On 2009/02/03, at 4:22 AM, <[email protected]>
<[email protected]> wrote:
A mix of units can be stated more simply as "mixed units" or more
precisely as "A mix of SI and not-SI units" rather than
"use of multiple systems" as you propose, which implies that more
than one "system" exists.
Gene.
Dear Gene,
Just a slight quibble with your post that I have been having trouble
with for a while.
When is something a unit and when is it not? I am referring to your
use of 'mixed units' and 'not-SI units'.
Suppose that I read 14 pounds somewhere. Is pounds a unit? It could
mean money in the UK, mass in Australia, weight in the USA, and the
pound could have various randomly generated or variable values. For
example, the pound of money in the UK varies with the exchange rate,
the pound of force is defined in newtons according to its current
location on the Earth and the local acceleration due to gravity. Of
course I might be able to guess from the context but this is
unnecessary using SI units as the names vary to give the clues:
kilograms for mass; newtons for force (weight); and the non-SI dollars
for money.
I tend to duck this issue by saying things like, 'Force can be
measured with the SI unit, newton, or with the old pre-metric measure,
pound', thus ducking the problem of knowing whether the pound is a
unit or not by calling it a measure. The use of the word measure also
provides for the fact that the definition of the pound has varied
throughout history according to the whimsy of politicians; if you call
it a measure rather than a unit I feel that I am, somehow or other,
providing for this linguistic variation.
It seems to me that one characteristic needed to be a unit is to have
a universally agreed (and maintained) definition. We all agree on
kilogram but only the 'English-speaking' nations have agreed on the
definition of a pound of mass (in 1959) in metric terms. It is highly
unlikely that any other nations will formally sign on to the 1959
agreement and it is probable that a pound of weight and a pound of
money will never be defined anywhere in the world.
Could you or anyone else tell me how you define what a 'unit' is please?
Cheers,
Pat Naughtin
PO Box 305 Belmont 3216,
Geelong, Australia
Phone: 61 3 5241 2008
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