The following is from my web page of metric history (not online at the
present time).
1795 France adopted the metric system. An attempt to carry a copy of the
earliest meter standard to the United States ended in tragic failure.
Not wishing to wait for the [Delambre-Méchain] survey to be completed,
France enacted a law requiring the use of the metric system, based on
provisional standards. A provisional meter standard was devised based on
an earlier survey of France, carried out from 1739 to 1744. Copies of
this meter, graduated rulers, and provisional kilogram standards were
manufactured and distributed to the 559 districts of the French
Republic. A botanist named Dambey embarked on a trip to the United
States with a copy of the provisional meter and kilogram standards.
However, the ship he was on was attacked by pirates in the Caribbean and
he was either captured or killed. The standards he carried were lost and
have never been found.
1799 Delambre and Méchain presented their survey results, which were
accepted at an international conference. Metric standards were
fabricated and certified.
Delambre and Méchain completed their work in surveying a meridian using
triangulation methods from Dunkerque to Barcelone and their work was
accepted at an international conference (conseil des Cinq Cents et au
Conseil des Anciens) after compromises were made regarding the
ellipticity of the Earth. The Earth's ellipticity actually causes an arc
minute of latitude to represent a gradually changing distance when
proceeding north or south. A mean value had to be derived and the amount
of ellipticity used in that calculation has a very small effect, but one
that precise metrologists care about. Delambre and Méchain deduced an
ellipticity of 1/150 and this was combined with the results of a survey
conducted in Peru between 1736 and 1740 to settle on an accepted value
of 1/334. Ever since this date the meter has remained the same size.
Different methods of defining the standard have evolved; these have
served only to make the standard for the meter more precise and have not
changed its length.
A new standard meter bar ("le mètre étalon" or "the archive meter") was
made of platinum, based on this survey, to represent ("realize", in
metrology terms) the standard meter. This and the new standard kilogram
were fabricated by Etienne Lenoir (a maker of astronomical instruments
and also the provisional meter and kilogram standards mentioned above)
and, after certification, deposited in the Archives of the Republic
(Archives de la République) on 1799 June 22. Copies of these would later
be given to the United States.
To amplify a bit, The original intent was to make the meter's definition
standard so that anyone on Earth could construct their own reference bar
from their own surveys of the Earth at any longitude. Amazingly, it had
already been postulated by Isaac Newton et al. for a century or so that
the Earth was in fact NOT a regular ellipsoid of revolution but was in
fact "lumpy" with "high" regions and "low" regions. In other words,
scientists had good reason even in 1790 that no two lines of longitude
are necessarily the same length. The D-M survey was thus to some extent
a fool's errand. Likewise the problems experienced in the southern part
of the survey, getting observation triangles to close, due to the mass
of mountains throwing off determination of the local vertical had also
been predicted. At least the D-M survey helped to bolster those
century-old predictions.
Briefly, the meter used today is the same as the 1799 meter to the
extent that each new definition maintained its size within the limits of
the uncertainty at each stage. Any inadvertent change in size over the
last 200 years is certainly smaller than the change in the U.S. yard "on
purpose" in 1959.
Jim
On 2010-10-18 1416, Pat Naughtin wrote:
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
Hear Pat speak at: http://www.youtube.com/watch?v=_lshRAPvPZY
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