Guido at Google

[Bengt Richter]

On Wed, 21 Dec 2005 21:47:29 -0500, Peter Hansen <peter at engcorp.com> wrote:

>Bengt Richter wrote:
>[roughly "an inch is not exactly 25.4mm"]
>> At least according to my dusty 37th Edition Handbook of Chemistry and Physics (c) 1955.
>> Maybe things have changed since then ;-)
>
>Wikipedia concurs with Jim, though it says the official change dates 
>from 1958.
>
>Better throw that old book out, as it's also likely to be missing any 
>reference to useful elements such as Lawrencium (1961), and Hassium 
>(1984), not to mention Ununnilium, Ununumium and Ununbium (94, 94, 96 
>respectively) or the most recently discovered element, which the PSU 
>tried to supp
>
I had been using 25.4mm/inch myself, but decided to look it up, and
found that I had been using the "wrong" value -- now actually proving
to be right after all, after the definition change of 1958(1959?).

Google found an NIST page:

    http://physics.nist.gov/Pubs/SP811/appenB.html

Where it says:
"""
    B.6 U.S. survey foot and mile

    The U. S. Metric Law of 1866 gave the relationship 1 m = 39.37 in (in is
    the unit symbol for the inch). From 1893 until 1959, the yard was defined
    as being exactly equal to (3600/3937) m, and thus the foot was defined as
    being exactly equal to (1200/3937) m.
    
    In 1959 the definition of the yard was changed to bring the U.S. yard and
    the yard used in other countries into agreement. Since then the yard has
    been defined as exactly equal to 0.9144 m, and thus the foot has been
    defined as exactly equal to 0.3048 m. At the same time it was decided that
    any data expressed in feet derived from geodetic surveys within the United
    States would continue to bear the relationship as defined in 1893, namely,
    1 ft = (1200/ 3937) m (ft is the unit symbol for the foot). The name of
    this foot is "U.S. survey foot," while the name of the new foot defined in
    1959 is "international foot." The two are related to each other through
    the expression 1 international foot = 0.999 998 U.S. survey foot exactly.

    In Sec. B.8 and Sec. B.9, the factors given are based on the international
    foot unless otherwise indicated. Users of this /Guide/ may also find
    the following summary of exact relationships helpful, where for
    convenience the symbols /ft/ and /mi,/ that is, ft and mi in
    italic type, indicate that it is the /U.S. survey foot/ or /U.S.
    survey mile/ that is meant rather than the international foot (ft) or
    international mile (mi), and where rd is the unit symbol for the rod and
    fur is the unit symbol for the furlong.

    1 /ft/ = (1200/3937) m 
    1 ft = 0.3048 m 
    1 ft = 0.999 998 /ft/ 
    1 rd, pole, or perch = 16 1/2 /ft/ 

    40 rd = 1 fur = 660 /ft/ 
    8 fur = 1 U.S. survey mile (also called "statute mile") = 1 /mi/ = 5280 /ft/
    1 fathom = 6 /ft/ 
    1 international mile = 1 mi = 5280 ft 
    272 1/4 /ft/**2 = 1 rd**2  

    160 rd**2 = 1 acre = 43 560ft**2 
    640 acre = 1 /mi/**2 
"""
(I changed italics to be indicated by /italic/ slashes, and superscript by **,
as well as changing special characters for a quarter and half to 1/4 and 1/2.
Hope I didn't typo ;-)

Anyway, 25.4 mm/inch it is. Nice to revert to that, after an unsettling diversion ;-)
NIST ought to have it right, right? Or is there an intelligent design version now? ;-/

Regards,
Bengt Richter