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From: Ross Smith <ross.smith@nz.eds.com>
Subject: Re: New IEC proposal: 1 kibibyte = 1024 bytes
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Rainer Thonnes wrote:
> 
> Going by the name of the old cgs and mks systems, one would have thought
> that the second should be the more fundamental, but SI is a bit coy in this
> respect.  There the second is listed as an elemental unit, and the frequency
> unit is expressed in terms of the second, but nevertheless the definition of
> the second is essentially in terms of the frequency of a particular type of
> radiation.
> 
> A similar oddity exists with respect to electric charge and current.  There
> exists a fundamental constant of nature, e, the charge of an electron, and
> yet they prefer to give the ampere rather than the coulomb the honour of
> being an elemental unit, and define the C as being an As, and to make
> matters worse, they've defined the A in terms of the force, of all things.

That's because SI is meant to be used in the real world :-) The
important question is not "Which units are 'really' fundamental?", but
"Which units can we measure with the greatest accuracy?" The
electromagnetic units are defined in terms of the force between two
currents, rather than the charge on the electron, because accurately
measuring the force between wires is easier than accurately counting
electrons on the macroscopic scale.

Much the same applies to the metre and the second. The original
definitions, when the metric system was first proposed in the 18th
century, defined the metre in terms of the Earth's circumference, and
the second in terms of the period of a one-metre pendulum. They've gone
through various other definitions as the accuracy and practicality of
various methods of measurement changed.

For a while the metre and second were defined in terms of the wavelength
and frequency of two different spectral lines, since they could be
measured very accurately. But now it turns out to be possible to measure
the speed of light more accurately than a wavelength, so the metre was
redefined in terms of the second, by fixing a value for the speed of
light. (If you want an accurate metre, you measure the speed of light in
terms of some local length standard in your laboratory; comparing the
result to the standard value tells you the exact ratio between your
local standard and the metre.)

Presumably, if the relevant technology changes so that some other form
of measurement becomes more accurate than light frequency and speed, the
units will be redefined to be based on the new measurements.

-- 
Ross Smith <mailto:ross.smith@nz.eds.com> ...... Wellington, New Zealand
     "I'm as interested as anybody else in all the things no decent
     person would be interested in."          -- Ashleigh Brilliant
