Metric essay

[Tom Parker]

Greg Hermann <bearbvd at sni.net> wrote:

>But I think one of these definitions may have changed recently too---how
>droll!! (Another droll one from the trolls at ISO!??!!) :-)

>I am not the least bit xenophobic or changeophobic--I just think that a
>system of weights and measures should be a constant, not a continuing work!


The definitions change to take advantage of better metrology.

A metre used to be defined by a bar of specially designed metal held in a
french fault. Whenever it was moved, there was one person to carry it and
another to catch it should the first drop it.

When it became possible to use light radidation as the standard, they set up
their interferometer and measured the standard metre and said that the metre
will now be X number of waves of light of a certain wavelength. The wavelength
was chosen because it is particularly easy to produce exactly.

Very few people will ever use the interferomtery standard, but when they do,
there is a standard so results can be compared.

The metre hasn't changed, except perhaps they rounded to a whole number of
wavelengths after measuring the old standard. The advantage of this is you can
write down exactly what a metre is, and someone can, from your description,
recreate an extrememly accurate standard.

There is no equivilent for mass. The kilogram is another lump of metal in a
french vault. You can't write x million atoms of an element makes a kilogram
becuase noone can yet count the atoms.

You can't write down how to make a kilogram standard, you can only say go to
the french and compare your standard with theirs and adjust it to suit.

(there are better ways to define the kilogram... but the primary standard is
still the french lump of metal).

--
Tom Parker - parkert at ihug.co.nz
           - http://www.geocities.com/MotorCity/Track/8381/