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Q: Comenserate time and length by pendulum period ( No Answer,   2 Comments )
Subject: Comenserate time and length by pendulum period
Category: Science > Instruments and Methods
Asked by: shamus0122-ga
List Price: $10.00
Posted: 24 Nov 2005 11:52 PST
Expires: 24 Dec 2005 11:52 PST
Question ID: 597191
In the metric system all units are defined in terms of various other
convenient physical constants
For Example: One degree C is 1/100 the distance between freezing and
boiling pure water at STP. Or calorie the amount of energy needed to
raise 1 cm cubed of water one degree C.
Since the 'second' is a unit of time given as the solar day divided by (
60 times 60 times 24 ).
And a pendulum at surface gravity with a 2 second period (a 1 second
half period ) has a lenght very close to a modern meter or about 39
Then why was this 'convenient fact', known to Galilao in the 1600's and
clock makers galore, NOT chosen by the French in the late 1700's when
the metric system was founded.
Instead of sub divisions of some terrestrial semi-arc, we could have
'length' tied to 'time' (the  number of human heartbeat over an
earthly day), comenserate with  the gravitational force at earth's
To my mind a much more compelling solution for establihing a 
'meaningful' refererence length.
There is no answer at this time.

Subject: Re: Comenserate time and length by pendulum period
From: kottekoe-ga on 24 Nov 2005 20:21 PST
Good question. Actually the meter was originally defined in terms of
the dimensions of the Earth, just as the second was defined in terms
of the Earth's rotational period. The meter was supposed to be
1/10,000,000 of the distance from the equator to the pole. Great
expeditions were formed to measure this distance going both North and
South from Paris. As it turns out, the accuracy of measurement even in
those days was good enough to get a different answer from the
Northerly surveying trip as from the Southerly one due to the
non-sphericity of the Earth. Ultimately, this definition was deemed
impractical and for many years the meter was defined as the length of
a particular rod maintained carefully in Paris. Your suggestion is a
great idea, but my guess is that it could not compete in accuracy with
the artifact in Paris.

In modern times, most of these definitions have been abandoned in
favor of more fundamental physical parameters that can be measured in
any laboratory. For a while the meter was defined as a certain number
of wavelengths of a particular atomic spectral line. The second is no
longer defined in terms of the rotation of the Earth, which changes as
the Earth slows down. The second is now a certain number oscillations
of a particular atomic spectral line and the meter is now determined
by the distance that light travels in a vacuum in a precisely defined
fraction of a second, thus defining the speed of light to be exactly
299,792,458 meters per second.

Since the second is no longer tied to the rotation of the Earth, the
average solar day is no longer exactly 24 hours, necessitating the
insertion of leap seconds, as will happen at the end of 2005.
Subject: Re: Comenserate time and length by pendulum period
From: knickers-ga on 30 Nov 2005 05:30 PST
The pendulum idea does not work because the value of G varies slightly
over the surface of the earth and depends on whether you are standing
in the vicinity of a big mountain or not. Hence you get slightly
different results. Nice idea though.

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