Re: Precision and the kilogram

[Gary Karshner <karshner@STMARYTX.EDU>]

Cliff,
        The deciliter of water has a similar problem to the meter as ten millionth
the distance from the pole to the equator on the meridian passing through
Paris. It is difficult to duplicate.
        Water is a very good polar solvent. One of the great quests of the
nineteenth century was to make pure water. Chemists never succeeded because
they always had to put the water in a container and the water would
dissolve some of that container. A platinum iridium cylinder is much less
pron to change, although I read where there is worry that the cylinder is
getting lighter with time.
        About thirty years a go there was a big controversy over something called
polywater. A small amount was found in the bottom of containers that
contained pure water. It turned out to be a rediscovery of what the
nineteenth century chemists had already discovered.

I hope this helps.

Gary

> 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?
Gary Karshner

St. Mary's University
San Antonio, Texas
KARSHNER@STMARYTX.EDU