science · 14 October 1960 · 64 years ago
In 1960, the fourth legal definition of the metre was made to be 1,650,763.73 wavelengths in vacuum of the orange-red light radiation of the krypton-86 atom (transition between levels 2p10 and 5d5). It was adopted at the 11th meeting of the Conférence Générale des Poids et Mesures (CGPM), which also specified the seven basic units of the SI system. It was superceded by a fifth definition of the metre on 20 Oct 1983, based on the speed of light. The definition in terms of wavelengths of the kryton spectral line wasthen 100 times more accurate than the previous third legal definition adopted in 1889. (The third legal definition on 28 Sep 1889, was based on the distance between two reference markson the International prototype metre bar, made of platinum-iridium, (90% Pt, 10% Ir), cross section X, at a 0 °C temperature.)
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