Editing Prototype (section)

== Metrology ==
<!-- This section is linked from [[Kilogram]] -->
In the science and practice of [[metrology]], a '''prototype''' is a human-made object that is used as the standard of [[measurement]] of some [[physical quantity]] to base all measurement of that physical quantity against.  Sometimes this standard object is called an '''artifact'''.  In the [[International System of Units]] ('''SI'''), there remains no prototype standard [[2019 revision of the SI|since May 20, 2019]].  Before that date, the last prototype used was the [[international prototype of the kilogram]], a solid [[platinum-iridium alloy|platinum-iridium]] cylinder kept at the [[Bureau International des Poids et Mesures]] (International Bureau of Weights and Measures) in [[Sèvres]] [[France]] (a suburb of [[Paris]]) that by [[definition]] was the mass of exactly one [[kilogram]].  Copies of this prototype are fashioned and issued to many nations to represent the national standard of the kilogram and are periodically compared to the Paris prototype.  Now the kilogram is redefined in such a way that the [[Planck constant]] {{Mvar|h}} is prescribed a value of exactly {{val|6.62607015|e=-34|u=joule-second (J⋅s)}}

Until 1960, the [[metre|meter]] was defined by a platinum-iridium prototype bar with two marks on it (that were, by definition, spaced apart by one meter), the [[international prototype of the metre]], and in 1983 the meter was redefined to be the distance in [[free space]] covered by [[speed of light|light]] in 1/299,792,458 of a [[second]] (thus ''defining'' the speed of light to be 299,792,458 meters per second).