<div dir="ltr">All,<div><br></div><div>Based on today's very interesting talk about measurement and Michelson -- I thought that the interesting history of the first standard [the "almighty metre"] might be of interest</div><div><br></div><div>The following is from the Wikipedia article</div><div><a href="https://en.wikipedia.org/wiki/Metre">https://en.wikipedia.org/wiki/Metre</a><br></div><div><br></div><div>A couple of highlights relevant to recent talks:</div><div><h3 style="color:rgb(0,0,0);margin:0.3em 0px 0px;padding-top:0.5em;padding-bottom:0px;overflow:hidden;font-size:1.2em;line-height:1.6;font-family:sans-serif"><span class="gmail-mw-headline" id="gmail-Wavelength_definition">Wavelength definition</span><span class="gmail-mw-editsection" style="font-size:small;font-weight:normal;margin-left:1em;vertical-align:baseline;line-height:1em;unicode-bidi:isolate;margin-right:0px"><span class="gmail-mw-editsection-bracket" style="margin-right:0.25em;color:rgb(84,89,93)">[</span><a href="https://en.wikipedia.org/w/index.php?title=Metre&action=edit&section=7" title="Edit section: Wavelength definition" style="text-decoration-line:none;color:rgb(6,69,173);background:none;white-space:nowrap">edit</a><span class="gmail-mw-editsection-bracket" style="margin-left:0.25em;color:rgb(84,89,93)">]</span></span></h3><p style="margin:0.5em 0px;color:rgb(32,33,34);font-family:sans-serif;font-size:17.5px">In 1873, <a href="https://en.wikipedia.org/wiki/James_Clerk_Maxwell" title="James Clerk Maxwell" style="text-decoration-line:none;color:rgb(6,69,173);background:none">James Clerk Maxwell</a> suggested that light emitted by an element be used as the standard both for the metre and for the second. These two quantities could then be used to define the unit of mass.<sup id="gmail-cite_ref-100" class="gmail-reference" style="line-height:1;unicode-bidi:isolate;white-space:nowrap;font-size:14px"><a href="https://en.wikipedia.org/wiki/Metre#cite_note-100" style="text-decoration-line:none;color:rgb(6,69,173);background:none">[100]</a></sup></p><p style="margin:0.5em 0px;color:rgb(32,33,34);font-family:sans-serif;font-size:17.5px">In 1893, the standard metre was first measured with an <a href="https://en.wikipedia.org/wiki/Interferometer" class="gmail-mw-redirect" title="Interferometer" style="text-decoration-line:none;color:rgb(6,69,173);background:none">interferometer</a> by <a href="https://en.wikipedia.org/wiki/Albert_Abraham_Michelson" class="gmail-mw-redirect" title="Albert Abraham Michelson" style="text-decoration-line:none;color:rgb(6,69,173);background:none">Albert A. Michelson</a>, the inventor of the device and an advocate of using some particular <a href="https://en.wikipedia.org/wiki/Wavelength" title="Wavelength" style="text-decoration-line:none;color:rgb(6,69,173);background:none">wavelength</a> of <a href="https://en.wikipedia.org/wiki/Light" title="Light" style="text-decoration-line:none;color:rgb(6,69,173);background:none">light</a> as a standard of length. By 1925, <a href="https://en.wikipedia.org/wiki/Interferometry" title="Interferometry" style="text-decoration-line:none;color:rgb(6,69,173);background:none">interferometry</a> was in regular use at the BIPM. However, the International Prototype Metre remained the standard until 1960, when the eleventh CGPM defined the metre in the new <a href="https://en.wikipedia.org/wiki/International_System_of_Units" title="" style="color:rgb(6,69,173);background:none">International System of Units</a> (SI) as equal to <span class="gmail-nowrap" style="white-space:nowrap">1<span style="margin-left:0.25em">650</span><span style="margin-left:0.25em">763</span>.73</span> <a href="https://en.wikipedia.org/wiki/Wavelength" title="Wavelength" style="text-decoration-line:none;color:rgb(6,69,173);background:none">wavelengths</a> of the <a href="https://en.wikipedia.org/wiki/Orange_(colour)" title="Orange (colour)" style="text-decoration-line:none;color:rgb(6,69,173);background:none">orange</a>-<a href="https://en.wikipedia.org/wiki/Red" title="Red" style="text-decoration-line:none;color:rgb(6,69,173);background:none">red</a> <a href="https://en.wikipedia.org/wiki/Emission_line" class="gmail-mw-redirect" title="Emission line" style="text-decoration-line:none;color:rgb(6,69,173);background:none">emission line</a> in the <a href="https://en.wikipedia.org/wiki/Electromagnetic_spectrum" title="Electromagnetic spectrum" style="text-decoration-line:none;color:rgb(6,69,173);background:none">electromagnetic spectrum</a> of the <a href="https://en.wikipedia.org/wiki/Krypton-86" class="gmail-mw-redirect" title="Krypton-86" style="text-decoration-line:none;color:rgb(6,69,173);background:none">krypton-86</a> <a href="https://en.wikipedia.org/wiki/Atom" title="Atom" style="text-decoration-line:none;color:rgb(6,69,173);background:none">atom</a> in a <a href="https://en.wikipedia.org/wiki/Vacuum" title="Vacuum" style="text-decoration-line:none;color:rgb(6,69,173);background:none">vacuum</a>.<sup id="gmail-cite_ref-Marion_101-0" class="gmail-reference" style="line-height:1;unicode-bidi:isolate;white-space:nowrap;font-size:14px"><a href="https://en.wikipedia.org/wiki/Metre#cite_note-Marion-101" style="text-decoration-line:none;color:rgb(6,69,173);background:none">[101]</a></sup></p></div><div><br></div><div><br></div><div><h3 style="color:rgb(0,0,0);margin:0.3em 0px 0px;padding-top:0.5em;padding-bottom:0px;overflow:hidden;font-size:1.2em;line-height:1.6;font-family:sans-serif"><span class="gmail-mw-headline" id="gmail-Timeline">Timeline</span><span class="gmail-mw-editsection" style="font-size:small;font-weight:normal;margin-left:1em;vertical-align:baseline;line-height:1em;unicode-bidi:isolate;margin-right:0px"><span class="gmail-mw-editsection-bracket" style="margin-right:0.25em;color:rgb(84,89,93)">[</span><a href="https://en.wikipedia.org/w/index.php?title=Metre&action=edit&section=9" title="Edit section: Timeline" style="text-decoration-line:none;color:rgb(6,69,173);background:none;white-space:nowrap">edit</a><span class="gmail-mw-editsection-bracket" style="margin-left:0.25em;color:rgb(84,89,93)">]</span></span></h3><table class="gmail-wikitable" style="font-size:17.5px;background-color:rgb(248,249,250);color:rgb(32,33,34);margin:1em 0px;border:1px solid rgb(162,169,177);border-collapse:collapse;font-family:sans-serif"><tbody><tr><th width="130pt" style="border:1px solid rgb(162,169,177);padding:0.2em 0.4em;background-color:rgb(234,236,240);text-align:center">Date</th><th style="border:1px solid rgb(162,169,177);padding:0.2em 0.4em;background-color:rgb(234,236,240);text-align:center">Deciding body</th><th style="border:1px solid rgb(162,169,177);padding:0.2em 0.4em;background-color:rgb(234,236,240);text-align:center">Decision</th></tr><tr><td style="border:1px solid rgb(162,169,177);padding:0.2em 0.4em">8 May 1790</td><td style="border:1px solid rgb(162,169,177);padding:0.2em 0.4em"><a href="https://en.wikipedia.org/wiki/National_Assembly_(French_Revolution)" title="National Assembly (French Revolution)" style="text-decoration-line:none;color:rgb(6,69,173);background:none">French National Assembly</a></td><td style="border:1px solid rgb(162,169,177);padding:0.2em 0.4em">The length of the new metre to be equal to the length of a <a href="https://en.wikipedia.org/wiki/Pendulum" title="Pendulum" style="text-decoration-line:none;color:rgb(6,69,173);background:none">pendulum</a> with a half-<a href="https://en.wikipedia.org/wiki/Period_(physics)" class="gmail-mw-redirect" title="Period (physics)" style="text-decoration-line:none;color:rgb(6,69,173);background:none">period</a> of one <a href="https://en.wikipedia.org/wiki/Second" title="Second" style="text-decoration-line:none;color:rgb(6,69,173);background:none">second</a>.<sup id="gmail-cite_ref-Larousse_36-1" class="gmail-reference" style="line-height:1;unicode-bidi:isolate;white-space:nowrap;font-size:14px"><a href="https://en.wikipedia.org/wiki/Metre#cite_note-Larousse-36" style="text-decoration-line:none;color:rgb(6,69,173);background:none">[36]</a></sup></td></tr><tr><td style="border:1px solid rgb(162,169,177);padding:0.2em 0.4em">30 Mar 1791</td><td style="border:1px solid rgb(162,169,177);padding:0.2em 0.4em">French National Assembly</td><td style="border:1px solid rgb(162,169,177);padding:0.2em 0.4em">Accepts the proposal by the <a href="https://en.wikipedia.org/wiki/French_Academy_of_Sciences" title="French Academy of Sciences" style="text-decoration-line:none;color:rgb(6,69,173);background:none">French Academy of Sciences</a> that the new definition for the metre be equal to one ten-millionth of the length of a great circle <a href="https://en.wikipedia.org/wiki/Circular_sector" title="Circular sector" style="text-decoration-line:none;color:rgb(6,69,173);background:none">quadrant</a> along the Earth's <a href="https://en.wikipedia.org/wiki/Meridian_(geography)" title="Meridian (geography)" style="text-decoration-line:none;color:rgb(6,69,173);background:none">meridian</a> through Paris, that is the distance from the equator to the north pole along that quadrant.<sup id="gmail-cite_ref-FOOTNOTEBigourdan190120–21_117-0" class="gmail-reference" style="line-height:1;unicode-bidi:isolate;white-space:nowrap;font-size:14px"><a href="https://en.wikipedia.org/wiki/Metre#cite_note-FOOTNOTEBigourdan190120%E2%80%9321-117" style="text-decoration-line:none;color:rgb(6,69,173);background:none">[117]</a></sup></td></tr><tr><td style="border:1px solid rgb(162,169,177);padding:0.2em 0.4em">1795</td><td colspan="2" style="border:1px solid rgb(162,169,177);padding:0.2em 0.4em">Provisional metre bar made of brass and based on <a href="https://en.wikipedia.org/wiki/Paris_meridian" title="Paris meridian" style="text-decoration-line:none;color:rgb(6,69,173);background:none">Paris meridan</a> arc (French: <i>Méridienne de France</i>) measured by <a href="https://en.wikipedia.org/wiki/Nicolas-Louis_de_Lacaille" title="Nicolas-Louis de Lacaille" style="text-decoration-line:none;color:rgb(6,69,173);background:none">Nicolas-Louis de Lacaillle</a> and <a href="https://en.wikipedia.org/wiki/C%C3%A9sar-Fran%C3%A7ois_Cassini_de_Thury" title="César-François Cassini de Thury" style="text-decoration-line:none;color:rgb(6,69,173);background:none">Cesar-François Cassini de Thury</a>, legally equal to 443.44 <a href="https://en.wikipedia.org/wiki/Line_(unit)" title="Line (unit)" style="text-decoration-line:none;color:rgb(6,69,173);background:none">lines</a> of the <i>toise du Pérou</i> (a standard <a href="https://en.wikipedia.org/wiki/Units_of_measurement_in_France_before_the_French_Revolution#Length" title="Units of measurement in France before the French Revolution" style="text-decoration-line:none;color:rgb(6,69,173);background:none">French unit of length</a> from 1766).<sup id="gmail-cite_ref-Larousse_36-2" class="gmail-reference" style="line-height:1;unicode-bidi:isolate;white-space:nowrap;font-size:14px"><a href="https://en.wikipedia.org/wiki/Metre#cite_note-Larousse-36" style="text-decoration-line:none;color:rgb(6,69,173);background:none">[36]</a></sup><sup id="gmail-cite_ref-Levallois_37-1" class="gmail-reference" style="line-height:1;unicode-bidi:isolate;white-space:nowrap;font-size:14px"><a href="https://en.wikipedia.org/wiki/Metre#cite_note-Levallois-37" style="text-decoration-line:none;color:rgb(6,69,173);background:none">[37]</a></sup><sup id="gmail-cite_ref-Wolf_82-1" class="gmail-reference" style="line-height:1;unicode-bidi:isolate;white-space:nowrap;font-size:14px"><a href="https://en.wikipedia.org/wiki/Metre#cite_note-Wolf-82" style="text-decoration-line:none;color:rgb(6,69,173);background:none">[82]</a></sup><sup id="gmail-cite_ref-entreprises_99-1" class="gmail-reference" style="line-height:1;unicode-bidi:isolate;white-space:nowrap;font-size:14px"><a href="https://en.wikipedia.org/wiki/Metre#cite_note-entreprises-99" style="text-decoration-line:none;color:rgb(6,69,173);background:none">[99]</a></sup> [The line was 1/864 of a <i>toise</i>.]</td></tr><tr><td style="border:1px solid rgb(162,169,177);padding:0.2em 0.4em">10 Dec 1799</td><td style="border:1px solid rgb(162,169,177);padding:0.2em 0.4em">French National Assembly</td><td style="border:1px solid rgb(162,169,177);padding:0.2em 0.4em">Specifies the platinum metre bar, presented on 22 June 1799 and deposited in the <a href="https://en.wikipedia.org/wiki/National_Archives_of_France" class="gmail-mw-redirect" title="National Archives of France" style="text-decoration-line:none;color:rgb(6,69,173);background:none">National Archives</a>, as the final standard. Legally equal to 443.296 lines on the <i>toise du Pérou</i>.<sup id="gmail-cite_ref-entreprises_99-2" class="gmail-reference" style="line-height:1;unicode-bidi:isolate;white-space:nowrap;font-size:14px"><a href="https://en.wikipedia.org/wiki/Metre#cite_note-entreprises-99" style="text-decoration-line:none;color:rgb(6,69,173);background:none">[99]</a></sup></td></tr><tr><td style="border:1px solid rgb(162,169,177);padding:0.2em 0.4em">24–28 Sept 1889</td><td style="border:1px solid rgb(162,169,177);padding:0.2em 0.4em">1st <a href="https://en.wikipedia.org/wiki/General_Conference_on_Weights_and_Measures" title="General Conference on Weights and Measures" style="text-decoration-line:none;color:rgb(6,69,173);background:none">General Conference on Weights and Measures</a> (CGPM)</td><td style="border:1px solid rgb(162,169,177);padding:0.2em 0.4em">Defines the metre as the distance between two lines on a standard bar of an alloy of <a href="https://en.wikipedia.org/wiki/Platinum" title="Platinum" style="text-decoration-line:none;color:rgb(6,69,173);background:none">platinum</a> with 10% <a href="https://en.wikipedia.org/wiki/Iridium" title="Iridium" style="text-decoration-line:none;color:rgb(6,69,173);background:none">iridium</a>, measured at the melting point of ice.<sup id="gmail-cite_ref-entreprises_99-3" class="gmail-reference" style="line-height:1;unicode-bidi:isolate;white-space:nowrap;font-size:14px"><a href="https://en.wikipedia.org/wiki/Metre#cite_note-entreprises-99" style="text-decoration-line:none;color:rgb(6,69,173);background:none">[99]</a></sup><sup id="gmail-cite_ref-118" class="gmail-reference" style="line-height:1;unicode-bidi:isolate;white-space:nowrap;font-size:14px"><a href="https://en.wikipedia.org/wiki/Metre#cite_note-118" style="text-decoration-line:none;color:rgb(6,69,173);background:none">[118]</a></sup></td></tr><tr><td style="border:1px solid rgb(162,169,177);padding:0.2em 0.4em">27 Sept – 6 Oct 1927</td><td style="border:1px solid rgb(162,169,177);padding:0.2em 0.4em">7th CGPM</td><td style="border:1px solid rgb(162,169,177);padding:0.2em 0.4em">Redefines the metre as the distance, at 0 <a href="https://en.wikipedia.org/wiki/Celsius" title="Celsius" style="text-decoration-line:none;color:rgb(6,69,173);background:none">°C</a> (273 <a href="https://en.wikipedia.org/wiki/Kelvin" title="Kelvin" style="text-decoration-line:none;color:rgb(6,69,173);background:none">K</a>), between the axes of the two central lines marked on the prototype bar of platinum-iridium, this bar being subject to one standard <a href="https://en.wikipedia.org/wiki/Atmospheric_pressure" title="Atmospheric pressure" style="text-decoration-line:none;color:rgb(6,69,173);background:none">atmosphere of pressure</a> and supported on two cylinders of at least 10 mm (1 cm) diameter, symmetrically placed in the same horizontal plane at a distance of 571 mm (57.1 cm) from each other.<sup id="gmail-cite_ref-119" class="gmail-reference" style="line-height:1;unicode-bidi:isolate;white-space:nowrap;font-size:14px"><a href="https://en.wikipedia.org/wiki/Metre#cite_note-119" style="text-decoration-line:none;color:rgb(6,69,173);background:none">[119]</a></sup></td></tr><tr><td style="border:1px solid rgb(162,169,177);padding:0.2em 0.4em">14 Oct 1960</td><td style="border:1px solid rgb(162,169,177);padding:0.2em 0.4em">11th CGPM</td><td style="border:1px solid rgb(162,169,177);padding:0.2em 0.4em">Defines the metre as <span class="gmail-nowrap" style="white-space:nowrap">1<span style="margin-left:0.25em">650</span><span style="margin-left:0.25em">763</span>.73</span> <a href="https://en.wikipedia.org/wiki/Wavelength" title="Wavelength" style="text-decoration-line:none;color:rgb(6,69,173);background:none">wavelengths</a> in a <a href="https://en.wikipedia.org/wiki/Vacuum" title="Vacuum" style="text-decoration-line:none;color:rgb(6,69,173);background:none">vacuum</a> of the <a href="https://en.wikipedia.org/wiki/Electromagnetic_radiation" title="Electromagnetic radiation" style="text-decoration-line:none;color:rgb(6,69,173);background:none">radiation</a> corresponding to the transition between the 2p<sup style="line-height:1;font-size:14px">10</sup> and 5d<sup style="line-height:1;font-size:14px">5</sup> quantum levels of the <a href="https://en.wikipedia.org/wiki/Krypton" title="Krypton" style="text-decoration-line:none;color:rgb(6,69,173);background:none">krypton</a>-86 <a href="https://en.wikipedia.org/wiki/Atom" title="Atom" style="text-decoration-line:none;color:rgb(6,69,173);background:none">atom</a>.<sup id="gmail-cite_ref-FOOTNOTEJudson1976_120-0" class="gmail-reference" style="line-height:1;unicode-bidi:isolate;white-space:nowrap;font-size:14px"><a href="https://en.wikipedia.org/wiki/Metre#cite_note-FOOTNOTEJudson1976-120" style="text-decoration-line:none;color:rgb(6,69,173);background:none">[120]</a></sup></td></tr><tr><td style="border:1px solid rgb(162,169,177);padding:0.2em 0.4em">21 Oct 1983</td><td style="border:1px solid rgb(162,169,177);padding:0.2em 0.4em">17th CGPM</td><td style="border:1px solid rgb(162,169,177);padding:0.2em 0.4em">Defines the metre as the length of the path travelled by <a href="https://en.wikipedia.org/wiki/Light" title="Light" style="text-decoration-line:none;color:rgb(6,69,173);background:none">light</a> in a vacuum during a time interval of <span role="math" class="gmail-sfrac gmail-tion" style="white-space:nowrap;display:inline-block;vertical-align:-0.5em;font-size:14.875px;text-align:center"><span class="gmail-num" style="display:block;line-height:1em;margin:0px 0.1em">1</span><span class="gmail-sr-only" style="border:0px;height:1px;overflow:hidden;padding:0px;width:1px">/</span><span class="gmail-den" style="display:block;line-height:1em;margin:0px 0.1em;border-top:1px solid">299 792 458</span></span> of a <a href="https://en.wikipedia.org/wiki/Second" title="Second" style="text-decoration-line:none;color:rgb(6,69,173);background:none">second</a>.<sup id="gmail-cite_ref-121" class="gmail-reference" style="line-height:1;unicode-bidi:isolate;white-space:nowrap;font-size:14px"><a href="https://en.wikipedia.org/wiki/Metre#cite_note-121" style="text-decoration-line:none;color:rgb(6,69,173);background:none">[121]</a></sup><sup id="gmail-cite_ref-122" class="gmail-reference" style="line-height:1;unicode-bidi:isolate;white-space:nowrap;font-size:14px"><a href="https://en.wikipedia.org/wiki/Metre#cite_note-122" style="text-decoration-line:none;color:rgb(6,69,173);background:none">[122]</a></sup></td></tr><tr><td style="border:1px solid rgb(162,169,177);padding:0.2em 0.4em">2002</td><td style="border:1px solid rgb(162,169,177);padding:0.2em 0.4em"><a href="https://en.wikipedia.org/wiki/International_Committee_for_Weights_and_Measures" class="gmail-mw-redirect" title="International Committee for Weights and Measures" style="text-decoration-line:none;color:rgb(6,69,173);background:none">International Committee for Weights and Measures</a> (CIPM)</td><td style="border:1px solid rgb(162,169,177);padding:0.2em 0.4em">Considers the metre to be a unit of <a href="https://en.wikipedia.org/wiki/Proper_length" title="Proper length" style="text-decoration-line:none;color:rgb(6,69,173);background:none">proper length</a> and thus recommends this definition be restricted to "lengths ℓ which are sufficiently short for the effects predicted by <a href="https://en.wikipedia.org/wiki/General_relativity" title="General relativity" style="text-decoration-line:none;color:rgb(6,69,173);background:none">general relativity</a> to be negligible with respect to the uncertainties of realisation".<sup id="gmail-cite_ref-taylor2008a77_123-0" class="gmail-reference" style="line-height:1;unicode-bidi:isolate;white-space:nowrap;font-size:14px"><a href="https://en.wikipedia.org/wiki/Metre#cite_note-taylor2008a77-123" style="text-decoration-line:none;color:rgb(6,69,173);background:none">[123]</a></sup></td></tr></tbody></table></div><div><br></div><div><br></div><div><table class="gmail-wikitable" style="font-size:17.5px;background-color:rgb(248,249,250);color:rgb(32,33,34);margin:0px auto;border:1px solid rgb(162,169,177);border-collapse:collapse;font-family:sans-serif"><caption style="font-weight:bold">Definitions of the metre since 1795<sup id="gmail-cite_ref-FOOTNOTECardarelli2003_124-0" class="gmail-reference" style="line-height:1;unicode-bidi:isolate;white-space:nowrap;font-weight:normal;font-size:14px"><a href="https://en.wikipedia.org/wiki/Metre#cite_note-FOOTNOTECardarelli2003-124" style="text-decoration-line:none;color:rgb(6,69,173);background:none">[124]</a></sup></caption><tbody><tr><th style="border:1px solid rgb(162,169,177);padding:0.2em 0.4em;background-color:rgb(234,236,240);text-align:center">Basis of definition</th><th style="border:1px solid rgb(162,169,177);padding:0.2em 0.4em;background-color:rgb(234,236,240);text-align:center">Date</th><th style="border:1px solid rgb(162,169,177);padding:0.2em 0.4em;background-color:rgb(234,236,240);text-align:center">Absolute<br>uncertainty</th><th style="border:1px solid rgb(162,169,177);padding:0.2em 0.4em;background-color:rgb(234,236,240);text-align:center">Relative<br>uncertainty</th></tr><tr><td style="border:1px solid rgb(162,169,177);padding:0.2em 0.4em"><span role="math" class="gmail-sfrac gmail-tion" style="white-space:nowrap;display:inline-block;vertical-align:-0.5em;font-size:14.875px;text-align:center"><span class="gmail-num" style="display:block;line-height:1em;margin:0px 0.1em">1</span><span class="gmail-sr-only" style="border:0px;height:1px;overflow:hidden;padding:0px;width:1px">/</span><span class="gmail-den" style="display:block;line-height:1em;margin:0px 0.1em;border-top:1px solid">10 000 000</span></span> part of the <a href="https://en.wikipedia.org/wiki/Circular_sector" title="Circular sector" style="text-decoration-line:none;color:rgb(6,69,173);background:none">quadrant</a> along the <a href="https://en.wikipedia.org/wiki/Meridian_(geography)" title="Meridian (geography)" style="text-decoration-line:none;color:rgb(6,69,173);background:none">meridian</a>, measurement by <a href="https://en.wikipedia.org/wiki/Jean_Baptiste_Joseph_Delambre" title="Jean Baptiste Joseph Delambre" style="text-decoration-line:none;color:rgb(6,69,173);background:none">Delambre</a> and <a href="https://en.wikipedia.org/wiki/Pierre_M%C3%A9chain" title="Pierre Méchain" style="text-decoration-line:none;color:rgb(6,69,173);background:none">Méchain</a> (443.296 lines)</td><td style="border:1px solid rgb(162,169,177);padding:0.2em 0.4em">1795</td><td style="border:1px solid rgb(162,169,177);padding:0.2em 0.4em">500–100<span class="gmail-nowrap" style="white-space:nowrap"> </span>μm</td><td style="border:1px solid rgb(162,169,177);padding:0.2em 0.4em"><span class="gmail-nowrap" style="white-space:nowrap">10<sup style="line-height:1;font-size:14px">−4</sup></span></td></tr><tr><td style="border:1px solid rgb(162,169,177);padding:0.2em 0.4em">First prototype <i><span title="French-language text"><i lang="fr"><a href="https://en.wikipedia.org/wiki/History_of_the_metre#M%C3%A8tre_des_Archives" title="History of the metre" style="text-decoration-line:none;color:rgb(6,69,173);background:none">Mètre des Archives</a></i></span></i> platinum bar standard</td><td style="border:1px solid rgb(162,169,177);padding:0.2em 0.4em">1799</td><td style="border:1px solid rgb(162,169,177);padding:0.2em 0.4em">50–10<span class="gmail-nowrap" style="white-space:nowrap"> </span>μm</td><td style="border:1px solid rgb(162,169,177);padding:0.2em 0.4em"><span class="gmail-nowrap" style="white-space:nowrap">10<sup style="line-height:1;font-size:14px">−5</sup></span></td></tr><tr><td style="border:1px solid rgb(162,169,177);padding:0.2em 0.4em">Platinum-iridium bar at melting point of ice (1st <a href="https://en.wikipedia.org/wiki/CGPM" class="gmail-mw-redirect" title="CGPM" style="text-decoration-line:none;color:rgb(6,69,173);background:none">CGPM</a>)</td><td style="border:1px solid rgb(162,169,177);padding:0.2em 0.4em">1889</td><td style="border:1px solid rgb(162,169,177);padding:0.2em 0.4em">0.2–0.1 μm (200–100 nm)</td><td style="border:1px solid rgb(162,169,177);padding:0.2em 0.4em"><span class="gmail-nowrap" style="white-space:nowrap">10<sup style="line-height:1;font-size:14px">−7</sup></span></td></tr><tr><td style="border:1px solid rgb(162,169,177);padding:0.2em 0.4em">Platinum-iridium bar at melting point of ice, atmospheric pressure, supported by two rollers (7th CGPM)</td><td style="border:1px solid rgb(162,169,177);padding:0.2em 0.4em">1927</td><td style="border:1px solid rgb(162,169,177);padding:0.2em 0.4em">n.a.</td><td style="border:1px solid rgb(162,169,177);padding:0.2em 0.4em">n.a.</td></tr><tr><td style="border:1px solid rgb(162,169,177);padding:0.2em 0.4em"><a href="https://en.wikipedia.org/wiki/Hyperfine_structure" title="Hyperfine structure" style="text-decoration-line:none;color:rgb(6,69,173);background:none">Hyperfine</a> atomic transition; <span class="gmail-nowrap" style="white-space:nowrap">1<span style="margin-left:0.25em">650</span><span style="margin-left:0.25em">763</span>.73</span> wavelengths of light from a specified transition in <a href="https://en.wikipedia.org/wiki/Krypton-86" class="gmail-mw-redirect" title="Krypton-86" style="text-decoration-line:none;color:rgb(6,69,173);background:none">krypton-86</a> (11th CGPM)</td><td style="border:1px solid rgb(162,169,177);padding:0.2em 0.4em">1960</td><td style="border:1px solid rgb(162,169,177);padding:0.2em 0.4em">4<span class="gmail-nowrap" style="white-space:nowrap"> </span>nm</td><td style="border:1px solid rgb(162,169,177);padding:0.2em 0.4em"><span class="gmail-nowrap" style="white-space:nowrap">4<span style="margin-left:0.25em;margin-right:0.15em">×</span>10<sup style="line-height:1;font-size:14px">−9</sup></span><sup id="gmail-cite_ref-125" class="gmail-reference" style="line-height:1;unicode-bidi:isolate;white-space:nowrap;font-size:14px"><a href="https://en.wikipedia.org/wiki/Metre#cite_note-125" style="text-decoration-line:none;color:rgb(6,69,173);background:none">[125]</a></sup></td></tr><tr><td style="border:1px solid rgb(162,169,177);padding:0.2em 0.4em">Length of the path travelled by light in a vacuum in <span role="math" class="gmail-sfrac gmail-tion" style="white-space:nowrap;display:inline-block;vertical-align:-0.5em;font-size:14.875px;text-align:center"><span class="gmail-num" style="display:block;line-height:1em;margin:0px 0.1em">1</span><span class="gmail-sr-only" style="border:0px;height:1px;overflow:hidden;padding:0px;width:1px">/</span><span class="gmail-den" style="display:block;line-height:1em;margin:0px 0.1em;border-top:1px solid">299 792 458</span></span> second (17th CGPM)</td><td style="border:1px solid rgb(162,169,177);padding:0.2em 0.4em">1983</td><td style="border:1px solid rgb(162,169,177);padding:0.2em 0.4em">0.1<span class="gmail-nowrap" style="white-space:nowrap"> </span>nm</td><td style="border:1px solid rgb(162,169,177);padding:0.2em 0.4em"><span class="gmail-nowrap" style="white-space:nowrap">10<sup style="line-height:1;font-size:14px">−10</sup></span></td></tr></tbody></table></div><div><br></div><div>and to further the obfuscation of the old standards with the new --</div><div><br></div><div>The kilogram is defined by taking the fixed numerical value of the Planck constant, ℎ, to be 6.626 070 15 × 10-34 when expressed in the unit J s, which is equal to kg m2 s−1, where the metre and the second are defined in terms of the speed of light, 𝒸, and the hyperfine transition frequency of the caesium-133 atom, ∆ν, respectively.<br></div><div><br></div><div> herewith a NIST publication all about the new Kg definition in terms of Planck's constant, the speed of light, and the hyperfine transition frequency for Cs133 <br></div><div><br></div><div><a href="https://www.nist.gov/system/files/documents/2019/09/05/si_k_trifold_brochure_web.pdf">https://www.nist.gov/system/files/documents/2019/09/05/si_k_trifold_brochure_web.pdf</a><br></div><div><br></div><div><br></div><div>Ted</div><div><br></div><div><br></div></div>