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Warp schrieb:
> Why can't the kilogram be defined as the weight of exactly 1 litre of
> pure water at a certain temperature? After all, that has been the de-facto
> definition for forever.
That was only the /initinal/ definition.
What reason would be there to stick to it - or choose a different
definition, for that matter?
There's only one: Reproducability.
If you can find a definition for a kilogram that /matches/ the initial
definition, but can be reprocuded with /higher precision/, then that
definition is both superior and "backward compatible".
It turned out that measuring the mass of 1 litre of water at a certain
temperature was subject to more error (possibly due to issues with
producing /really/ pure water, and reproducing /exactly/ the desired
temperature) than "copying" the mass of some sample entity made of a
robust, non-corrosive material.
Note that while it was found that the "primary copies" of the kilogram
exhibit a "drift" relative to the Grand K, but nobody is presently able
to tell whether the Grand K also exhibits an /absolute/ drift. If pure
water would provide a reference of adequate precision (or even anywhere
close), I guess scientists would already have checked for absolute
drifts using this method.
Also note that there is no such thing as a "de-facto definition"; there
are "practical realizations" that differ from the official definition,
but I guess none is based on the initial definition these days, except
maybe as makeshift references for low-precision applications.
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