On 2009/04/06, at 12:24 AM, John M. Steele wrote:
I'd like to comment on this idea of the changing inch as it
illustrates the problem of being too liberal in rounding a
conversion factor. Measured data should be rounded consistent with
its measurement accuracy. However, conversions are either declared
values (legal definitions) or the highest accuracy a lab is capable
of. Destroying that accuracy is frought with with risk.
The UK had their own problem with the "incredible shrinking yard."
However, according to NIST SP447 (downloadable from their site) p.
21, the US yard prototype was measured in 1893 just before the
Mendenhall order. It was determined to be 0.914 399 80 m, only 0.22
parts per million different from the International foot adopted in
1959.
Yet, the Mendenhall order established a rounded value 1 m = 39.37
inches (equivalent to 0.914 401 83 m) which led to a greater
difference with the UK yard, and became so entrenched that we are
still cursed with it today in the form of the US Survey foot. They
knew it was wrong, but the chose to adopt a definition that had good
enough accuracy for commercial purposes from the Metric Act of 1866.
The Mendenhall order freed us from separate physical standards for
Customary measure, which is a good thing. However, the wrong choice
of value, and a worse choice than they were capable of making at the
time has not worked out entirely well.
In any case the difference between Survey and International foot is
only 2 parts per million, affecting volumetric measure 6 ppm, so it
hasn't changed much. I disagree that we should round conversion
factors willy-nilly to a "nice" round number when we know better.
Dear John,
Thanks for these thoughts. My comment is contained in a new title for
this subject: 'The inch is no cinch!'
Cheers,
Pat Naughtin
PO Box 305 Belmont 3216,
Geelong, Australia
Phone: 61 3 5241 2008
Metric system consultant, writer, and speaker, Pat Naughtin, has
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