Dear All,
From time to time I read the line (in various sources) that goes
something like:
The metre has never changed in length, only the definition has been
rewritten to provide better accuracy and precision.
This poses the question, "Which is the metre we use?"
A brief chronology goes like this:
1790 May 8 The French National Assembly decides that the
length of the metre will be equal to the length of a pendulum with a
half-period of one second.
1791 March 30 Recognising that a universal standard of
measurement was needed, the French National Assembly accepted the
proposal by the French Academy of Sciences that the new definition for
the metre be equal to one ten-millionth of the length of the Earth's
meridian along a quadrant of the Earth, that is the distance from the
equator to the north pole. To determine this length about one tenth of
the Earth's circumference, from Dunkirk, in France, to Barcelona, in
Spain, was meticulously measured. Due to lack of knowledge of the true
shape of the Earth, this definition was never implemented.
1795 A provisional metre bar was constructed of brass.
1799 December 10 The French National Assembly specifies a
platinum metre bar deposited in the National Archives, as the final
standard of length for the whole world.A platinum rod was produced in
Paris that was exactly one metre from end to end. This became the
master standard for the whole world's measuring system. Known as the
'Mètre des Archives' it became the "universal measure" that is the
basis for the metric system we now all use.
From this it looks like the pendulum was never officially recognised
and neither was the metre derived from this measurement of the Earth.
Does this mean that the length of the 1795 brass bar is still the
world standard for the length of a metre assuming that it is true all
subsequent definitions of the metre have kept this same length (as the
1795 brass bar) while only the words of the definitions changed to
achieve greater accuracy and precision.
Am I misreading this history? Is the 1799 platinum bar the one that
takes precedence?
Cheers,
Pat Naughtin
Author of the ebook, Metrication Leaders Guide, see
http://metricationmatters.com/MetricationLeadersGuideInfo.html
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