Warp schrieb:

>> It may be that it's too difficult to measure "pure water" at "exactly 4 
>> degrees celsius" for modern measurement precision.
> 
>   Not more difficult than measuring the exact speed of light (which is needed
> to define the unit of length) or an exact magnetic force (which is needed to
> define the unit of electric current).

It's not a matter of /exact/ measurement, but of /precise/ measurement 
instead.

Vacuum speed of light is (comparatively) simple to measure at high 
precision; it seems to me there are not many sources of errors in 
experiments except the vacuum (scientists are pretty good at that 
nowadays), the stability of the distances in your experiment (which will 
be what you'll measure), and the quality of your time reference (note 
that the second is the SI base unit that can presently be measured with 
the highest precision of all).

With a liter of pure water at exactly 4 degrees celsius, one problem 
you'll have is to exactly hit the 4 degrees celsius. Another problem is 
to /get/ really pure water, and /keep/ it pure. Yet another problem is 
that you'll have to define the exact isotopic composition of the water.

Pressure is another thing, as I guess even pure water is only 
theoretically incompressible.

Then there's the shape of the container. You need to make sure that it 
/precisely/ holds 1 litre when it is at 4 degrees celsius /and/ filled. 
Note that you must also account for any openings used to fill it with 
water: If you want them to be completely filled with water when 
measuring, you need to know their precise dimensions up until the valve, 
while making sure you have no residual water outside the valve. If you 
choose to not have such openings to be completely filled, you must take 
into account capillary effcts to compute the actual volume of the water, 
which then depends on the material and even surface structure of the 
container...

Sounds like quite some fun.

>   Given that the object which is currently the measurement of 1 kg changes
> weight every time it's measured, does it really matter? It would simply be
> one more change, but then it would be fixed, and that's it.

Nobody /knows/ whether if it actually changes weight. All they know is 
that the "primary copies" do change weight with respect to one another 
and the Prototype.

And then look at the order of magnitude of those changes.

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