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href="/web/20200202034301/https://www.bipm.org/en/conference-centre/diary/" target="_self">List of meetings</a></li> <li><a href="/web/20200202034301/https://www.bipm.org/en/conference-centre/" target="_self">BIPM Conference Centre</a></li> <li><a href="/web/20200202034301/https://www.bipm.org/en/conference-centre/directions/" target="_self">How to get to the BIPM</a></li> <li><a href="/web/20200202034301/https://www.bipm.org/en/cgpm-2018/" target="_self">General Conference (CGPM)</a></li> <li><a href="/web/20200202034301/https://www.bipm.org/en/committees/cipm/" target="_self">International Committee (CIPM)</a></li> <li><a href="/web/20200202034301/https://www.bipm.org/en/committees/cc/" target="_self">Consultative Committees and Working Groups</a></li> <li><a href="/web/20200202034301/https://www.bipm.org/en/conference-centre/bipm-workshops/" target="_self">BIPM Workshops</a></li> <li><a href="/web/20200202034301/https://www.bipm.org/en/conference-centre/other-events/" target="_self">Other BIPM events</a></li> <li><a href="/web/20200202034301/https://www.bipm.org/en/publications/" target="_self">Meeting reports</a></li> <li><a href="/web/20200202034301/https://www.bipm.org/en/worldwide-metrology/metrology-events.html" target="_self">External events</a></li> </ul> </li> </ul> </div> </td> </tr> </table>  <table width="1000" border="0" cellspacing="0" cellpadding="0"> <tr> <td width="10" height="50"><img src="/web/20200202034301im_/https://www.bipm.org/utils/common/img/pix.gif" width="10" height="50"></td> <td align="left" width="100%" height="50" background="/web/20200202034301im_/https://www.bipm.org/utils/common/img/cadre_trait.gif"> <table width="100%" border="0" background="/web/20200202034301im_/https://www.bipm.org/metrology/length/units.html"> <tr> <td align="left" class="GrosTitre"><div name="exalead-titre">Length: Unit of length (metre)</div></td> <td align="right" nowrap> <a href="/web/20200202034301/https://www.bipm.org/metrologie/length/units.html" class="GrosTitreVO"></a> </td> </tr> </table> </td> <td width="10" height="50"><img src="/web/20200202034301im_/https://www.bipm.org/utils/common/img/pix.gif" width="10" height="50"></td> </tr> </table>  <script language="JavaScript"> function GoToURL(formURL) { x = formURL.publication.selectedIndex if ( x == -1 ) { alert ("Nothing Selected") } else { location.href = formURL.publication.options[x].value } } $(function() { $( "#tabs" ).tabs(); }); </script> <table width="1000" border="0" cellspacing="0" cellpadding="0" valign="top"> <tr valign="top"> <td width="15"><img src="/web/20200202034301im_/https://www.bipm.org/utils/common/img/pix.gif" width="15"></td> <td width="100%" align="left" valign="top"> <div id="tabs"> <ul> <li><a href="#si-brochure">Base unit</a></li> <li><a href="#history">Historical perspective</a></li> <li><a href="#resolutions"> CGPM/CIPM decisions</a></li> <li><a href="#mep"><a href="/web/20200202034301/https://www.bipm.org/en/publications/mises-en-pratique/">Practical realization</a></a></li> <li><a href="#committees"><a href="/web/20200202034301/https://www.bipm.org/en/committees/cc/ccl/">Committees</a></a></li> <li><a href="#overview"><a href=".">Overview</a></a></li> </ul> <div id="si-brochure"> <p>Definition agreed by the 26th CGPM (November 2018), implemented 20 May 2019:</p> <br> <ul class="txt12pt"> <p><b>The metre, symbol m, is the SI unit of length. It is defined by taking the fixed numerical value of the speed of light in vacuum <i>c</i> to be 299 792 458 when expressed in the unit m s<sup>1</sup>, where the second is defined in terms of the caesium frequency <img src="/web/20200202034301im_/https://www.bipm.org/utils/special/16/bold/delta_maj.gif" alt="Delta"><img src="/web/20200202034301im_/https://www.bipm.org/utils/special/16/ital-bold/nu.gif" alt="nu"><sub>Cs</sub>.</b></p> <table align="left"> <tr> <td><img src="/web/20200202034301im_/https://www.bipm.org/utils/common/img/rev-si/SI-colour-m_c.jpg" width="250" height="250" alt=""></td> <td width="15"></td> <td valign="center"> <p>This definition implies the exact relation <i>c</i> = <nobr>299 792 458 m s<sup>1</sup>.</nobr> Inverting this relation gives an exact expression for the metre in terms of the defining constants <i>c</i> and <img src="/web/20200202034301im_/https://www.bipm.org/utils/special/14/delta_maj.gif" alt="Delta"><img src="/web/20200202034301im_/https://www.bipm.org/utils/special/14/ital/nu.gif" alt="nu"><sub>Cs</sub>:</p> <img src="/web/20200202034301im_/https://www.bipm.org/utils/common/img/rev-si/metre1.jpg" width="541.2" height="76.4" alt=""> <p>The effect of this definition is that one metre is the length of the path travelled by light in vacuum during a time interval with duration of <nobr>1/299 792 458</nobr> of a second.</p> </td> </tr> </table> </ul> <img src="/web/20200202034301im_/https://www.bipm.org/utils/common/img/pix.gif" width="100%" height="30" alt=""> <div class="txt11ptr"> See more: <a href="/web/20200202034301/https://www.bipm.org/en/publications/si-brochure/" class="intros">SI Brochure</a> / <a href="/web/20200202034301/https://www.bipm.org/en/measurement-units/#si-base-units" class="intros">SI base units</a></div> </div> <div id="mep"></div> <div id="resolutions"> <table width="100%" cellpadding="5" border="0"> <tr> <td class="SousTitre"><img src="/web/20200202034301im_/https://www.bipm.org/utils/common/img/fleche_jaune.gif" width="29" height="14" alt="">Decisions relating to the base units of the SI<br> <div align="right">SI Brochure, from <a href="/web/20200202034301/https://www.bipm.org/en/publications/si-brochure/appendix1.html" class="SousTitre">Appendix 1</a></div> </td> </tr> </table> <ul><p><img src="/web/20200202034301im_/https://www.bipm.org/utils/common/img/bullet1.gif" width="16" height="9" alt="">Length:</p> <table width="100%" bgcolor="#829EBA" border="0" cellpadding="5"> <tr> <td width="20" bgcolor="#829EBA" valign="top"> <img src="/web/20200202034301im_/https://www.bipm.org/utils/common/img/flc_dir.gif" width="18" height="18" alt=""></td> <td bgcolor="#DBE3EB" width="120" class="txt11tpl" valign="top"><a href="/web/20200202034301/https://www.bipm.org/en/CGPM/db/1/1/" class="intros">1st CGPM, 1889</a></td> <td bgcolor="#DBE3EB" class="intros"><a href="/web/20200202034301/https://www.bipm.org/en/CGPM/db/1/1/" class="intros">sanction of the prototype metre</a></td> </tr> <tr> <td bgcolor="#829EBA" valign="top"> <img src="/web/20200202034301im_/https://www.bipm.org/utils/common/img/flc_dir.gif" width="18" height="18" alt=""></td> <td bgcolor="#DBE3EB" width="120" class="txt11tpl" valign="top"><a href="/web/20200202034301/https://www.bipm.org/en/CGPM/db/7/1/" class="intros">7th CGPM, 1927</a></td> <td bgcolor="#DBE3EB" class="intros"><a href="/web/20200202034301/https://www.bipm.org/en/CGPM/db/7/1/" class="intros">definition and use of the prototype metre</a></td> </tr> <tr> <td bgcolor="#829EBA" valign="top"> <img src="/web/20200202034301im_/https://www.bipm.org/utils/common/img/flc_dir.gif" width="18" height="18" alt=""></td> <td bgcolor="#DBE3EB" width="120" class="txt11ptl" valign="top"><a href="/web/20200202034301/https://www.bipm.org/en/CGPM/db/10/6/" class="intros">10th CGPM, 1954</a></td> <td bgcolor="#DBE3EB" class="intros"><a href="/web/20200202034301/https://www.bipm.org/en/CGPM/db/10/6/" class="intros">metre adopted as a base unit</a></td> </tr> <tr> <td bgcolor="#829EBA" valign="top"> <img src="/web/20200202034301im_/https://www.bipm.org/utils/common/img/flc_dir.gif" width="18" height="18" alt=""></td> <td bgcolor="#DBE3EB" class="txt11ptl" valign="top"><a href="/web/20200202034301/https://www.bipm.org/en/CGPM/db/11/6/" class="intros">11th CGPM, 1960</a></td> <td bgcolor="#DBE3EB" class="intros"><a href="/web/20200202034301/https://www.bipm.org/en/CGPM/db/11/6/" class="intros">redefinition of the metre in terms of krypton 86 radiation</a></td> </tr> <tr> <td bgcolor="#829EBA" valign="top"> <img src="/web/20200202034301im_/https://www.bipm.org/utils/common/img/flc_dir.gif" width="18" height="18" alt=""></td> <td bgcolor="#DBE3EB" class="txt11ptl" valign="top"><a href="/web/20200202034301/https://www.bipm.org/en/CGPM/db/15/2/" class="intros">15th CGPM, 1975</a></td> <td bgcolor="#DBE3EB" class="intros"><a href="/web/20200202034301/https://www.bipm.org/en/CGPM/db/15/2/" class="intros">recommends value for the speed of light</a></td> </tr> <tr> <td bgcolor="#829EBA" valign="top"> <img src="/web/20200202034301im_/https://www.bipm.org/utils/common/img/flc_dir.gif" width="18" height="18" alt=""></td> <td bgcolor="#DBE3EB" class="txt11ptl" valign="top"><a href="/web/20200202034301/https://www.bipm.org/en/CGPM/db/17/1/" class="intros">17th CGPM, 1983</a></td> <td bgcolor="#DBE3EB" class="intros">redefinition of the metre using the speed of light,<br/> <a href="/web/20200202034301/https://www.bipm.org/en/CGPM/db/17/1/" class="intros">realization of the definition of the metre</a></td> </tr> <tr> <td bgcolor="#829EBA" valign="top"> <img src="/web/20200202034301im_/https://www.bipm.org/utils/common/img/flc_dir.gif" width="18" height="18"></td> <td bgcolor="#DBE3EB" class="txt11ptl" valign="top"><a href="/web/20200202034301/https://www.bipm.org/utils/en/pdf/CI-2002-1-EN.pdf" class="intros">CIPM, 2002</a></td> <td bgcolor="#DBE3EB" class="intros"><a href="/web/20200202034301/https://www.bipm.org/utils/en/pdf/CI-2002-1-EN.pdf" class="intros">specifies the rules for the practical realization of the definition of the metre</a></td> </tr> <tr> <td bgcolor="#829EBA" valign="top"> <img src="/web/20200202034301im_/https://www.bipm.org/utils/common/img/flc_dir.gif" width="18" height="18"> </td> <td bgcolor="#DBE3EB" class="txt11ptl" valign="top"><a href="/web/20200202034301/https://www.bipm.org/utils/en/pdf/CI-2003-1-EN.pdf" class="intros">CIPM, 2003</a></td> <td bgcolor="#DBE3EB" class="intros"><a href="/web/20200202034301/https://www.bipm.org/utils/en/pdf/CI-2003-1-EN.pdf" class="intros">revision of the list of recommended radiations</a></td> </tr> <tr> <td bgcolor="#829EBA" valign="top"> <img src="/web/20200202034301im_/https://www.bipm.org/utils/common/img/flc_dir.gif" width="18" height="18"> </td> <td bgcolor="#DBE3EB" class="txt11ptl" valign="top"><a href="/web/20200202034301/https://www.bipm.org/utils/en/pdf/CI-2005-3-EN.pdf" class="intros">CIPM, 2005</a></td> <td bgcolor="#DBE3EB" class="intros"><a href="/web/20200202034301/https://www.bipm.org/utils/en/pdf/CI-2005-3-EN.pdf" class="intros">revision of the list of recommended radiations</a></td> </tr> <tr> <td bgcolor="#829EBA" valign="top"> <img src="/web/20200202034301im_/https://www.bipm.org/utils/common/img/flc_dir.gif" width="18" height="18"> </td> <td bgcolor="#DBE3EB" class="txt11ptl" valign="top"><a href="/web/20200202034301/https://www.bipm.org/utils/en/pdf/CI-2007-1-EN.pdf" class="intros">CIPM, 2007</a></td> <td bgcolor="#DBE3EB" class="intros"><a href="/web/20200202034301/https://www.bipm.org/utils/en/pdf/CI-2007-1-EN.pdf" class="intros">revision of the list of recommended radiations</a></td> </tr> <tr> <td bgcolor="#829EBA" valign="top"> <img src="/web/20200202034301im_/https://www.bipm.org/utils/common/img/flc_dir.gif" width="18" height="18"> </td> <td bgcolor="#DBE3EB" class="txt11ptl" valign="top"><a href="/web/20200202034301/https://www.bipm.org/en/CGPM/db/23/9/" class="intros">23rd CGPM, 2007</a></td> <td bgcolor="#DBE3EB" class="intros"><a href="/web/20200202034301/https://www.bipm.org/en/CGPM/db/23/9/" class="intros">revision of the <i>mise en pratique</i> of the definition of the metre and development of new optical frequency standards</a></td> </tr> <tr> <td bgcolor="#829EBA" valign="top"> <img src="/web/20200202034301im_/https://www.bipm.org/utils/common/img/flc_dir.gif" width="18" height="18"> </td> <td bgcolor="#DBE3EB" class="txt11ptl" valign="top"><a href="/web/20200202034301/https://www.bipm.org/utils/en/pdf/CI-2009-2-EN.pdf" class="intros">CIPM, 2009</a></td> <td bgcolor="#DBE3EB" class="intros"><a href="/web/20200202034301/https://www.bipm.org/utils/en/pdf/CI-2009-2-EN.pdf" class="intros">updates to the list of standard frequencies</a></td> </tr> <tr> <td bgcolor="#829EBA" valign="top"> <img src="/web/20200202034301im_/https://www.bipm.org/utils/common/img/flc_dir.gif" width="18" height="18" alt=""></td> <td bgcolor="#DBE3EB" width="120" class="txt11ptl" valign="top"><a href="/web/20200202034301/https://www.bipm.org/en/CGPM/db/24/1/" class="intros">24th CGPM, 2011</a></td> <td bgcolor="#DBE3EB" class="intros"><a href="/web/20200202034301/https://www.bipm.org/en/CGPM/db/24/1/" class="intros">on the possible future revision of the International System of Units, the SI</a></td> </tr> <tr> <td bgcolor="#829EBA" valign="top"> <img src="/web/20200202034301im_/https://www.bipm.org/utils/common/img/flc_dir.gif" width="18" height="18"> </td> <td bgcolor="#DBE3EB" class="txt11ptl" valign="top"><a href="/web/20200202034301/https://www.bipm.org/en/CGPM/db/24/8/" class="intros">24th CGPM, 2011</a></td> <td bgcolor="#DBE3EB" class="intros"><a href="/web/20200202034301/https://www.bipm.org/en/CGPM/db/24/8/" class="intros">revision of the <i>mise en pratique</i> of the definition of the metre and development of new optical frequency standards</a></td> </tr> <tr> <td valign="top"> <img src="/web/20200202034301im_/https://www.bipm.org/utils/common/img/flc_dir.gif" width="18" height="18"> </td> <td bgcolor="#DBE3EB" class="txt11ptl" valign="top"><a href="/web/20200202034301/https://www.bipm.org/utils/en/pdf/CI-2013-1-EN.pdf" class="intros">CIPM, 2013</a></td> <td bgcolor="#DBE3EB" class="intros"><a href="/web/20200202034301/https://www.bipm.org/utils/en/pdf/CI-2013-1-EN.pdf" class="intros">updates to the list of standard frequencies</a></td> </tr> <tr> <td valign="top"> <img src="/web/20200202034301im_/https://www.bipm.org/utils/common/img/flc_dir.gif" width="18" height="18" alt=""></td> <td bgcolor="#DBE3EB" width="120" class="txt11ptl" valign="top"><a href="/web/20200202034301/https://www.bipm.org/en/CGPM/db/26/1/" class="intros">26th CGPM, 2018</a></td> <td bgcolor="#DBE3EB" class="intros"><a href="/web/20200202034301/https://www.bipm.org/en/CGPM/db/26/1/" class="intros">revision of the International System of Units, the SI</a></td> </tr> </table> </ul> </div> <div id="history"> <ul class="txt12pt"> <p>The 1889 definition of the metre, namely, the length of the international prototype of platinum-iridium, was replaced by the 11th CGPM (1960) using a definition based on the wavelength of the radiation corresponding to a particular transition in krypton 86. This change was adopted in order to improve the accuracy with which the definition of the metre could be realized, this being achieved using an interferometer with a travelling microscope to measure the optical path difference as the fringes were counted. In turn, this was replaced in 1983 by the 17th CGPM (Resolution 1) with a definition referenced to the distance that light travels in vacuum in a specified interval of time. The original international prototype of the metre, which was sanctioned by the 1st CGPM in 1889, is still kept at the BIPM under conditions specified in 1889. In order to make clear its dependence on the fixed numerical value of the speed of light, <i>c</i>, the wording of the definition was changed in Resolution 1 of the 26th CGPM (2018).</p> <hr width="30%" color="#829EBA" align="left"> <ul class="txt11ptl"> <li><a href="/web/20200202034301/https://www.bipm.org/en/CGPM/db/1/1/" class="intros">Resolution of the 1st CGPM (1889)</a>; CR p.34</li> <li><a href="/web/20200202034301/https://www.bipm.org/en/CGPM/db/11/6/" class="intros">Resolution 6 of the 11th CGPM (1960)</a>; CR p.85</li> <li><a href="/web/20200202034301/https://www.bipm.org/en/CGPM/db/17/1/" class="intros">Resolution 1 of the 17th CGPM (1983)</a>; CR p.98; <a href="https://web.archive.org/web/20200202034301/https://doi.org/10.1088/0026-1394/20/1/005" class="intros" target="new"><i>Metrologia</i>, 1984, <b>20</b>, 25</a></li> <li><a href="/web/20200202034301/https://www.bipm.org/en/CGPM/db/26/1/" class="intros">Resolution 1 of the 26th CGPM (2018)</a>; <a href="https://web.archive.org/web/20200202034301/https://doi.org/10.1088/1681-7575/ab0013" class="intros" target="new"><i>Metrologia</i>, 2019, <b>56</b>, 022001</a> </ul> </ul> <img src="/web/20200202034301im_/https://www.bipm.org/utils/common/img/pix.gif" width="100%" height="30" alt=""> <div class="txt11ptr"> See more: <a href="/web/20200202034301/https://www.bipm.org/en/measurement-units/history-si/" class="intros">History of the SI</a> / <a href="/web/20200202034301/https://www.bipm.org/en/measurement-units/history-si/evolution-metre.html" class="intros">The BIPM and the evolution of the definition of the metre</a> </div> </div> <div id="committees"></div> </div> </td> <td width="10"><img src="/web/20200202034301im_/https://www.bipm.org/utils/common/img/pix.gif" width="10" alt=""></td> </tr> <tr> <td height="5" colspan="3"><img src="/web/20200202034301im_/https://www.bipm.org/utils/common/img/pix.gif" width="10" height="5"></td> </tr> </table> </td></tr></table>   <table 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href="/web/20200202034301/https://www.bipm.org/en/committees/cc/ccqm/" target="_self">Committee structure (CCQM)</a></li> <li><a href="/web/20200202034301/https://www.bipm.org/metrology/chemistry-biology/#strategy" target="_self">Strategic plan</a></li> <li><a href="/web/20200202034301/https://www.bipm.org/metrology/chemistry-biology/units.html" target="_self">Unit for amount of substance (mole)</a></li> <li><a href="/web/20200202034301/https://www.bipm.org/en/bipm/chemistry/" target="_self">BIPM laboratory work</a></li> <li><a href="/web/20200202034301/https://www.bipm.org/en/bipm-services/comparisons/#QM" target="_self">Comparisons piloted by the BIPM</a></li> <li><a href="/web/20200202034301/https://www.bipm.org/en/bipm-services/calibrations/#chemistry" target="_self">BIPM calibrations: National ozone standards</a></li> <li><a href="/web/20200202034301/https://www.bipm.org/metrology/chemistry-biology/publications.html" target="_self">BIPM publications in the scientific literature</a></li> <li><a href="/web/20200202034301/https://www.bipm.org/metrology/chemistry-biology/liaisons.html" target="_self">BIPM liaison and coordination work</a></li> <li><a href="/web/20200202034301/https://www.bipm.org/en/bipm/people/rwielgosz.html" target="_self">BIPM contact</a></li> </ul> </li> <li><a id="" href="/web/20200202034301/https://www.bipm.org/metrology/ionizing-radiation/" target="_self">RI</a> <ul> <li><a href="/web/20200202034301/https://www.bipm.org/metrology/ionizing-radiation/" target="_self">Metrology in the field of Ionizing Radiation</a></li> <li><a href="/web/20200202034301/https://www.bipm.org/en/committees/cc/ccri/" target="_self">Committee structure (CCRI)</a></li> <li><a href="/web/20200202034301/https://www.bipm.org/metrology/ionizing-radiation/#strategy" target="_self">Strategic plan</a></li> <li><a href="/web/20200202034301/https://www.bipm.org/metrology/ionizing-radiation/units.html" target="_self">SI units</a></li> <li><a href="/web/20200202034301/https://www.bipm.org/en/bipm/ionizing/radionuclides/sir/" target="_self">International Reference System (SIR)</a></li> <li><a href="/web/20200202034301/https://www.bipm.org/en/bipm/ionizing/" target="_self">BIPM laboratory work</a></li> <li><a href="/web/20200202034301/https://www.bipm.org/en/bipm-services/comparisons/#RI" target="_self">Ongoing BIPM comparisons</a></li> <li><a href="/web/20200202034301/https://www.bipm.org/en/bipm-services/calibrations/#ionizing-radiation" target="_self">BIPM calibrations: Ionization chambers</a></li> <li><a href="/web/20200202034301/https://www.bipm.org/metrology/ionizing-radiation/publications.html" target="_self">BIPM publications in the scientific literature</a></li> <li><a href="/web/20200202034301/https://www.bipm.org/en/publications/scientific-output/monographies-ri.html" target="_self"><i>Monographies</i> BIPM</a></li> <li><a href="/web/20200202034301/https://www.bipm.org/metrology/ionizing-radiation/liaisons.html" target="_self">BIPM liaison and coordination work</a></li> <li><a href="/web/20200202034301/https://www.bipm.org/en/bipm/people/steven.judge.html" target="_self">BIPM contact</a></li> </ul> </li> <li><a id="" href="/web/20200202034301/https://www.bipm.org/metrology/thermometry/" target="_self">T</a> <ul> <li><a href="/web/20200202034301/https://www.bipm.org/metrology/thermometry/" target="_self">Metrology in Thermometry</a></li> <li><a href="/web/20200202034301/https://www.bipm.org/en/committees/cc/cct/" target="_self">Committee structure (CCT)</a></li> <li><a href="/web/20200202034301/https://www.bipm.org/metrology/thermometry/#strategy" target="_self">Strategic plan</a></li> <li><a href="/web/20200202034301/https://www.bipm.org/metrology/thermometry/units.html" target="_self">SI base unit (kelvin)</a></li> <li><a href="/web/20200202034301/https://www.bipm.org/en/bipm/people/susanne.picard.html" target="_self">BIPM contact</a></li> </ul> </li> <li><a id="" href="/web/20200202034301/https://www.bipm.org/metrology/time-frequency/" target="_self">TF</a> <ul> <li><a href="/web/20200202034301/https://www.bipm.org/metrology/time-frequency/" target="_self">Time and Frequency Metrology</a></li> <li><a href="/web/20200202034301/https://www.bipm.org/en/committees/cc/cctf/" target="_self">Committee structure (CCTF)</a></li> <li><a href="/web/20200202034301/https://www.bipm.org/metrology/time-frequency/#strategy" target="_self">Strategic plan</a></li> <li><a href="/web/20200202034301/https://www.bipm.org/en/bipm-services/timescales/time-server.html" target="_self">What time is it?</a></li> <li><a href="/web/20200202034301/https://www.bipm.org/en/bipm-services/timescales/" target="_self">Timescales</a></li> <li><a href="/web/20200202034301/https://www.bipm.org/metrology/time-frequency/units.html" target="_self">Unit of time (second)</a></li> <li><a href="/web/20200202034301/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/20200202034301/https://www.bipm.org/en/bipm/tai/" target="_self">BIPM technical work</a></li> <li><a href="/web/20200202034301/https://www.bipm.org/jsp/en/TimeCalibrations.jsp" target="_self">BIPM calibrations for time transfer</a></li> <li><a href="/web/20200202034301/https://www.bipm.org/metrology/time-frequency/publications.html" target="_self">BIPM technical publications</a></li> <li><a href="/web/20200202034301/https://www.bipm.org/en/bipm/tai/annual-report.html" target="_self">BIPM Annual Report on Time Activities</a></li> <li><a href="/web/20200202034301/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/20200202034301/http://webtai.bipm.org/database/html/" target="_self">Time Department Database</a></li> <li><a href="/web/20200202034301/https://www.bipm.org/metrology/time-frequency/liaisons.html" target="_self">BIPM liaison and coordination work</a></li> <li><a href="/web/20200202034301/https://www.bipm.org/en/bipm/people/patrizia.tavella.html" target="_self">BIPM contact</a></li> </ul> </li> <li><a id="" href="/web/20200202034301/https://www.bipm.org/en/measurement-units/" target="_self">U</a> <ul> <li><a href="/web/20200202034301/https://www.bipm.org/en/measurement-units/" target="_self">Measurement units: the International System (SI)</a></li> <li><a href="/web/20200202034301/https://www.bipm.org/en/publications/si-brochure/" target="_self">SI Brochure</a></li> <li><a href="/web/20200202034301/https://www.bipm.org/en/measurement-units/#si-base-units" target="_self">SI base units</a></li> <li><a href="/web/20200202034301/https://www.bipm.org/en/measurement-units/#prefixes" target="_self">SI prefixes</a></li> <li><a href="/web/20200202034301/https://www.bipm.org/en/committees/cc/ccu/" target="_self">Consultative Committee for Units (CCU)</a></li> <li><a href="/web/20200202034301/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/20200202034301/http://physics.nist.gov/cuu/Constants/index.html" target="new">CODATA recommended values of the Fundamental Physical Constants</a></li> <li><a href="/web/20200202034301/https://www.bipm.org/extra/codata-tgfc/" target="_self">CODATA Task Group on Fundamental Constants</a></li> <li><a href="/web/20200202034301/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 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