Excellent post, Joe. Thanks for sharing it. I stand corrected, it seems we're more... "advanced" on this than I thought, we're actually fast approaching 9-decimal-place accuracy. I just wished we'd reach it by other means instead of using the second for it...
Marcus On Wed, 27 Mar 2002 21:18:38 Joseph B. Reid wrote: >Ma Be quoted me in USMA 19107: > >>>The advantage of these definitions of the second and the metre is that they >>>do not depend on the preservation of physical prototyypes. They can be >>>replicated by any well equipped standardizing laboratory anywhere in the >>>world. > >And added: > >>Perhaps so, but what was wrong with the previous one, that didn't need the >>second for its definition? We're told that this was largely to increase >>accuracy to 8 decimal places or something to that effect. Now, does this >>mean we may not have another similar alternative? Just wondering... > > >The metre has had several definitions, each one more precise than the >precding one. > >In 1793 it one ten-millionth of the distance from the north pole to the >equator, accurate to 1 0.8 mm. > >In 1799 it became "l'italon des Archives", a platinum bar, accurate to 1 10 5m. > >The next standard metre was constructed in 1889. It was an X-shsped bar of >90% platinum and 10% iridiium. It was accurate to 1 0.1 5m. > >In 1906 the angstrom was defined in terms of the wavelength of the red >spectral line of cadmium. This, in effect, defined the metre within 1 10 >nm. > >In 1957 the CIPM declared the metre to be equal to 1 650 763.73 wavelengths >in vacuo of the radiation corresponding to the transition between the level >2p10 and 5d5 of the krypton 86 atom. This definition had an estimated >precision of 1 4 nm. > >In 1983 the CGPM decided that "The metre is the length of the path >travelled by light in vacuum during a time interval of 1/299 792 458 of a >second". > >In view of the variety of codings for special characters, I list the ones I >have used in the above posting: > 5 is Greek mu > 1 is plus or minus >The accuracy estimates are my interpretation of those given in another >format by Maurice DANLOUX DUMESNILS in his "Itude critique du systhme >mitrique", 1962, Gauthier-Villars & Cie. > >Joseph B.Reid >17 Glebe Road West >Toronto M5P 1C8 TEL. 416-486-6071 > > See Dave Matthews Band live or win a signed guitar http://r.lycos.com/r/bmgfly_mail_dmb/http://win.ipromotions.com/lycos_020201/splash.asp
