Markus Kuhn wrote:
>
> Jim McCracken wrote on 2002-07-31 13:45 UTC:
> > The inch was not set at 25.4 millimeters centuries ago, but in the
> > 1950s.
>
> Was setting the inch to 25.6 mm or 25.0 mm seriously discussed as an
> option at this or at any earlier opportunity? Is the rationale for the
> choice of 25.4 mm for the inch documented somewhere in more details, as
> well as the history of previous definitions for the inch?
>
> When was the inch defined in the US for the first time with an accuracy
> of better then 0.1 mm? the practice of length measurement with a
> relative error below 1% surely must predate the 1950s significantly ...
> even centuries ...
My understanding is that the definition of the yard (from which the
definitions of the foot and the inch are derived) was chosen to fall
between the U.S. yard and the U.K. yard in size. In otherwords, it is a
compromise value.
In the U.S., the yard was defined in 1893 as being exactly (3937/3600)
m and in 1959 it was redefined to be exactly 0.9144 m. Surveyors
objected, so they were allowed to retain the old mile and foot
definitions---e.g., (3937/1200) m for the foot---and that is the basis
for the U.S. survey foot and survey (or statute) mile. Those differ from
the new definitions by about 2 mm/km.
Citations are available at my web page
http://www.cofc.edu/~frysingj/background.htm
Jim
--
James R. Frysinger
Lifetime Certified Advanced Metrication Specialist
Senior Member, IEEE
http://www.cofc.edu/~frysingj
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