Re: Precision and the kilogram

[brian whatcott <inet@INTELLISYS.NET>]

At 19:50 9/2/99 -0500, Cliff Parker wrote:
> I have been describing the historical development of the metric system
> to my students this week.  One thing has always bothered me.  Perhaps
> someone can provided some insight.
>
> Why was the definition of a kilogram changed from the mass of one
> deciliter of water 4 C to the mass of the prototype in France?  It seems
> to me that the mass of water would have the advantage of being portable
> and offer a definition of similar precision to that given for the
> meter.  What am I not considering?

I'll take a shot at this. Water is densest at about 4degC, so if a
particular volume is going to be the basis then this would be the
appropriate temperature.

 It's easy to think of a liter as 1000 cubic centimeters,
each of which, if composed of water, weighs a gram or thereabouts.
 So the order of magnitude you have in mind is a deci- , a tenth
of 1000 grams, 100 grams in fact. So something is not quite right here
with your narrative description.

I suppose a turning point was the realisation that a liter of water does
 not weigh quite exactly the same as a 1 kg mass. The defined quantity
for mass, originally intended to be the mass of a cubic decimeter of
water at maximal density was found to be 28 parts per million too big.

 The choice was made in favor of the kilogram in the form of the standard
cylinder of platinum-iridium at Sevres.
In 1964 the liter was redefined as the cubic decimeter of water
but it was disrecommended for precise work.

 Hence the metric standards are finally arbitrary as to length, mass
and time but well specified in terms of stable natural periodicities
where possible.




brian whatcott <inet@intellisys.net>
Altus OK