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<html> <head><script type="text/javascript" src="/_static/js/bundle-playback.js?v=HxkREWBo" charset="utf-8"></script> <script type="text/javascript" src="/_static/js/wombat.js?v=txqj7nKC" charset="utf-8"></script> <script>window.RufflePlayer=window.RufflePlayer||{};window.RufflePlayer.config={"autoplay":"on","unmuteOverlay":"hidden"};</script> <script type="text/javascript" src="/_static/js/ruffle/ruffle.js"></script> <script type="text/javascript"> __wm.init("https://web.archive.org/web"); __wm.wombat("https://www.bipm.org/metrology/length/","20200201205912","https://web.archive.org/","web","/_static/", "1580590752"); </script> <link rel="stylesheet" type="text/css" href="/_static/css/banner-styles.css?v=S1zqJCYt" /> <link rel="stylesheet" type="text/css" href="/_static/css/iconochive.css?v=3PDvdIFv" />  <meta http-equiv="X-UA-Compatible" content="IE=8"/> <title>BIPM - length</title> <meta name="description" content="Length"> <script language="javascript" 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href="/web/20200201205912/https://www.bipm.org/en/about-us/" title="BIPM home page"><img src="/web/20200201205912im_/https://www.bipm.org/utils/common/img/logoBIPM-RB.jpg" border="0"></a></td> <td width="530" valign="center" align="center" class="intro">    the intergovernmental organization through which Member States act together<br>      on matters related to measurement science and measurement standards.</td> <td width="270" align="center" valign="top"> <script language="javascript"> function ActionForm(f) { f.q.style.background = "white"; } </script> <form name="search" action="https://web.archive.org/web/20200201205912/https://search.bipm.org/bipm/en/C="> <table width="180" align="right" valign="top" border="0"> <tr height="50"> <td valign="bottom" class="intro">Search facility: <input onclick="ActionForm(this.form)" style="vertical-align: middle;background-image:url(/web/20200201205912im_/https://www.bipm.org/utils/en/img/bipm_portal.gif); background-repeat:no-repeat; 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get to the BIPM</a></li> <li><a href="/web/20200201205912/https://www.bipm.org/en/about-us/pavillon-de-breteuil/" target="_self">History of the Pavillon de Breteuil</a></li> </ul> </li> <li><a href="/web/20200201205912/https://www.bipm.org/en/worldwide-metrology/" target="_self"> Worldwide metrology</a> <ul> <li><a href="/web/20200201205912/https://www.bipm.org/en/worldwide-metrology/" target="_self">What is metrology?</a></li> <li><a href="/web/20200201205912/https://www.bipm.org/en/cipm-mra/" target="_self">International equivalence of measurements<br>(CIPM MRA)</a></li> <li><a href="/web/20200201205912/https://www.bipm.org/en/worldwide-metrology/metre-convention/" target="_self">The Metre Convention</a></li> <li><a href="/web/20200201205912/https://www.bipm.org/en/worldwide-metrology/cgpm/" target="_self">The General Conference (CGPM)</a></li> <li><a href="/web/20200201205912/https://www.bipm.org/en/committees/cipm/" target="_self">The International Committee (CIPM)</a></li> <li><a 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href="/web/20200201205912/https://www.bipm.org/en/cipm-mra/" target="_self"> International equivalence</a> <ul> <li><a href="/web/20200201205912/https://www.bipm.org/en/cipm-mra/" target="_self">What is the CIPM MRA?</a></li> <li><a href="https://web.archive.org/web/20200201205912/https://www.bipm.org/kcdb" target="_self">CIPM MRA database <img src="/web/20200201205912im_/https://www.bipm.org/utils/common/img2/logo_kcdb_pt.gif"></a></li> <li><a href="/web/20200201205912/https://www.bipm.org/en/cipm-mra/participation/" target="_self">Participating laboratories</a></li> <li><a href="/web/20200201205912/https://www.bipm.org/en/cipm-mra/cipm-mra-documents/" target="_self">CIPM MRA documents</a></li> <li><a href="/web/20200201205912/https://www.bipm.org/en/cipm-mra/approval-process.html" target="_self">CMC approval process</a></li> <li><a href="/web/20200201205912/https://www.bipm.org/en/cipm-mra/logo/" target="_self">Use of the CIPM MRA logo</a></li> <li><a href="/web/20200201205912/https://www.bipm.org/en/committees/jc/jcrb/" target="_self">JCRB: Joint Committee of the RMOs and the BIPM</a></li> </ul> </li> <li><a href="/web/20200201205912/https://www.bipm.org/en/measurement-units/" target="_self"> SI units</a> <ul> <li><a href="/web/20200201205912/https://www.bipm.org/en/measurement-units/" target="_self">The International System of Units (SI)</a></li> <li><a href="/web/20200201205912/https://www.bipm.org/en/publications/si-brochure/" target="_self">SI Brochure</a></li> <li><a href="/web/20200201205912/https://www.bipm.org/en/publications/si-brochure/appendix1.html" target="_self">Decisions of the CGPM and the CIPM</a></li> <li><a href="/web/20200201205912/https://www.bipm.org/en/publications/mises-en-pratique/" target="_self">How to realize the definitions in practice</a></li> <li><a href="/web/20200201205912/https://www.bipm.org/en/measurement-units/base-units.html" target="_self">Unit definitions</a></li> <li><a href="/web/20200201205912/https://www.bipm.org/en/measurement-units/prefixes.html" target="_self">SI prefixes</a></li> <li><a href="/web/20200201205912/https://www.bipm.org/en/si-download-area/" target="_self">Revised SI: Download area</a></li> <li><a href="/web/20200201205912/https://www.bipm.org/en/measurement-units/history-si/" target="_self">Brief history of the SI</a></li> </ul> </li> <li><a href="/web/20200201205912/https://www.bipm.org/en/bipm-services/" target="_self"> Services</a> <ul> <li><a href="/web/20200201205912/https://www.bipm.org/en/bipm-services/comparisons/" target="_self">Comparisons piloted by the BIPM</a></li> <li><a href="/web/20200201205912/https://www.bipm.org/en/bipm-services/calibrations/" target="_self">Calibration and measurement services</a></li> <li><a href="/web/20200201205912/https://www.bipm.org/en/bipm/tai/" target="_self">Timescales</a></li> <li><a href="https://web.archive.org/web/20200201205912/https://www.bipm.org/kcdb" target="_self"><img src="/web/20200201205912im_/https://www.bipm.org/utils/common/img2/logo_kcdb_pt.gif" alt="KCDB" vspace="2"></a></li> <li><a href="https://web.archive.org/web/20200201205912/https://www.bipm.org/jctlm/" target="_self"><img src="/web/20200201205912im_/https://www.bipm.org/utils/common/img2/logo_jctlm_pt.gif" alt="JCTLM" vspace="2"></a></li> <li><a href="https://web.archive.org/web/20200201205912/http://imrr.bipm.org/" target="_self"><img src="/web/20200201205912im_/https://www.bipm.org/utils/common/img2/logo_imrr_pt.gif" alt="IMRR" vspace="2"></a></li> <li><a href="/web/20200201205912/https://www.bipm.org/en/publications/si-brochure/" target="_self">SI Brochure</a></li> <li><a href="https://web.archive.org/web/20200201205912/http://iopscience.iop.org/0026-1394/" target="metrologia"><i>Metrologia</i></a></li> <li><a href="/web/20200201205912/https://www.bipm.org/en/conference-centre/" target="_self">Meetings</a></li> <li><a href="/web/20200201205912/https://www.bipm.org/en/publications/mises-en-pratique/standard-frequencies.html" target="_self">Recommended values of standard frequencies</a></li> <li><a href="/web/20200201205912/https://www.bipm.org/en/bipm/ionizing/radionuclides/sir/" target="_self">International Reference System (SIR)</a></li> </ul> </li> <li><a href="/web/20200201205912/https://www.bipm.org/en/publications/" target="_self"> Publications</a> <ul> <li><a href="/web/20200201205912/https://www.bipm.org/en/publications/si-brochure/" target="_self">SI Brochure</a></li> <li><a href="/web/20200201205912/https://www.bipm.org/en/publications/mises-en-pratique/" target="_self"><i>Mises en pratique</i></a></li> <li><a href="/web/20200201205912/https://www.bipm.org/en/publications/directors-report/" target="_self">Annual Director's Report</a></li> <li><a href="/web/20200201205912/https://www.bipm.org/en/committees/cipm/publications-cipm.html#ra" target="_self">Financial Report</a></li> <li><a href="/web/20200201205912/https://www.bipm.org/en/committees/cipm/publications-cipm.html#no" target="_self">Notification of Contributions</a></li> <li><a href="/web/20200201205912/https://www.bipm.org/en/publications/" target="_self">Official proceedings</a></li> <li><a href="/web/20200201205912/https://www.bipm.org/en/worldwide-metrology/cgpm/resolutions.html" target="_self">Resolutions of the General Conference</a></li> <li><a href="/web/20200201205912/https://www.bipm.org/en/publications/open-literature/" target="_self">Scientific output</a></li> <li><a href="https://web.archive.org/web/20200201205912/http://iopscience.iop.org/0026-1394/" target="metrologia"><i>Metrologia</i></a></li> <li><a href="/web/20200201205912/https://www.bipm.org/en/publications/guides/" target="_self">Guides in Metrology</a></li> </ul> </li> <li><a href="/web/20200201205912/https://www.bipm.org/en/conference-centre/diary/" target="_self"> Meetings</a> <ul> <li><a href="/web/20200201205912/https://www.bipm.org/en/conference-centre/diary/" target="_self">List of meetings</a></li> <li><a href="/web/20200201205912/https://www.bipm.org/en/conference-centre/" target="_self">BIPM Conference Centre</a></li> <li><a href="/web/20200201205912/https://www.bipm.org/en/conference-centre/directions/" target="_self">How to get to the BIPM</a></li> <li><a href="/web/20200201205912/https://www.bipm.org/en/cgpm-2018/" target="_self">General Conference (CGPM)</a></li> <li><a href="/web/20200201205912/https://www.bipm.org/en/committees/cipm/" target="_self">International Committee (CIPM)</a></li> <li><a href="/web/20200201205912/https://www.bipm.org/en/committees/cc/" target="_self">Consultative Committees and Working Groups</a></li> <li><a href="/web/20200201205912/https://www.bipm.org/en/conference-centre/bipm-workshops/" target="_self">BIPM Workshops</a></li> <li><a href="/web/20200201205912/https://www.bipm.org/en/conference-centre/other-events/" target="_self">Other BIPM events</a></li> <li><a href="/web/20200201205912/https://www.bipm.org/en/publications/" target="_self">Meeting reports</a></li> <li><a href="/web/20200201205912/https://www.bipm.org/en/worldwide-metrology/metrology-events.html" target="_self">External events</a></li> </ul> </li> </ul> </div> </td> </tr> </table>   <center> <table width="1000" border="0" cellspacing="0" cellpadding="0"> <tr> <td width="10" height="50"><img src="/web/20200201205912im_/https://www.bipm.org/utils/common/img/pix.gif" width="10" height="50"></td> <td align="left" width="100%" height="50" background="/web/20200201205912im_/https://www.bipm.org/utils/common/img/cadre_trait.gif"> <table width="100%" border="0" background="/web/20200201205912im_/https://www.bipm.org/metrology/length/"> <tr> <td align="left" class="GrosTitre"><div name="exalead-titre">International metrology in the field of Length</div></td> <td align="right" nowrap>        </td> </tr> </table> </td> <td width="10" height="50"><img src="/web/20200201205912im_/https://www.bipm.org/utils/common/img/pix.gif" width="10" height="50"></td> </tr> </table> </center>  <script> $(function() { $( "#tabs2" ).tabs(); }); </script>  <center> <table width="1000" border="0" cellspacing="0" cellpadding="0"> <tr> <td width="10"><img src="/web/20200201205912im_/https://www.bipm.org/utils/common/img/pix.gif" width="10"></td> <td valign="top" width="100%"> <div id="tabs2"> <ul> <li><a href="#introduction">Overview</a></li> <li><a href="#units"><a href="units.html">Units</a></a></li> <li><a href="#committees"><a href="/web/20200201205912/https://www.bipm.org/en/committees/cc/ccl/">Committee structure</a></a></li> <li><a href="#standard-frequencies"><a href="/web/20200201205912/https://www.bipm.org/en/publications/mises-en-pratique/standard-frequencies.html">Recommended values of standard frequencies</a></a></li> <li><a href="#strategy">Strategic plan</a></li> </ul> <div id="introduction"> <table width="100%" cellpadding="15"><tr><td> <p><img src="/web/20200201205912im_/https://www.bipm.org/utils/common/img/fleche_jaune.gif" width="29" height="14" alt=""> Dimensional metrology has always played an important role in society. Indeed, with the increase of international trade seen in the 19th century, the need for international agreement on length and mass measurements formed the starting point for the global homogenization of units as embodied in the Metre Convention.</p> <p>Over the years, the definition of the metre has served as a model. The International Prototype of the Metre was one of two International Prototypes deposited at the BIPM headquarters in 1889. In 1960 the metre became the first SI unit to be defined based on a quantum phenomenon, and in 1983 it became the first SI unit to be defined by fixing the numerical value of a fundamental constant: the speed of light. Today, adoption of similar definitions for other SI base units seems to be the way forward in the planned future global revision of the SI.</p> <p>Today, the realization of the metre is essentially based on laser frequency standards, which are increasingly accompanied by techniques to directly generate optical frequencies from the second in terms of optical frequency comb systems.</p> <p>From the early seventies through till 2006 the BIPM piloted a long line of laser frequency comparisons, providing traceability and validation of national length standards. Although today there is no active technical programme in length measurements at the BIPM headquarters, international agreement in length metrology remains essential and the BIPM continues to coordinate international agreement in this area, cooperating with the CCL to provide the basis for homogeneous realization of the metre world-wide.</p> </td></tr></table> </div> <div id="strategy"> <p class="txt12ptg">CCL strategy:</p> <ul class="txt12pt"> <li><a href="/web/20200201205912/https://www.bipm.org/utils/en/pdf/CCL-strategy-document.pdf" class="introGras"> CCL strategic plan (2018), 27 pp <img src="/web/20200201205912im_/https://www.bipm.org/utils/common/img/pdf.gif" border="0"></a></li> <br> <li><a href="/web/20200201205912/https://www.bipm.org/utils/en/pdf/CCL-strategy-summary.pdf" class="introGras"> Summary document (2018), 1 p <img src="/web/20200201205912im_/https://www.bipm.org/utils/common/img/pdf.gif" border="0"></a></li> </ul> </div> </div> </td> <td width="10"><img src="/web/20200201205912im_/https://www.bipm.org/utils/common/img/pix.gif" width="10"></td> <td width="200" valign="top"> <table width="180" border="0" cellspacing="0" cellpadding="4" bgcolor="#829EBA"> <tr> <td width="100%" height="100%" valign="top"> <table width="100%" border="0" cellspacing="0" cellpadding="0" bgcolor="#FFFFFF"> <tr> <td width="180" height="100%" valign="top"> <table width="180" border="0" cellpadding="3" bgcolor="#FFFFFF" cellspacing="0" height="100%"> <tr align="left" valign="middle" bgcolor="#00498F"> <td height="25" valign="top" bgcolor="#829EBA" class="cadreTrt"><img src="/web/20200201205912im_/https://www.bipm.org/utils/common/img/flc_dir.gif" width="18" height="18" alt=""> Length </td> </tr> <tr> <td height="100%" valign="top" class="txt11ptl" align="left"> <img src="/web/20200201205912im_/https://www.bipm.org/utils/common/img/bullet1.gif" width="16" height="9" alt=""><a class="introGras" href="/web/20200201205912/https://www.bipm.org/metrology/length/">Overview</a><br> <img src="/web/20200201205912im_/https://www.bipm.org/utils/common/img/bullet1.gif" width="16" height="9" alt=""><a class="introGras" href="units.html">Unit of length (metre)</a><br> <img src="/web/20200201205912im_/https://www.bipm.org/utils/common/img/bullet1.gif" width="16" height="9" alt=""><a class="introGras" href="/web/20200201205912/https://www.bipm.org/en/committees/cc/ccl/">Committee structure</a><br> <img src="/web/20200201205912im_/https://www.bipm.org/utils/common/img/bullet1.gif" width="16" height="9" alt=""><a class="introGras" href="/web/20200201205912/https://www.bipm.org/en/publications/mises-en-pratique/standard-frequencies.html">Recommended values of standard frequencies</a><br> <img src="/web/20200201205912im_/https://www.bipm.org/utils/common/img/bullet1.gif" width="16" height="9" alt=""><a class="introGras" href="/web/20200201205912/https://www.bipm.org/en/committees/cc/ccl/#cc-strategy">Strategic plan</a><br> </td> </tr> </table> </td> </tr> </table> </td> </tr> </table> <img src="/web/20200201205912im_/https://www.bipm.org/utils/common/img/pix.gif" width="100%" height="5" alt=""> <table width="180" border="0" cellspacing="0" cellpadding="4" bgcolor="#829EBA"> <tr> <td width="100%" height="100%" valign="top"> <table width="100%" border="0" cellspacing="0" cellpadding="0" bgcolor="#FFFFFF"> <tr> <td width="180" height="100%" valign="top"> <table width="180" border="0" cellpadding="3" bgcolor="#FFFFFF" cellspacing="0" height="100%"> <tr align="left" valign="middle" bgcolor="#00498F"> <td height="25" valign="top" bgcolor="#829EBA" class="cadreTrt"><img src="/web/20200201205912im_/https://www.bipm.org/utils/common/img/flc_dir.gif" width="18" height="18" alt=""> International equivalence </td> </tr> <tr> <td height="100%" valign="top" class="txt11ptl" align="left" bgcolor="#F2F5F8"> <a class="introGras" href="https://web.archive.org/web/20200201205912/http://www.bipm.org/kcdb"><img src="/web/20200201205912im_/https://www.bipm.org/utils/common/img/logos/KCDB2.0.png" width="160" border="0"></a><br> </td> </tr> </table> </td> </tr> </table> </td> </tr> </table> <img src="/web/20200201205912im_/https://www.bipm.org/utils/common/img/pix.gif" width="100%" height="5" alt=""> <table width="180" border="0" cellspacing="0" cellpadding="4" bgcolor="#829EBA"> <tr> <td width="100%" height="100%" valign="top"> <table width="100%" border="0" cellspacing="0" cellpadding="0" bgcolor="#FFFFFF"> <tr> <td width="180" height="100%" valign="top"> <table width="180" border="0" cellpadding="3" bgcolor="#FFFFFF" cellspacing="0" height="100%"> <tr align="left" valign="middle" bgcolor="#00498F"> <td height="25" valign="top" bgcolor="#829EBA" class="cadreTrt"> <img src="/web/20200201205912im_/https://www.bipm.org/utils/common/img/flc_dir.gif" width="18" height="18"> At the BIPM</td> </tr> <tr> <td height="100%" valign="top" align="left"> <img src="/web/20200201205912im_/https://www.bipm.org/utils/common/img/bullet1.gif" width="16" height="9" alt=""><a class="introGras" href="/web/20200201205912/https://www.bipm.org/en/bipm/people/gpanfilo.html">BIPM contact person</a><br> </td> </tr> </table> </td> </tr> </table> </td> </tr> </table> </td> <td width="10"><img src="/web/20200201205912im_/https://www.bipm.org/utils/common/img/pix.gif" width="10"></td> </tr>  <tr> <td colspan="3" height="5"><img src="/web/20200201205912im_/https://www.bipm.org/utils/common/img/pix.gif" width="10" height="5"></td> </tr> </table></center>   <table width="100%" bgcolor="#D3B657"> <tr> <td width="15%"></td> <td width="1000"> <table width="1000" border="0" cellspacing="0" cellpadding="0"> <tr> <td nowrap width="100%"> <style media="all" type="text/css"></style> <div class="menu3"> <ul> <li><a href="/web/20200201205912/https://www.bipm.org/en/worldwide-metrology/" target="_self">Metrology area:</a></li> <li><a id="" href="/web/20200201205912/https://www.bipm.org/metrology/auv/" target="_self">AUV</a> <ul> <li><a href="/web/20200201205912/https://www.bipm.org/metrology/auv/" target="_self">Metrology in the field of Acoustics, Ultrasound and Vibration</a></li> <li><a href="/web/20200201205912/https://www.bipm.org/en/committees/cc/ccauv/" target="_self">Committee structure (CCAUV)</a></li> <li><a href="/web/20200201205912/https://www.bipm.org/metrology/auv/#strategy" target="_self">Strategic plan</a></li> <li><a href="/web/20200201205912/https://www.bipm.org/en/publications/si-brochure/" target="_self">Units (SI Brochure)</a></li> <li><a href="/web/20200201205912/https://www.bipm.org/en/bipm/people/gpanfilo.html" target="_self">BIPM contact</a></li> </ul> </li> <li><a id="" href="/web/20200201205912/https://www.bipm.org/metrology/electricity-magnetism/" target="_self">EM</a> <ul> <li><a href="/web/20200201205912/https://www.bipm.org/metrology/electricity-magnetism/" target="_self">Metrology in the field of Electricity and Magnetism</a></li> <li><a href="/web/20200201205912/https://www.bipm.org/en/committees/cc/ccem/" target="_self">Committee structure (CCEM)</a></li> <li><a href="/web/20200201205912/https://www.bipm.org/metrology/electricity-magnetism/#strategy" target="_self">Strategic plan</a></li> <li><a href="/web/20200201205912/https://www.bipm.org/metrology/electricity-magnetism/units.html" target="_self">Units for electrical quantities</a></li> <li><a href="/web/20200201205912/https://www.bipm.org/en/bipm/electricity/" target="_self">BIPM laboratory work</a></li> <li><a href="/web/20200201205912/https://www.bipm.org/en/bipm-services/comparisons/#PM" target="_self">Ongoing BIPM comparisons</a></li> <li><a href="/web/20200201205912/https://www.bipm.org/en/bipm-services/calibrations/#electricity" target="_self">BIPM calibrations of electrical standards</a></li> <li><a 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href="/web/20200201205912/https://www.bipm.org/en/publications/mises-en-pratique/standard-frequencies.html" target="_self">Recommended values of standard frequencies</a></li> <li><a href="/web/20200201205912/https://www.bipm.org/en/bipm/people/gpanfilo.html" target="_self">BIPM contact</a></li> </ul> </li> <li><a id="" href="/web/20200201205912/https://www.bipm.org/metrology/mass/" target="_self">M</a> <ul> <li><a href="/web/20200201205912/https://www.bipm.org/metrology/mass/" target="_self">Mass Metrology</a></li> <li><a href="/web/20200201205912/https://www.bipm.org/en/committees/cc/ccm/" target="_self">Committee structure (CCM)</a></li> <li><a href="/web/20200201205912/https://www.bipm.org/metrology/mass/#strategy" target="_self">Strategic plan</a></li> <li><a href="/web/20200201205912/https://www.bipm.org/metrology/mass/units.html" target="_self">Unit of mass (kilogram)</a></li> <li><a href="/web/20200201205912/https://www.bipm.org/en/bipm/mass/" target="_self">BIPM laboratory work</a></li> <li><a href="/web/20200201205912/https://www.bipm.org/en/bipm/mass/calibrations_mass/" target="_self">Calibration services</a></li> <li><a href="/web/20200201205912/https://www.bipm.org/metrology/mass/publications.html" target="_self">BIPM publications in the scientific literature</a></li> <li><a href="/web/20200201205912/https://www.bipm.org/metrology/mass/contact.html" target="_self">BIPM contact</a></li> </ul> </li> <li><a id="" href="/web/20200201205912/https://www.bipm.org/metrology/photometry-radiometry/" target="_self">PR</a> <ul> <li><a href="/web/20200201205912/https://www.bipm.org/metrology/photometry-radiometry/" target="_self">Metrology in the field of Photometry and Radiometry</a></li> <li><a href="/web/20200201205912/https://www.bipm.org/en/committees/cc/ccpr/" target="_self">Committee structure (CCPR)</a></li> <li><a href="/web/20200201205912/https://www.bipm.org/utils/en/pdf/CCPR-strategy-document.pdf" target="_self">Strategic plan</a></li> <li><a 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href="/web/20200201205912/https://www.bipm.org/metrology/chemistry-biology/units.html" target="_self">Unit for amount of substance (mole)</a></li> <li><a href="/web/20200201205912/https://www.bipm.org/en/bipm/chemistry/" target="_self">BIPM laboratory work</a></li> <li><a href="/web/20200201205912/https://www.bipm.org/en/bipm-services/comparisons/#QM" target="_self">Comparisons piloted by the BIPM</a></li> <li><a href="/web/20200201205912/https://www.bipm.org/en/bipm-services/calibrations/#chemistry" target="_self">BIPM calibrations: National ozone standards</a></li> <li><a href="/web/20200201205912/https://www.bipm.org/metrology/chemistry-biology/publications.html" target="_self">BIPM publications in the scientific literature</a></li> <li><a href="/web/20200201205912/https://www.bipm.org/metrology/chemistry-biology/liaisons.html" target="_self">BIPM liaison and coordination work</a></li> <li><a href="/web/20200201205912/https://www.bipm.org/en/bipm/people/rwielgosz.html" target="_self">BIPM contact</a></li> </ul> </li> <li><a id="" href="/web/20200201205912/https://www.bipm.org/metrology/ionizing-radiation/" target="_self">RI</a> <ul> <li><a href="/web/20200201205912/https://www.bipm.org/metrology/ionizing-radiation/" target="_self">Metrology in the field of Ionizing Radiation</a></li> <li><a href="/web/20200201205912/https://www.bipm.org/en/committees/cc/ccri/" target="_self">Committee structure (CCRI)</a></li> <li><a href="/web/20200201205912/https://www.bipm.org/metrology/ionizing-radiation/#strategy" target="_self">Strategic plan</a></li> <li><a href="/web/20200201205912/https://www.bipm.org/metrology/ionizing-radiation/units.html" target="_self">SI units</a></li> <li><a href="/web/20200201205912/https://www.bipm.org/en/bipm/ionizing/radionuclides/sir/" target="_self">International Reference System (SIR)</a></li> <li><a href="/web/20200201205912/https://www.bipm.org/en/bipm/ionizing/" target="_self">BIPM laboratory work</a></li> <li><a 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href="/web/20200201205912/https://www.bipm.org/metrology/thermometry/" target="_self">T</a> <ul> <li><a href="/web/20200201205912/https://www.bipm.org/metrology/thermometry/" target="_self">Metrology in Thermometry</a></li> <li><a href="/web/20200201205912/https://www.bipm.org/en/committees/cc/cct/" target="_self">Committee structure (CCT)</a></li> <li><a href="/web/20200201205912/https://www.bipm.org/metrology/thermometry/#strategy" target="_self">Strategic plan</a></li> <li><a href="/web/20200201205912/https://www.bipm.org/metrology/thermometry/units.html" target="_self">SI base unit (kelvin)</a></li> <li><a href="/web/20200201205912/https://www.bipm.org/en/bipm/people/susanne.picard.html" target="_self">BIPM contact</a></li> </ul> </li> <li><a id="" href="/web/20200201205912/https://www.bipm.org/metrology/time-frequency/" target="_self">TF</a> <ul> <li><a href="/web/20200201205912/https://www.bipm.org/metrology/time-frequency/" target="_self">Time and Frequency Metrology</a></li> <li><a href="/web/20200201205912/https://www.bipm.org/en/committees/cc/cctf/" target="_self">Committee structure (CCTF)</a></li> <li><a href="/web/20200201205912/https://www.bipm.org/metrology/time-frequency/#strategy" target="_self">Strategic plan</a></li> <li><a href="/web/20200201205912/https://www.bipm.org/en/bipm-services/timescales/time-server.html" target="_self">What time is it?</a></li> <li><a href="/web/20200201205912/https://www.bipm.org/en/bipm-services/timescales/" target="_self">Timescales</a></li> <li><a href="/web/20200201205912/https://www.bipm.org/metrology/time-frequency/units.html" target="_self">Unit of time (second)</a></li> <li><a href="/web/20200201205912/https://www.bipm.org/en/publications/mises-en-pratique/standard-frequencies.html" target="_self">Recommended values of standard frequencies</a></li> <li><a href="/web/20200201205912/https://www.bipm.org/en/bipm/tai/" target="_self">BIPM technical work</a></li> <li><a href="/web/20200201205912/https://www.bipm.org/jsp/en/TimeCalibrations.jsp" target="_self">BIPM calibrations for time transfer</a></li> <li><a href="/web/20200201205912/https://www.bipm.org/metrology/time-frequency/publications.html" target="_self">BIPM technical publications</a></li> <li><a href="/web/20200201205912/https://www.bipm.org/en/bipm/tai/annual-report.html" target="_self">BIPM Annual Report on Time Activities</a></li> <li><a href="/web/20200201205912/https://www.bipm.org/en/bipm-services/timescales/time-ftp.html" target="_self">FTP server of the BIPM Time Department</a></li> <li><a href="https://web.archive.org/web/20200201205912/http://webtai.bipm.org/database/html/" target="_self">Time Department Database</a></li> <li><a href="/web/20200201205912/https://www.bipm.org/metrology/time-frequency/liaisons.html" target="_self">BIPM liaison and coordination work</a></li> <li><a href="/web/20200201205912/https://www.bipm.org/en/bipm/people/patrizia.tavella.html" target="_self">BIPM contact</a></li> </ul> </li> <li><a id="" href="/web/20200201205912/https://www.bipm.org/en/measurement-units/" target="_self">U</a> <ul> <li><a href="/web/20200201205912/https://www.bipm.org/en/measurement-units/" target="_self">Measurement units: the International System (SI)</a></li> <li><a href="/web/20200201205912/https://www.bipm.org/en/publications/si-brochure/" target="_self">SI Brochure</a></li> <li><a href="/web/20200201205912/https://www.bipm.org/en/measurement-units/#si-base-units" target="_self">SI base units</a></li> <li><a href="/web/20200201205912/https://www.bipm.org/en/measurement-units/#prefixes" target="_self">SI prefixes</a></li> <li><a href="/web/20200201205912/https://www.bipm.org/en/committees/cc/ccu/" target="_self">Consultative Committee for Units (CCU)</a></li> <li><a href="/web/20200201205912/https://www.bipm.org/metrology/units/liaisons.html" target="_self">BIPM liaison and coordination work</a></li> <li><a href="https://web.archive.org/web/20200201205912/http://physics.nist.gov/cuu/Constants/index.html" target="new">CODATA recommended values of the Fundamental Physical Constants</a></li> <li><a href="/web/20200201205912/https://www.bipm.org/extra/codata-tgfc/" target="_self">CODATA Task Group on Fundamental Constants</a></li> <li><a href="/web/20200201205912/https://www.bipm.org/en/bipm/people/estefania.de.mirandes.html" target="_self">BIPM contact</a></li> </ul> </li> </ul> </div> </td> </tr> </table> </td> <td width="15%"></td> </tr> </table>   <table width="100%" bgcolor="#193B7F" alt="mid blue"><tr><td> <center> <table width="1000" bgcolor="#193B7F" border="0" cellspacing="0" cellpadding="0"> <tr height="60"> <td width="20"><img src="/web/20200201205912im_/https://www.bipm.org/utils/common/img/pix.gif" width="20" height="60" alt=""></td> <td width="470"><div align="left"><a href="/web/20200201205912/https://www.bipm.org/en/conference-centre/directions/" class="navigationBlanc">BIPM - 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