From: Ron Kean (ronkean@juno.com)
Date: Mon Jul 19 1999 - 17:02:02 MDT
On Mon, 19 Jul 1999 16:58:18 -0400 "John Clark" <jonkc@worldnet.att.net>
writes:
> Ron Kean <ronkean@juno.com> Wrote:
>
> >According to the CRC Handbook of Chemistry and Physics, the
>second is the> >duration of 9,192,631,770 periods of the radiation
corresponding >to the> >transition between the two hyperfine levels of
the ground state >of the> >atom of Cesium 133.
>
>Ok, but how long is a meter? If you define it as 1/299782458
that should be 1/299792458
of the >distance>light moves in one second that means that as your
experimental
>procedures>improve and you know the exact speed of light better the
length of a
>meter changes.
>
Yes, that sounds correct. However I did not make the rules. The
definition does not originate with me. The definition comes from the CRC
Handbook, and ultimately by international scientific agreement, probably
under the BIPM. It seems equally true that as measurement technology
improves, the length of the meter, as measured, would also change under
any of the previous definitions of the meter.
>You can't define everything as you wish and remain consistent.
Please don't blame me; I did not make the definition. My guess is that
by the mid 1980s the speed of light had been measured to an accuracy as
fine as the precision of the Krypton 86 definition of the meter, so it
was decided to make the second and the speed of light the primary
standards for defining the meter, rather than using the second and the
meter as primary standards, as under the older system. I don't see any
inconsistency under either system. Under the old system the speed of
light 'changes' as measurement become more refined. Under the new system
the meter 'changes' as measurement become more refined. I suspect the
sentiment was that it is better to have something fundamental such as the
speed of light remain constant by definition, rather than have something
artificial (the meter) be arbitrarily held constant which results in a
fundamental constant (the speed of light) changing. And since under the
old system the meter was defined in a way that required a physical
measurement to reproduce, the meter would change anyway as the
measurement technology changed. So the new system would seem to be an
improvement.
In the >last century>the Indiana state legislature tried to pass a law
decreeing that the >exact value of PI>was 4. It missed becoming law by
one vote, but probably wouldn't have >worked anyway.
>
> John K Clark jonkc@att.net
The version of the story I heard was that Kentucky or Tennessee had
considered defining pi as 3. Legal definitions of units are usually made
to provide a basis for settling commercial disputes, and it does not
really matter to physics that those legal definitions are somewhat
inaccurate, nor does it matter much to commerce, since for commerce it is
more important to have an agreed and easily implemented definition than
it is for the definition to be highly accurate.
Back in the 1790s the US by law defined the gallon as 231 cubic inches.
Since then, the length of the inch has changed several times. So every
time that happened the gallon changed. But it caused no commercial
problem because the changes were minuscule. The Imperial (British,
Canadian) gallon was defined in a very different way. That gallon was
the volume taken by 10 pounds of water at 62 degrees F.
Likewise, defining pi as 3 may have been proposed for some commercial
uses, not for scientific use. 22/7 is a good approximation of pi, but at
the time lawmakers may have thought that fractions are too complicated
for many people (including, perhaps, them) and that a 'simple' definition
is better. If a commodity such as rope is being sold in a coil with a
known number of turns, its length can be estimated by counting the turns,
measuring the diameter of the coil, and multiplying by pi.
Ron Kean
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.
.
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