Ian Lynch wrote:
this is agreed to be unsatisfactory,
Yes it would be difficult to see how the measurement might be checked
without losing the odd atom or two :-)
Wikpedia says that the lump of metal gains mass because it absorbs
atmospheric contamination. So they have this complicated technique to
clean it, and the technique removes about 5-60 micrograms of
contaminants. I imagine that you have to worry about not removing too
much stuff every time you clean it.
They actually have several replicas around the world, for mutual
calibration. But it turns out that these masses have been diverging from
each other. The base prototype (the one that truly defines the Kg) has
lost about 50 micrograms in its life compared to the replicas. Nobody
knows why. So they also don't know whether the main prototype has lost
mass or whether the replicas have gained mass (maybe they replicas don't
get cleaned as well?). And of course, it's possible that they are all
diverging one way or the other, and we wouldn't know.
There is an additional 30 microgram monthly variation in the proptotypes
which is not explained either.
Daniel.
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