
How Did The Meter Get Its Length? - srikar
http://www.npr.org/2014/06/23/324738251/how-did-the-meter-get-its-length
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ColinWright
Roughly ...

They were going to use a length such that a pendulum would have a 1 second
half period, or 2 second full period. That varies according to where you are,
but it's pretty good. This means that _g=(pi)^2,_ because the period of a
pendulum T is given by _2(pi)sqrt(L /g)_.

Since that varies according to the local value of _g_ , they then used one
ten-millionth ( 10^-7 ) of the distance from the North Pole to the Equator
running through Paris. They actually did that by extrapolating a shorter
distance, but it means the Earth is, "by definition", 40 million metres
circumference. They created a platinum bar of the right length and declared
that to be the answer.

Turns out it's wrong, but they never bothered to fix it.

However, in 1960 the eleventh CGPM defined the metre to be 1,650,763.73
wavelengths of the orange-red emission line in the electromagnetic spectrum of
the krypton-86 atom in a vacuum. Obviously much easier to use.

