Dear Bill,
I think that you are right in saying Jefferson used a pendulum with a
period of 2 seconds for his 'seconds pendulum' with a length of about
a metre. Others, particularly Prieur in France, used a pendulum about
250 mm long to give a period of about 1 second.
By the way, do you know of a reliable web source where I can find the
values of g, the acceleration due to gravity, for various places
around the world? When I searched, I found a lot of physics teachers
with good values for their class room locations – but little else.
Only one of these gave a (calculated) value for the North Pole (9.8640
m/s^2) and the equator (9.7982 m/s^2); see http://www.ac.wwu.edu/~vawter/PhysicsNet/Topics/Gravity/AccOfGravity.html
This reference at http://wiki.answers.com/Q/Which_Europe_Cities_have_exactly_9.80665_MS_of_acceleration_due_to_gravity
doesn't help much!
http://mtp.jpl.nasa.gov/notes/altitude/altitude.html states that
9.80665 is the gravity at sea level at 45.542 degrees latitude, and
one might assume that the definition was intended to be what it is at
45 degrees exactly. I assume what you're really asking is how was
9.80665 chosen, and this should answer that. But anyway, major
European cities within a degree of 45N: Bordeaux, Lyon Turin, Milan,
Belgrade, Bucharest. Cities along this line of latitude on other
continents: Novorossiysk, Krasnodar, Halifax, Green Bay, Minneapolis,
Portland. Cities near 45S: Dunedin (NZ). Rawson (Argentina).
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
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On 2009/09/30, at 02:31 , Bill Hooper wrote:
On Sep 28 , at 7:14 PM, Pat Naughtin wrote:
Dear All,
Does anyone know anything of the history of the 'plummet'?
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;
That's interesting, Pat.
I had heard that Jefferson's efforts were the reverse; namely, that
the second should be defined as the time of the half period of a
pendulum of some specific length.
If the second is defined as the time for the half period of a simple
pendulum (on Earth, etc.) with a length of exactly 1 m, then (using
the modern definition of the metre) the second would be 1.0035
seconds (of the currently defined kind).
If the currently defined second would be used to define the metre,
the metre would be 0.9929 metres (of the present kind).
Bill Hooper
1810 mm tall (using the presently defined metre) or
1797 mm if the metre were defined in terms of the pendulum.
Fernandina Beach, Florida, USA
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SImplification Begins With SI.
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