On 10/13/09 10:29, Warp wrote:
> clipka<ano### [at] anonymousorg>  wrote:
>> 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.
>
>    I don't see how that is different from the current method, ie. measuring
> the weight of that one object at 4 degrees celsius.

	Who said you have to do it at 4 Celsius? Its density varies with 
temperature, not its mass.

>    Except that with water you don't have to rely on one specific object which
> is unique and there exists only one in the world.

	Yes, but clipka's points still stand. It's likely harder to get the 
right purity, volume, temperature, and pressure. That one object may 
have variations, but perhaps those variations are smaller than the 
accuracy of purity, volume, temperature and pressure all put together.

>> 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.
>
>    Not much different from defining length in relation to the speed of
> light. If you want to measure it, you need precise timing and precise
> length measurements.

	We have had precise length measurements for a long, long time. Precise 
timing may be another story.

>> 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.
>
>    At least with the water the definition would be fixed to one single
> weight. Then it's only a question of how accurately it can me measured,
> which is no different from all the other units.

	True.

-- 
An unbreakable toy is useful for breaking other toys.

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