Re: Precision and the kilogram

[Leigh Palmer <palmer@SFU.CA>]

> 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 don't know that it was changed. It's been referred to a mass standard
all my conscious life.

A deciliter is not an SI unit; one would have to refer it to another
volume unit, the cubic meter. This would make it a secondary standard
right off the top.

It seems to me that a deciliter of water at 4 degrees C is a sloppy and
inconvenient thing to work with. A solid metal is a better choice if one
wishes to use it as a standard of weight (which is what it was intended
to be) and compare it gravimetrically with secondary standards. It is
also difficult to work with in a standard state, which I suppose would
be saturated with air at one atmosphere. The isotopic composition of
water and hence its density varies from source to source, and isotopic
composition is not easy to monitor.

It would seem that disadvantages of water as a standard are many and
decisive. The standard kilogram works well.

Leigh