Metric essay

[Robertson, Nigel]

At last someone with a true grip on the situation.

Incidentally, I believe that people at the National Physics Lab in the UK
have been developing a replacement definition for the kilogram and is based
on forces produced by an electromagnet.  This force can be related to
voltage and current which are better defined than the lump of metal platinum
under the glass dome at NPL which is slowly evaporating away

Nigel

> -----Original Message-----
> From:	Tom Parker [SMTP:parkert at geocities.com]
> Sent:	07 June 1999 02:13
> To:	Greg Hermann
> Subject:	Re: Metric essay
> 
> 
> 
> 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/