Another drawback to the "seconds pendulum", as that was called was that
its period depended on the local acceleration due to gravity. By the
time of the French revolution, it was well known that this acceleration
varied with latitude and with elevation. Isaac Newton had also pointed
out that local topography, such as nearby mountains, had some effect on
the local "vertical" as indicated by a plumb bob -- which in fact is
what threw a wrench in the southern half of the "great survey" as it
entered the Pyrenees. (Local vertical was used for the astronomical
observations of latitude in lieu of horizon since the survey was done on
land, not at sea.) Yet another problem was the effect of temperature
changes on the length of the pendulum arm. By the end of the 18th
century, compound bimetallic arms were used in finer clocks to
compensate for that. The seconds pendulum, however, used an
approximation that the pendulum arm's mass was insignificant compared to
the mass of the bob, making it a "simple pendulum" whereas the
bimetallic compound arms definitely ventured into the physics of the
"physical pendulum".
The above arguments were raised most often in the discussion about
whether to use astronomical observations or pendula to define the
second, but the logic works both ways. In short, the committee discarded
the pendulum as a precise realization means for any standard.
Jim
Pat Naughtin wrote:
Dear All,
Does anyone know anything of the history of the 'plummet'?
Apparently the plummet was used in Wellington's armies as an instrument
to measure the rate at which soldiers marched. The plummet was a piece
of cord attached to a lead shot that was then used as a pendulum.
Some approximate values are:
1000 mm plummet was used for 60 steps per minute
600 mm plummet was used for 75 steps per minute
300 mm plummet was used for 108 steps per minute
250 mm plummet was used for 120 steps per minute
(The last of these is probably the most common rate used by military
services today)
As the plummet was in common use in 1812, my question relates to how
long before 1812, this pendulum method was in use for military marching.
If this technique was available in 1790, for example, then it would have
had a significant influence on the metric debate about whether to use
the plummet pendulum or the size of the meridian as the basis for the
length of the metre. This debate centred around Borda who wanted to
market his 'repeating circle' and Thomas Jefferson who favored the
pendulum method because of its universal availability and its
portability; sadly perhaps, Borda won that round!
See http://www.gap-system.org/~history/Biographies/Borda.html where you
will find:
When Borda was made Chairman of the Commission of Weights and Measures,
which had as its members Condorcet
<http://www.gap-system.org/~history/Mathematicians/Condorcet.html>,
Lavoisier, Laplace
<http://www.gap-system.org/~history/Mathematicians/Laplace.html> and Legendre
<http://www.gap-system.org/~history/Mathematicians/Legendre.html>, he
soon put his accurate surveying instrument to good use. The Commission
was set up in 1790 to bring in a uniform system of measurement. It
considered a proposal which had already been made to the French
government to base the metre on the length of a pendulum which beat at
the rate of one second. This proposal had found favour with Britain and
the United States who considered it a truly international measure.
Borda, however, reported on the 19 March 1791 that the Commission had
decided on a different standard, namely that one metre should be one ten
millionth of the distance from the North Pole to the equator. His
argument against the pendulum standard was that it based one unit on
another, which might itself change, and also that the second itself was
an arbitrary unit based on the division of a day by 12 × 60 × 60. Borda
argued that the day should be divided into 10 hours with an hour divided
into 100 minutes each of 100 seconds. Under Borda's leadership the
project to accurately measure the distance from the North Pole to the
equator using the Borda repeating circle was carried out.
Cheers,
Pat Naughtin
Author of the ebook, /Metrication Leaders Guide,/ that you can obtain
from http://metricationmatters.com/MetricationLeadersGuideInfo.html
PO Box 305 Belmont 3216,
Geelong, Australia
Phone: 61 3 5241 2008
Metric system consultant, writer, and speaker, Pat Naughtin, has helped
thousands of people and hundreds of companies upgrade to the modern
metric system smoothly, quickly, and so economically that they now save
thousands each year when buying, processing, or selling for their
businesses. Pat provides services and resources for many different
trades, crafts, and professions for commercial, industrial and
government metrication leaders in Asia, Europe, and in the USA. Pat's
clients include the Australian Government, Google, NASA, NIST, and the
metric associations of Canada, the UK, and the USA.
See http://www.metricationmatters.com
<http://www.metricationmatters.com/>for more metrication information,
contact Pat at [email protected]
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