resolution 1, CR, 97)</i> </td> <td>1 / <span class="nowrap"><span data-sort-value="7007100000000000000♠"></span>10<span style="margin-left:.25em;">000</span><span style="margin-left:.25em;">000</span></span> of the distance from the <a href="/wiki/Earth" title="Earth">Earth</a>'s equator to the North Pole measured on the circumference through <a href="/wiki/Paris" title="Paris">Paris</a>. </td> <td style="text-align:center">L </td></tr> <tr> <td><a href="/wiki/Kilogram" title="Kilogram">kilogram</a> </td> <td style="text-align: center">kg </td> <td><a href="/wiki/Mass" title="Mass">mass</a> </td> <td>"The kilogram is the unit of mass; it is equal to the mass of the international prototype of the kilogram."<br /><i>3rd CGPM (1901, CR, 70)</i> </td> <td>The mass of one <a href="/wiki/Litre" title="Litre">litre</a> of <a href="/wiki/Water" title="Water">water</a> at the temperature of melting ice. A litre is one thousandth of a cubic metre. </td> <td style="text-align:center">M </td></tr> <tr> <td><a href="/wiki/Second" title="Second">second</a> </td> <td style="text-align: center">s </td> <td><a href="/wiki/Time" title="Time">time</a> </td> <td>"The second is the duration of <span class="nowrap"><span data-sort-value="7009919263177000000♠"></span>9<span style="margin-left:.25em;">192</span><span style="margin-left:.25em;">631</span><span style="margin-left:.25em;">770</span></span> periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom."<br /><i>13th CGPM (1967/68, Resolution 1; CR, 103)</i><br />"This definition refers to a caesium atom at rest at a temperature of 0 K."<br /><i>(Added by CIPM in 1997)</i> </td> <td>The day is divided in 24 hours, each hour divided in 60 minutes, each minute divided in 60 seconds.<br />A second is 1 / (24 × 60 × 60) of the <a href="/wiki/Day" title="Day">day</a>. </td> <td style="text-align:center">T </td></tr> <tr> <td><a href="/wiki/Ampere" title="Ampere">ampere</a> </td> <td style="text-align: center">A </td> <td><a href="/wiki/Electric_current" title="Electric current">electric current</a> </td> <td>"The ampere is that constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross-section, and placed 1 metre apart in vacuum, would produce between these conductors a force equal to <span class="nowrap"><span data-sort-value="6993200000000000000♠"></span>2<span style="margin-left:0.25em;margin-right:0.15em;">×</span>10<sup>−7</sup></span> <a href="/wiki/Newton_(unit)" title="Newton (unit)">newton</a> per metre of length."<br /><i>9th CGPM (1948)</i> </td> <td>The original "Absolute Ampere" was defined as 0.1 <a href="/wiki/Electromagnetic_unit" class="mw-redirect" title="Electromagnetic unit">Electromagnetic units</a>.<br />The original "International Ampere" was defined electrochemically as the current required to deposit 1.118 milligrams of silver per second from a solution of <a href="/wiki/Silver_nitrate" title="Silver nitrate">silver nitrate</a>. Compared to the SI ampere, the difference is 0.015%. </td> <td style="text-align:center">I </td></tr> <tr> <td><a href="/wiki/Kelvin" title="Kelvin">kelvin</a> </td> <td style="text-align: center">K </td> <td><a href="/wiki/Thermodynamic_temperature" title="Thermodynamic temperature">thermodynamic temperature</a> </td> <td>"The kelvin, unit of thermodynamic temperature, is the fraction 1 / <span class="nowrap"><span data-sort-value="7002273160000000000♠"></span>273.16</span> of the thermodynamic temperature of the <a href="/wiki/Triple_point" title="Triple point">triple point</a> of water."<br /><i>13th CGPM (1967/68, Resolution 4; CR, 104)</i><br />"This definition refers to water having the isotopic composition defined exactly by the following amount of substance ratios: <span class="nowrap">0.000 155 76</span> mole of <sup>2</sup>H per mole of <sup>1</sup>H, <span class="nowrap">0.000 379 9</span> mole of <sup>17</sup>O per mole of <sup>16</sup>O, and <span class="nowrap">0.002 005 2</span> mole of <sup>18</sup>O per mole of <sup>16</sup>O."<br /><i>(Added by CIPM in 2005)</i> </td> <td>The <a href="/wiki/Celsius_scale" class="mw-redirect" title="Celsius scale">Celsius scale</a>: the Kelvin scale uses the degree Celsius for its unit increment, but is a thermodynamic scale (0 K is <a href="/wiki/Absolute_zero" title="Absolute zero">absolute zero</a>). </td> <td style="text-align:center">Θ </td></tr> <tr> <td><a href="/wiki/Mole_(unit)" title="Mole (unit)">mole</a> </td> <td style="text-align: center">mol </td> <td><a href="/wiki/Amount_of_substance" title="Amount of substance">amount of substance</a> </td> <td>"1. The mole is the amount of substance of a system which contains as many elementary entities as there are atoms in 0.012 kilogram of carbon 12; its symbol is 'mol'.<br />2. When the mole is used, the elementary entities must be specified and may be atoms, molecules, ions, electrons, other particles, or specified groups of such particles."<br /><i>14th CGPM (1971, Resolution 3; CR, 78)</i><br />"In this definition, it is understood that unbound atoms of carbon 12, at rest and in their ground state, are referred to."<br /><i>(Added by CIPM in 1980)</i> </td> <td><a href="/wiki/Atomic_weight" class="mw-redirect" title="Atomic weight">Atomic weight</a> or <a href="/wiki/Molecular_weight" class="mw-redirect" title="Molecular weight">molecular weight</a> divided by the <a href="/wiki/Molar_mass_constant" title="Molar mass constant">molar mass constant</a>, 1 g/mol. </td> <td style="text-align:center">N </td></tr> <tr> <td><a href="/wiki/Candela" title="Candela">candela</a> </td> <td style="text-align: center">cd </td> <td><a href="/wiki/Luminous_intensity" title="Luminous intensity">luminous intensity</a> </td> <td>"The candela is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency <span class="nowrap"><span data-sort-value="7014540000000000000♠"></span>540<span style="margin-left:0.25em;margin-right:0.15em;">×</span>10<sup>12</sup></span> hertz and that has a radiant intensity in that direction of 1 / 683 watt per <a href="/wiki/Steradian" title="Steradian">steradian</a>."<br /><i>16th CGPM (1979, Resolution 3; CR, 100)</i> </td> <td>The <a href="/wiki/Candlepower" title="Candlepower">candlepower</a>, which is based on the light emitted from a burning candle of standard properties. </td> <td style="text-align:center">J </td></tr> <tr> <th>Name </th> <th>Symbol </th> <th>Measure </th> <th>Pre-2019 (2005) formal definition<sup id="cite_ref-SIBrochure_2-3" class="reference"><a href="#cite_note-SIBrochure-2"><span class="cite-bracket">[</span>2<span class="cite-bracket">]</span></a></sup> </th> <th>Historical origin / justification </th> <th><a href="/wiki/Dimensional_analysis" title="Dimensional analysis">Dimension<br />symbol</a> </th></tr> </tbody></table> <div class="mw-heading mw-heading2"><h2 id="Notes">Notes</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Historical_definitions_of_the_SI_base_units&action=edit&section=3" title="Edit section: Notes"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1239543626"><div class="reflist reflist-lower-alpha"> <div class="mw-references-wrap"><ol class="references"> <li id="cite_note-8"><span class="mw-cite-backlink"><b><a href="#cite_ref-8">^</a></b></span> <span class="reference-text">It is known as the International Prototype of the Kilogram.</span> </li> </ol></div></div> <div class="mw-heading mw-heading2"><h2 id="References">References</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Historical_definitions_of_the_SI_base_units&action=edit&section=4" title="Edit section: References"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1239543626"><div class="reflist"> <div class="mw-references-wrap"><ol class="references"> <li id="cite_note-NIST330-1"><span class="mw-cite-backlink">^ <a href="#cite_ref-NIST330_1-0"><sup><i><b>a</b></i></sup></a> <a href="#cite_ref-NIST330_1-1"><sup><i><b>b</b></i></sup></a></span> <span class="reference-text"><style data-mw-deduplicate="TemplateStyles:r1238218222">.mw-parser-output cite.citation{font-style:inherit;word-wrap:break-word}.mw-parser-output .citation q{quotes:"\"""\"""'""'"}.mw-parser-output .citation:target{background-color:rgba(0,127,255,0.133)}.mw-parser-output .id-lock-free.id-lock-free a{background:url("//upload.wikimedia.org/wikipedia/commons/6/65/Lock-green.svg")right 0.1em center/9px no-repeat}.mw-parser-output .id-lock-limited.id-lock-limited a,.mw-parser-output .id-lock-registration.id-lock-registration a{background:url("//upload.wikimedia.org/wikipedia/commons/d/d6/Lock-gray-alt-2.svg")right 0.1em center/9px no-repeat}.mw-parser-output .id-lock-subscription.id-lock-subscription a{background:url("//upload.wikimedia.org/wikipedia/commons/a/aa/Lock-red-alt-2.svg")right 0.1em center/9px no-repeat}.mw-parser-output .cs1-ws-icon a{background:url("//upload.wikimedia.org/wikipedia/commons/4/4c/Wikisource-logo.svg")right 0.1em center/12px no-repeat}body:not(.skin-timeless):not(.skin-minerva) .mw-parser-output .id-lock-free a,body:not(.skin-timeless):not(.skin-minerva) .mw-parser-output .id-lock-limited a,body:not(.skin-timeless):not(.skin-minerva) .mw-parser-output .id-lock-registration a,body:not(.skin-timeless):not(.skin-minerva) .mw-parser-output .id-lock-subscription a,body:not(.skin-timeless):not(.skin-minerva) .mw-parser-output .cs1-ws-icon a{background-size:contain;padding:0 1em 0 0}.mw-parser-output .cs1-code{color:inherit;background:inherit;border:none;padding:inherit}.mw-parser-output .cs1-hidden-error{display:none;color:var(--color-error,#d33)}.mw-parser-output .cs1-visible-error{color:var(--color-error,#d33)}.mw-parser-output .cs1-maint{display:none;color:#085;margin-left:0.3em}.mw-parser-output .cs1-kern-left{padding-left:0.2em}.mw-parser-output .cs1-kern-right{padding-right:0.2em}.mw-parser-output .citation .mw-selflink{font-weight:inherit}@media screen{.mw-parser-output .cs1-format{font-size:95%}html.skin-theme-clientpref-night .mw-parser-output .cs1-maint{color:#18911f}}@media screen and (prefers-color-scheme:dark){html.skin-theme-clientpref-os .mw-parser-output .cs1-maint{color:#18911f}}</style><cite id="CITEREFDavid_B._NewellEite_Tiesinga2019" class="citation book cs1">David B. 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(20 May 1975). <a rel="nofollow" class="external text" href="https://archive.org/details/internationalbur420page"><i>The International Bureau of Weights and Measures 1875–1975: NBS Special Publication 420</i></a>. <a href="/wiki/Washington,_D.C." title="Washington, D.C.">Washington, D.C.</a>: <a href="/wiki/National_Bureau_of_Standards" class="mw-redirect" title="National Bureau of Standards">National Bureau of Standards</a>. pp. <a rel="nofollow" class="external text" href="https://archive.org/details/internationalbur420page/page/238">238</a>–244.</cite><span title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Abook&rft.genre=book&rft.btitle=The+International+Bureau+of+Weights+and+Measures+1875%E2%80%931975%3A+NBS+Special+Publication+420&rft.place=Washington%2C+D.C.&rft.pages=238-244&rft.pub=National+Bureau+of+Standards&rft.date=1975-05-20&rft_id=https%3A%2F%2Farchive.org%2Fdetails%2Finternationalbur420page&rfr_id=info%3Asid%2Fen.wikipedia.org%3AHistorical+definitions+of+the+SI+base+units" class="Z3988"></span></span> </li> <li id="cite_note-NIST_2018-11-9"><span class="mw-cite-backlink"><b><a href="#cite_ref-NIST_2018-11_9-0">^</a></b></span> <span class="reference-text"><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1238218222"><cite id="CITEREFMaterese2018" class="citation news cs1">Materese, Robin (16 November 2018). <a rel="nofollow" class="external text" href="https://www.nist.gov/news-events/news/2018/11/historic-vote-ties-kilogram-and-other-units-natural-constants">"Historic Vote Ties Kilogram and Other Units to Natural Constants"</a>. <i>NIST</i><span class="reference-accessdate">. Retrieved <span class="nowrap">16 November</span> 2018</span>.</cite><span title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Ajournal&rft.genre=article&rft.jtitle=NIST&rft.atitle=Historic+Vote+Ties+Kilogram+and+Other+Units+to+Natural+Constants&rft.date=2018-11-16&rft.aulast=Materese&rft.aufirst=Robin&rft_id=https%3A%2F%2Fwww.nist.gov%2Fnews-events%2Fnews%2F2018%2F11%2Fhistoric-vote-ties-kilogram-and-other-units-natural-constants&rfr_id=info%3Asid%2Fen.wikipedia.org%3AHistorical+definitions+of+the+SI+base+units" class="Z3988"></span></span> </li> <li id="cite_note-10"><span class="mw-cite-backlink"><b><a href="#cite_ref-10">^</a></b></span> <span class="reference-text"><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1238218222"><cite id="CITEREFMcKenzie1961" class="citation book cs1">McKenzie, A. E. E. (1961). <a rel="nofollow" class="external text" href="https://books.google.com/books?id=m7c6AAAAIAAJ&pg=PA322"><i>Magnetism and Electricity</i></a>. <a href="/wiki/Cambridge_University_Press" title="Cambridge University Press">Cambridge University Press</a>. p. 322.</cite><span title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Abook&rft.genre=book&rft.btitle=Magnetism+and+Electricity&rft.pages=322&rft.pub=Cambridge+University+Press&rft.date=1961&rft.aulast=McKenzie&rft.aufirst=A.+E.+E.&rft_id=https%3A%2F%2Fbooks.google.com%2Fbooks%3Fid%3Dm7c6AAAAIAAJ%26pg%3DPA322&rfr_id=info%3Asid%2Fen.wikipedia.org%3AHistorical+definitions+of+the+SI+base+units" class="Z3988"></span></span> </li> </ol></div></div> <div class="navbox-styles"><style data-mw-deduplicate="TemplateStyles:r1129693374">.mw-parser-output .hlist dl,.mw-parser-output .hlist ol,.mw-parser-output .hlist ul{margin:0;padding:0}.mw-parser-output .hlist dd,.mw-parser-output .hlist dt,.mw-parser-output .hlist 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href="/wiki/Template:SI_units" title="Template:SI units"><abbr title="View this template">v</abbr></a></li><li class="nv-talk"><a href="/wiki/Template_talk:SI_units" title="Template talk:SI units"><abbr title="Discuss this template">t</abbr></a></li><li class="nv-edit"><a href="/wiki/Special:EditPage/Template:SI_units" title="Special:EditPage/Template:SI units"><abbr title="Edit this template">e</abbr></a></li></ul></div><div id="SI_units" style="font-size:114%;margin:0 4em"><a href="/wiki/International_System_of_Units" title="International System of Units">SI units</a></div></th></tr><tr><th scope="row" class="navbox-group" style="width:1%"><a href="/wiki/SI_base_unit" title="SI base unit">Base units</a></th><td class="navbox-list-with-group navbox-list navbox-odd" style="width:100%;padding:0"><div style="padding:0 0.25em"> <ul><li><a href="/wiki/Ampere" title="Ampere">ampere</a></li> <li><a href="/wiki/Candela" title="Candela">candela</a></li> <li><a href="/wiki/Kelvin" title="Kelvin">kelvin</a></li> <li><a href="/wiki/Kilogram" title="Kilogram">kilogram</a></li> <li><a href="/wiki/Metre" title="Metre">metre</a></li> <li><a href="/wiki/Mole_(unit)" title="Mole (unit)">mole</a></li> <li><a href="/wiki/Second" title="Second">second</a></li></ul> </div></td></tr><tr><th scope="row" class="navbox-group" style="width:1%"><a href="/wiki/SI_derived_unit" title="SI derived unit">Derived units <br />with special names</a></th><td class="navbox-list-with-group navbox-list navbox-even" style="width:100%;padding:0"><div style="padding:0 0.25em"> <ul><li><a href="/wiki/Becquerel" title="Becquerel">becquerel</a></li> <li><a href="/wiki/Coulomb" title="Coulomb">coulomb</a></li> <li><a href="/wiki/Celsius" title="Celsius">degree Celsius</a></li> <li><a href="/wiki/Farad" title="Farad">farad</a></li> <li><a href="/wiki/Gray_(unit)" title="Gray (unit)">gray</a></li> <li><a href="/wiki/Henry_(unit)" title="Henry (unit)">henry</a></li> <li><a href="/wiki/Hertz" title="Hertz">hertz</a></li> <li><a href="/wiki/Joule" title="Joule">joule</a></li> <li><a href="/wiki/Katal" title="Katal">katal</a></li> <li><a href="/wiki/Lumen_(unit)" title="Lumen (unit)">lumen</a></li> <li><a href="/wiki/Lux" title="Lux">lux</a></li> <li><a href="/wiki/Newton_(unit)" title="Newton (unit)">newton</a></li> <li><a href="/wiki/Ohm" title="Ohm">ohm</a></li> <li><a href="/wiki/Pascal_(unit)" title="Pascal (unit)">pascal</a></li> <li><a href="/wiki/Radian" title="Radian">radian</a></li> <li><a href="/wiki/Siemens_(unit)" title="Siemens (unit)">siemens</a></li> <li><a href="/wiki/Sievert" title="Sievert">sievert</a></li> <li><a href="/wiki/Steradian" title="Steradian">steradian</a></li> <li><a href="/wiki/Tesla_(unit)" title="Tesla (unit)">tesla</a></li> <li><a href="/wiki/Volt" title="Volt">volt</a></li> <li><a href="/wiki/Watt" title="Watt">watt</a></li> <li><a href="/wiki/Weber_(unit)" title="Weber (unit)">weber</a></li></ul> </div></td></tr><tr><th scope="row" class="navbox-group" style="width:1%"><a href="/wiki/Non-SI_units_mentioned_in_the_SI" title="Non-SI units mentioned in the SI">Other accepted units</a></th><td class="navbox-list-with-group navbox-list navbox-odd" style="width:100%;padding:0"><div style="padding:0 0.25em"> <ul><li><a href="/wiki/Astronomical_unit" title="Astronomical unit">astronomical unit</a></li> <li><a href="/wiki/Dalton_(unit)" title="Dalton (unit)">dalton</a></li> <li><a href="/wiki/Day" title="Day">day</a></li> <li><a href="/wiki/Decibel" title="Decibel">decibel</a></li> <li><a href="/wiki/Degree_(angle)" title="Degree (angle)">degree of arc</a></li> <li><a href="/wiki/Electronvolt" title="Electronvolt">electronvolt</a></li> <li><a href="/wiki/Hectare" title="Hectare">hectare</a></li> <li><a href="/wiki/Hour" title="Hour">hour</a></li> <li><a href="/wiki/Litre" title="Litre">litre</a></li> <li><a href="/wiki/Minute" title="Minute">minute</a></li> <li><a href="/wiki/Minute_and_second_of_arc" title="Minute and second of arc">minute and second of arc</a></li> <li><a href="/wiki/Neper" title="Neper">neper</a></li> <li><a href="/wiki/Tonne" title="Tonne">tonne</a></li></ul> </div></td></tr><tr><th scope="row" class="navbox-group" style="width:1%">See also</th><td class="navbox-list-with-group navbox-list navbox-even" style="width:100%;padding:0"><div style="padding:0 0.25em"> <ul><li><a href="/wiki/Conversion_of_units" title="Conversion of units">Conversion of units</a></li> <li><a href="/wiki/Metric_prefix" title="Metric prefix">Metric prefixes</a></li> <li><a class="mw-selflink selflink">Historical definitions of the SI base units</a></li> <li><a href="/wiki/2019_revision_of_the_SI" title="2019 revision of the SI">2019 revision of the SI</a></li> <li><a href="/wiki/System_of_units_of_measurement" title="System of units of measurement">System of units of measurement</a></li></ul> </div></td></tr><tr><td class="navbox-abovebelow" colspan="2"><div> <ul><li><span class="noviewer" 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