Metre

==== Wavelength definition ====
In 1873, [[James Clerk Maxwell]] suggested that light emitted by an element be used as the standard both for the unit of length and for the second. These two quantities could then be used to define the unit of mass.<ref>{{cite book |url=https://archive.org/details/electricandmagne01maxwrich |title=A Treatise On Electricity and Magnetism |first=James Clerk |last=Maxwell |author-link=James Clerk Maxwell |publisher=MacMillan and Co. |location=London |volume=1 |year=1873 |page=3}}</ref> About the unit of length he wrote:

{{blockquote|text=In the present state of science the most universal standard of length which we could assume would be the wave length in vacuum of a particular kind of light, emitted by some widely diffused substance such as sodium, which has well-defined lines in its spectrum. Such a standard would be independent of any changes in the dimensions of the earth, and should be adopted by those who expect their writings to be more permanent than that body.|author=James Clerk Maxwell|title=''[[A Treatise on Electricity and Magnetism]]''|source=3rd edition, Vol. 1, p. 3}}

[[Charles Sanders Peirce]]’s work promoted the advent of American science at the forefront of global metrology. Alongside his intercomparisons of artifacts of the metre and contributions to gravimetry through improvement of reversible pendulum, Peirce was the first to tie experimentally the metre to the wave length of a spectral line. According to him the standard length might be compared with that of a wave of light identified by a line in the [[Sunlight|solar spectrum]]. Albert Michelson soon took up the idea and improved it.<ref name=":4">{{Cite journal |last=Crease |first=Robert P. |date=2009-12-01 |title=Charles Sanders Peirce and the first absolute measurement standard |url=https://doi.org/10.1063/1.3273015 |journal=Physics Today |volume=62 |issue=12 |pages=39–44 |doi=10.1063/1.3273015 |bibcode=2009PhT....62l..39C |s2cid=121338356 |issn=0031-9228}}</ref><ref>{{Cite journal |last=Lenzen |first=Victor F. |date=1965 |title=The Contributions of Charles S. Peirce to Metrology |url=https://www.jstor.org/stable/985776 |journal=Proceedings of the American Philosophical Society |volume=109 |issue=1 |pages=29–46 |jstor=985776 |issn=0003-049X}}</ref>

In 1893, the standard metre was first measured with an [[interferometer]] by [[Albert Abraham Michelson|Albert A. Michelson]], the inventor of the device and an advocate of using some particular [[wavelength]] of [[light]] as a standard of length. By 1925, [[interferometry]] 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 [[International System of Units]] (SI) as equal to {{val|1650763.73}} [[wavelength]]s of the [[orange (colour)|orange]]-[[red]] [[emission line]] in the [[electromagnetic spectrum]] of the [[krypton-86]] [[atom]] in [[vacuum]].<ref name="Marion">{{cite book |last=Marion |first=Jerry B. |title=Physics For Science and Engineering |year=1982 |publisher=CBS College Publishing |isbn=978-4-8337-0098-6 |page=3}}</ref>