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Q: Comenserate time and length by pendulum period ( No Answer,   2 Comments )
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 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 inches: 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 surface. To my mind a much more compelling solution for establihing a 'meaningful' refererence length.```
 ```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.```
 ```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.```