Definition:Metre/Historical Note

Historical Note on Metre

 * 1671: measured the length $L$ of a pendulum whose period is $1$ second, and proposed a unit of Measurement $2 L$, which was to be called the universal toise.


 * 1675: suggested the term metre as the length of this pendulum.

It differs from the modern metre by half a centimetre.

It was soon established that as Acceleration Due to Gravity varies considerably according to location, this was not a sustainable definition to maintain a standard.


 * 19 March 1791: the suggested that the metre should be defined as $10^{-7}$ the distance from the Earth's equator, through  to the North Pole (at sea level).


 * 26 March 1791: this proposal was adopted.


 * 1805: brought a standard metre made in Paris to the United States. He designed a baseline apparatus which instead of bringing different bars in actual contact during measurements, used only one bar calibrated on the metre and optical contact. Thus the metre became the unit of length for geodesy in the United States.


 * 1873: suggested using light emitted by an element as the standard both for the metre and for the second.


 * 1889: The international prototype metre was defined as the distance between two lines on a standard bar composed of an alloy of $90 \%$ platinum and $10 \%$ iridium, measured at the melting point of ice.


 * 1960: The metre was defined in the new International System of Units as equal to $1 \, 650 \, 763 \cdotp 73$ wavelengths of the orange-red light emitted by the transition $2 P_{10} - 5 D_5$ in the electromagnetic spectrum of the krypton-$86$ atom in a vacuum.


 * 1983: defined as the distance travelled by light in vacuum in $\dfrac 1 {299 \ 792 \ 458}$ of a second.