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Conference (CGPM)</a></li> <li><a href="/web/20200202204815/https://www.bipm.org/en/committees/cipm/" target="_self">International Committee (CIPM)</a></li> <li><a href="/web/20200202204815/https://www.bipm.org/en/committees/cc/" target="_self">Consultative Committees and Working Groups</a></li> <li><a href="/web/20200202204815/https://www.bipm.org/en/conference-centre/bipm-workshops/" target="_self">BIPM Workshops</a></li> <li><a href="/web/20200202204815/https://www.bipm.org/en/conference-centre/other-events/" target="_self">Other BIPM events</a></li> <li><a href="/web/20200202204815/https://www.bipm.org/en/publications/" target="_self">Meeting reports</a></li> <li><a href="/web/20200202204815/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/20200202204815im_/https://www.bipm.org/utils/common/img/pix.gif" width="10" height="50"></td> <td align="left" width="100%" height="50" background="/web/20200202204815im_/https://www.bipm.org/utils/common/img/cadre_trait.gif"> <table width="100%" border="0" background="/web/20200202204815im_/https://www.bipm.org/metrology/time-frequency/units.html"> <tr> <td align="left" class="GrosTitre"><div name="exalead-titre">Time and frequency: SI unit of time (second)</div></td> <td align="right" nowrap> <a href="/web/20200202204815/https://www.bipm.org/metrologie/time-frequency/units.html" class="GrosTitreVO"></a> </td> </tr> </table> </td> <td width="10" height="50"><img src="/web/20200202204815im_/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/20200202204815im_/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/20200202204815/https://www.bipm.org/en/publications/mises-en-pratique/">Practical realization</a></a></li> <li><a href="#secondary-representations"><a href="/web/20200202204815/https://www.bipm.org/en/publications/mises-en-pratique/standard-frequencies.html">Secondary representations</a></a></li> <li><a href="#committees"><a href="/web/20200202204815/https://www.bipm.org/en/committees/cc/cctf/">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 second, symbol s, is the SI unit of time. It is defined by taking the fixed numerical value of the caesium frequency <img src="/web/20200202204815im_/https://www.bipm.org/utils/special/16/bold/delta_maj.gif" alt="Delta"><img src="/web/20200202204815im_/https://www.bipm.org/utils/special/16/ital-bold/nu.gif" alt="nu"><sub>Cs</sub>, the unperturbed ground-state hyperfine transition frequency of the caesium-133 atom, to be <nobr>9 192 631 770</nobr> when expressed in the unit Hz, which is equal to s<sup>1</sup>.</b></p> <table align="left"> <tr> <td><img src="/web/20200202204815im_/https://www.bipm.org/utils/common/img/rev-si/SI-colour-s_delta-nu.jpg" width="250" height="250" alt=""></td> <td width="15"></td> <td valign="center"> <p>This definition implies the exact relation <img src="/web/20200202204815im_/https://www.bipm.org/utils/special/14/delta_maj.gif" alt="Delta"><img src="/web/20200202204815im_/https://www.bipm.org/utils/special/14/ital/nu.gif" alt="nu"><sub>Cs</sub> = 9 192 631 770 Hz. Inverting this relation gives an expression for the unit second in terms of the defining constant <nobr><img src="/web/20200202204815im_/https://www.bipm.org/utils/special/14/delta_maj.gif" alt="Delta"><img src="/web/20200202204815im_/https://www.bipm.org/utils/special/14/ital/nu.gif" alt="nu"><sub>Cs</sub>:</nobr></p> <center><img src="/web/20200202204815im_/https://www.bipm.org/utils/common/img/rev-si/seconde1.jpg" width="164.4" height="70.8" alt=""></center> or <center><img src="/web/20200202204815im_/https://www.bipm.org/utils/common/img/rev-si/seconde2.jpg" width="164.4" height="70.8" alt=""></center> <p>The effect of this definition is that the second is equal to the duration of <nobr>9 192 631 770</nobr> periods of the radiation corresponding to the transition between the two hyperfine levels of the unperturbed ground state of the <sup>133</sup>Cs atom.</p> </td> </tr> </table> </ul> <img src="/web/20200202204815im_/https://www.bipm.org/utils/common/img/pix.gif" width="100%" height="30" alt=""> <div class="txt11ptr"> See more: <a href="/web/20200202204815/https://www.bipm.org/en/publications/si-brochure/" class="intros">SI Brochure</a> / <a href="/web/20200202204815/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/20200202204815im_/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/20200202204815/https://www.bipm.org/en/publications/si-brochure/appendix1.html" class="SousTitre">Appendix 1</a></div> </td> </tr> </table> <ul><p><img src="/web/20200202204815im_/https://www.bipm.org/utils/common/img/bullet1.gif" width="16" height="9" alt="">Time:</p> <table width="100%" bgcolor="#829EBA" border="0" cellpadding="5"> <tr> <td bgcolor="#829EBA" valign="top"> <img src="/web/20200202204815im_/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/20200202204815/https://www.bipm.org/en/CGPM/db/10/6/" class="intros">10th CGPM, 1954</a></td> <td bgcolor="#DBE3EB" class="intro"><a href="/web/20200202204815/https://www.bipm.org/en/CGPM/db/10/6/" class="intros">second adopted as a base unit</a></td> </tr> <tr> <td width="20" bgcolor="#829EBA" valign="top"> <img src="/web/20200202204815im_/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/20200202204815/https://www.bipm.org/en/CIPM/db/1956/1/" class="intros">CIPM, 1956</a></td> <td bgcolor="#DBE3EB" class="intro"><a href="/web/20200202204815/https://www.bipm.org/en/CIPM/db/1956/1/" class="intros">definition of the second as a fraction of the tropical year 1900</a></td> </tr> <tr> <td valign="top"> <img src="/web/20200202204815im_/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/20200202204815/https://www.bipm.org/en/CGPM/db/11/9/" class="intros">11th CGPM, 1960</a></td> <td bgcolor="#DBE3EB" class="intro"><a href="/web/20200202204815/https://www.bipm.org/en/CGPM/db/11/9/" class="intros">ratifies the CIPM 1956 definition of the second</a></td> </tr> <tr> <td bgcolor="#829EBA" valign="top"> <img src="/web/20200202204815im_/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/20200202204815/https://www.bipm.org/en/CIPM/db/1964/0/" class="intros">CIPM, 1964</a></td> <td bgcolor="#DBE3EB" class="intro"><a href="/web/20200202204815/https://www.bipm.org/en/CIPM/db/1964/0/" class="intros">declares the caesium 133 hyperfine transition to be the recommended standard</a></td> </tr> <tr> <td bgcolor="#829EBA" valign="top"> <img src="/web/20200202204815im_/https://www.bipm.org/utils/common/img/flc_dir.gif" width="18" height="18" alt=""></td> <td bgcolor="#DBE3EB" width="120" class="txt11ptl"><a href="/web/20200202204815/https://www.bipm.org/en/CGPM/db/12/5/" class="intros">12th CGPM, 1964</a></td> <td bgcolor="#DBE3EB" class="intro"><a href="/web/20200202204815/https://www.bipm.org/en/CGPM/db/12/5/" class="intros">empowers the CIPM to investigate atomic and molecular frequency standards</a></td> </tr> <tr> <td bgcolor="#829EBA" valign="top"> <img src="/web/20200202204815im_/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/20200202204815/https://www.bipm.org/en/CGPM/db/13/1/" class="intros">13th CGPM, 1967/68</a></td> <td bgcolor="#DBE3EB" class="intro"><a href="/web/20200202204815/https://www.bipm.org/en/CGPM/db/13/1/" class="intros">defines the second in terms of the caesium transition</a></td> </tr> <tr> <td bgcolor="#829EBA" valign="top"> <img src="/web/20200202204815im_/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/20200202204815/https://www.bipm.org/en/committees/cc/cctf/ccds-1970.html" class="intros">CCDS, 1970</a></td> <td bgcolor="#DBE3EB" class="intro"><a href="/web/20200202204815/https://www.bipm.org/en/committees/cc/cctf/ccds-1970.html" class="intros">defines International Atomic Time, TAI</a></td> </tr> <tr> <td bgcolor="#829EBA" valign="top"> <img src="/web/20200202204815im_/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/20200202204815/https://www.bipm.org/en/CGPM/db/14/1/" class="intros">14th CGPM, 1971</a></td> <td bgcolor="#DBE3EB" class="intro"><a href="/web/20200202204815/https://www.bipm.org/en/CGPM/db/14/1/" class="intros">requests the CIPM to define and establish International Atomic Time, TAI </a></td> </tr> <tr> <td bgcolor="#829EBA" valign="top"> <img src="/web/20200202204815im_/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/20200202204815/https://www.bipm.org/en/CGPM/db/15/5/" class="intros">15th CGPM, 1975</a></td> <td bgcolor="#DBE3EB" class="intro"><a href="/web/20200202204815/https://www.bipm.org/en/CGPM/db/15/5/" class="intros">endorses the use of Coordinated Universal Time, UTC </a></td> </tr> <tr> <td bgcolor="#829EBA" valign="top"> <img src="/web/20200202204815im_/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/20200202204815/https://www.bipm.org/utils/en/pdf/CI-2006-1-EN.pdf" class="intros">CIPM, 2006</a></td> <td bgcolor="#DBE3EB" class="intro"><a href="/web/20200202204815/https://www.bipm.org/utils/en/pdf/CI-2006-1-EN.pdf" class="intros" secondary representations of the second < a></td> </tr> <tr> <td bgcolor="#829EBA" valign="top"> <img src="/web/20200202204815im_/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/20200202204815/https://www.bipm.org/en/CGPM/db/23/9/" class="intros">23rd CGPM, 2007</a></td> <td bgcolor="#DBE3EB" class="intro"><a href="/web/20200202204815/https://www.bipm.org/en/CGPM/db/23/9/" class="intros">on the revision of the <i>mise en pratique</i> of the definition of the metre and the development of new optical frequency standards</a></td> </tr> <tr> <td bgcolor="#829EBA" valign="top"> <img src="/web/20200202204815im_/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/20200202204815/https://www.bipm.org/utils/en/pdf/CI-2009-2-EN.pdf" class="intros">CIPM, 2009</a></td> <td bgcolor="#DBE3EB" class="intro"><a href="/web/20200202204815/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/20200202204815im_/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/20200202204815/https://www.bipm.org/en/CGPM/db/24/1/" class="intros">24th CGPM, 2011</a></td> <td bgcolor="#DBE3EB" class="intro"><a href="/web/20200202204815/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/20200202204815im_/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/20200202204815/https://www.bipm.org/en/CGPM/db/24/8/" class="intros">24th CGPM, 2011</a></td> <td bgcolor="#DBE3EB" class="intro"><a href="/web/20200202204815/https://www.bipm.org/en/CGPM/db/24/8/" class="intros">on the revision of the <i>mise en pratique</i> of the metre and the development of new optical frequency standards</a></td> </tr> <tr> <td bgcolor="#829EBA" valign="top"> <img src="/web/20200202204815im_/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/20200202204815/https://www.bipm.org/utils/en/pdf/CI-2013-1-EN.pdf" class="intros">CIPM, 2013</a></td> <td bgcolor="#DBE3EB" class="intro"><a href="/web/20200202204815/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 bgcolor="#829EBA" valign="top"> <img src="/web/20200202204815im_/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/20200202204815/https://www.bipm.org/utils/en/pdf/CIPMOutcomes/CI-2015-Recommendation-2.pdf" class="intros">CIPM, 2015</a></td> <td bgcolor="#DBE3EB" class="intro"><a href="/web/20200202204815/https://www.bipm.org/utils/en/pdf/CIPMOutcomes/CI-2015-Recommendation-2.pdf" class="intros">updates to the list of standard frequencies</a></td> </tr> <tr> <td valign="top"> <img src="/web/20200202204815im_/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/20200202204815/https://www.bipm.org/en/CGPM/db/26/1/" class="intros">26th CGPM, 2018</a></td> <td bgcolor="#DBE3EB" class="intro"><a href="/web/20200202204815/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>Before 1960, the unit of time the second, was defined as the fraction 1/86 400 of the mean solar day. The exact definition of "mean solar day" was left to astronomers. However measurements showed that irregularities in the rotation of the Earth made this an unsatisfactory definition. In order to define the unit of time more precisely, the 11th CGPM (1960) adopted a definition given by the International Astronomical Union based on the tropical year 1900. Experimental work, however, had already shown that an atomic standard of time, based on a transition between two energy levels of an atom or a molecule, could be realized and reproduced much more accurately. Considering that a very precise definition of the unit of time is indispensable for science and technology, the 13th CGPM (1967-1968) chose a new definition of the second referenced to the frequency of the ground state hyperfine transition in the caesium 133 atom. A revised more precise wording of this same definition now in terms of a fixed numerical value of the unperturbed ground-state hyperfine transition frequency of the caesium 133 atom, <img src="/web/20200202204815im_/https://www.bipm.org/utils/special/14/delta_maj.gif" alt="Delta"><img src="/web/20200202204815im_/https://www.bipm.org/utils/special/14/nu.gif" alt="nu"><sub>Cs</sub>, was adopted in Resolution 1 of the 26th CGPM (2018).</p> <hr width="30%" color="#829EBA" align="left"> <ul class="txt11ptl"> <li><a href="/web/20200202204815/https://www.bipm.org/en/CGPM/db/11/9/" class="intros">Resolution 9 of the 11th CGPM (1960)</a>; CR p.86</li> <li><a href="/web/20200202204815/https://www.bipm.org/en/CGPM/db/13/1/" class="intros">Resolution 1 of the 13th CGPM (1967-1968)</a>; CR p.103; <a href="https://web.archive.org/web/20200202204815/https://doi.org/10.1088/0026-1394/4/1/006" class="intros" target="new"><i>Metrologia</i>, 1968, <b>4</b>, 43</a></li> <li><a href="/web/20200202204815/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/20200202204815/https://doi.org/10.1088/1681-7575/ab0013" class="intros" target="new"><i>Metrologia</i>, 2019, <b>56</b>, 022001</a></li> </ul> </ul> <img src="/web/20200202204815im_/https://www.bipm.org/utils/common/img/pix.gif" width="100%" height="30" alt=""> <div class="txt11ptr"> See more: <a href="/web/20200202204815/https://www.bipm.org/en/measurement-units/history-si/" class="intros">History of the SI</a> </div> </div> <div id="committees"></div> </div> </td> <td width="10"><img src="/web/20200202204815im_/https://www.bipm.org/utils/common/img/pix.gif" width="10" alt=""></td> </tr> <tr> <td height="5" colspan="3"><img src="/web/20200202204815im_/https://www.bipm.org/utils/common/img/pix.gif" width="10" height="5"></td> </tr> </table> </td></tr></table>   <table width="100%" bgcolor="#D3B657"> <tr> <td width="15%"></td> <td 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literature</a></li> <li><a href="/web/20200202204815/https://www.bipm.org/metrology/chemistry-biology/liaisons.html" target="_self">BIPM liaison and coordination work</a></li> <li><a href="/web/20200202204815/https://www.bipm.org/en/bipm/people/rwielgosz.html" target="_self">BIPM contact</a></li> </ul> </li> <li><a id="" href="/web/20200202204815/https://www.bipm.org/metrology/ionizing-radiation/" target="_self">RI</a> <ul> <li><a href="/web/20200202204815/https://www.bipm.org/metrology/ionizing-radiation/" target="_self">Metrology in the field of Ionizing Radiation</a></li> <li><a href="/web/20200202204815/https://www.bipm.org/en/committees/cc/ccri/" target="_self">Committee structure (CCRI)</a></li> <li><a href="/web/20200202204815/https://www.bipm.org/metrology/ionizing-radiation/#strategy" target="_self">Strategic plan</a></li> <li><a href="/web/20200202204815/https://www.bipm.org/metrology/ionizing-radiation/units.html" target="_self">SI units</a></li> <li><a href="/web/20200202204815/https://www.bipm.org/en/bipm/ionizing/radionuclides/sir/" target="_self">International Reference System (SIR)</a></li> <li><a href="/web/20200202204815/https://www.bipm.org/en/bipm/ionizing/" target="_self">BIPM laboratory work</a></li> <li><a href="/web/20200202204815/https://www.bipm.org/en/bipm-services/comparisons/#RI" target="_self">Ongoing BIPM comparisons</a></li> <li><a href="/web/20200202204815/https://www.bipm.org/en/bipm-services/calibrations/#ionizing-radiation" target="_self">BIPM calibrations: Ionization chambers</a></li> <li><a href="/web/20200202204815/https://www.bipm.org/metrology/ionizing-radiation/publications.html" target="_self">BIPM publications in the scientific literature</a></li> <li><a href="/web/20200202204815/https://www.bipm.org/en/publications/scientific-output/monographies-ri.html" target="_self"><i>Monographies</i> BIPM</a></li> <li><a href="/web/20200202204815/https://www.bipm.org/metrology/ionizing-radiation/liaisons.html" target="_self">BIPM liaison and coordination work</a></li> <li><a href="/web/20200202204815/https://www.bipm.org/en/bipm/people/steven.judge.html" target="_self">BIPM contact</a></li> </ul> </li> <li><a id="" href="/web/20200202204815/https://www.bipm.org/metrology/thermometry/" target="_self">T</a> <ul> <li><a href="/web/20200202204815/https://www.bipm.org/metrology/thermometry/" target="_self">Metrology in Thermometry</a></li> <li><a href="/web/20200202204815/https://www.bipm.org/en/committees/cc/cct/" target="_self">Committee structure (CCT)</a></li> <li><a href="/web/20200202204815/https://www.bipm.org/metrology/thermometry/#strategy" target="_self">Strategic plan</a></li> <li><a href="/web/20200202204815/https://www.bipm.org/metrology/thermometry/units.html" target="_self">SI base unit (kelvin)</a></li> <li><a href="/web/20200202204815/https://www.bipm.org/en/bipm/people/susanne.picard.html" target="_self">BIPM contact</a></li> </ul> </li> <li><a id="current" href="/web/20200202204815/https://www.bipm.org/metrology/time-frequency/" target="_self">TF</a> <ul> <li><a href="/web/20200202204815/https://www.bipm.org/metrology/time-frequency/" target="_self">Time and Frequency Metrology</a></li> <li><a href="/web/20200202204815/https://www.bipm.org/en/committees/cc/cctf/" target="_self">Committee structure (CCTF)</a></li> <li><a href="/web/20200202204815/https://www.bipm.org/metrology/time-frequency/#strategy" target="_self">Strategic plan</a></li> <li><a href="/web/20200202204815/https://www.bipm.org/en/bipm-services/timescales/time-server.html" target="_self">What time is it?</a></li> <li><a href="/web/20200202204815/https://www.bipm.org/en/bipm-services/timescales/" target="_self">Timescales</a></li> <li><a href="/web/20200202204815/https://www.bipm.org/metrology/time-frequency/units.html" target="_self">Unit of time (second)</a></li> <li><a href="/web/20200202204815/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/20200202204815/https://www.bipm.org/en/bipm/tai/" target="_self">BIPM technical work</a></li> <li><a href="/web/20200202204815/https://www.bipm.org/jsp/en/TimeCalibrations.jsp" target="_self">BIPM calibrations for time transfer</a></li> <li><a href="/web/20200202204815/https://www.bipm.org/metrology/time-frequency/publications.html" target="_self">BIPM technical publications</a></li> <li><a href="/web/20200202204815/https://www.bipm.org/en/bipm/tai/annual-report.html" target="_self">BIPM Annual Report on Time Activities</a></li> <li><a href="/web/20200202204815/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/20200202204815/http://webtai.bipm.org/database/html/" target="_self">Time Department Database</a></li> <li><a href="/web/20200202204815/https://www.bipm.org/metrology/time-frequency/liaisons.html" target="_self">BIPM liaison and coordination work</a></li> <li><a href="/web/20200202204815/https://www.bipm.org/en/bipm/people/patrizia.tavella.html" target="_self">BIPM contact</a></li> </ul> </li> <li><a id="" href="/web/20200202204815/https://www.bipm.org/en/measurement-units/" target="_self">U</a> <ul> <li><a href="/web/20200202204815/https://www.bipm.org/en/measurement-units/" target="_self">Measurement units: the International System (SI)</a></li> <li><a href="/web/20200202204815/https://www.bipm.org/en/publications/si-brochure/" target="_self">SI Brochure</a></li> <li><a href="/web/20200202204815/https://www.bipm.org/en/measurement-units/#si-base-units" target="_self">SI base units</a></li> <li><a href="/web/20200202204815/https://www.bipm.org/en/measurement-units/#prefixes" target="_self">SI prefixes</a></li> <li><a href="/web/20200202204815/https://www.bipm.org/en/committees/cc/ccu/" target="_self">Consultative Committee for Units (CCU)</a></li> <li><a href="/web/20200202204815/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/20200202204815/http://physics.nist.gov/cuu/Constants/index.html" target="new">CODATA recommended values of the Fundamental Physical Constants</a></li> <li><a href="/web/20200202204815/https://www.bipm.org/extra/codata-tgfc/" target="_self">CODATA Task Group on Fundamental Constants</a></li> <li><a href="/web/20200202204815/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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