[time-nuts] Standards for units
Arnold Tibus
Arnold.Tibus at gmx.de
Tue Apr 3 09:08:51 EDT 2007
Hello to all,
and thank you very much Enrico, a very nice and comprehensive
document.
There is a book on the market (2005) "The Measure of all Things" by
Ken Alder, Free Press New York, translated version in german (2006)
"Das Mass der Welt". It tells on about 500 pages quite precise and
very detailed (based on a lot of original french and other documents)
the long history and big work principally of J.- B.-J. Delambre, P.- F.-A. Méchain
and J.-J. Lalande fighting for the definition and introduction of the meter
in the 18. century.
Worth to be read I believe.
I like all the discussions on this Time-Nuts platform. Concerning the
relation between inch and meter I thought that the definition
is fixed long time ago.
When the meter was defined to be multiples of the wavelength
of light in vacuum, in 1959, the relationship between inches and
centimeters was redefined to be that one inch is equal to 2.540
centimeters, exactly. The new foot, derived from 1 inch = 2.54 cm (exactly),
is referred to as the "international foot".
There are a lot of informations about to find in the internet, but to be on the safe
side I had a look on the NIST homepage:
http://emtoolbox.nist.gov/Faq/Faq.asp#FAQ-Miscellaneous-6
(") Quote:
What is the definition of the meter and the inch?
The inch has always been tied to the meter in the U.S. After the Civil War, the Surveyor General of the U.S. set the inch as 39.37 inches to the meter. The British had a yard bar, the Canadians had 25.4 mm per inch,
and the other English speaking nations had chosen one of these or some variation. They were all close, but not exactly the same. The yard bar was in fact shrinking. In 1959, a conference attended by the directors of
the National Measurement Institutes from all of the English speaking countries, those who still used inches, met to standardize the inch. The 25.4 mm per inch (exactly) was chosen, and since then this is the inch used
by NIST (formerly NBS). Except . . . Yes, there is always an exception. Surveyors still use the old inch because all of the mountains of measurements in place defining the positions of everything in the US are in these
inches, and the job of changing them was considered too large a task. So surveyors still use the surveyor inch, 39.37 inches per meter.
(") End of quote.
and :
http://physics.nist.gov/cuu/Units/meter.html
(") Quote:
...in 1983 the CGPM replaced this latter definition by the following definition:
The meter is the length of the path travelled by light in vacuum during a time interval of 1/299 792 458 of a second.
Note that the effect of this definition is to fix the speed of light in vacuum at exactly 299 792 458 m·s-1. The original international prototype of the meter, which was sanctioned by the 1st CGPM in 1889, is still kept
at the BIPM under the conditions specified in 1889.
(")
End of quote.
As well:
Eric Weisstein's World of Physics
http://scienceworld.wolfram.com/physics/Inch.html
An nonmetric unit of length, originally defined as the lengths of
three "average size" barleycorns laid end-to-end, but now more
rationally defined as 2.54 cm. An older definition no longer used
was 1 meter = 39.37 inches, giving 2.54000508 cm/inch.
Surprisingly, these two conversions are both exact. For details on
the current definition of the inch, see
A Tale of Two Feet OR The Case of The Double Standard - T. J. Keefe
http://physics.ccri.cc.ri.us/keefe/twofeet.htm
Finally, the conversion is finally in a concrete form, as long the speed of light
in vacuum does not turn out to be not constant ;-)
keep watching the time and units,
regards
Arnold, DK2WT
On Tue, 3 Apr 2007 11:37:16 +0200, Enrico Rubiola wrote:
>Dear all,
>I have read some weird discussion about measurement units.
>There is a wonderful book I come across, by Francois Caldarelli
>You may take a look
>http://rubiola.org/shared/caldarelli.pdf
>then it's up to you....
>Best
>Enrico Rubiola
>professor of electronics
>web: http://rubiola.org
>e-mail: rubiola at femto-st.fr
>FEMTO-ST Institute
>32 av. de l'Observatoire
>25044 Besancon, FRANCE
>voice: +33(0)381.853940 (E.Rubiola)
>voice: +33(0)381.853999 (switchboard)
>fax: +33(0)381.853998
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