On 10/12/09 10:17, Warp wrote:
> Mike Raiford<"m[raiford]!at"@gmail.com>  wrote:
>> http://blogs.ngm.com/blog_central/2009/10/a-grander-k.html
>
>    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.

	Perhaps they could _change_ the definition to that, but I'm willing to 
bet that it's off by a bit from the current definition.

-- 
A tautology is a thing which is tautological.

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Neeum Zawan <m.n### [at] ieeeorg> wrote:
>         Perhaps they could _change_ the definition to that, but I'm willing to 
> bet that it's off by a bit from the current definition.

  AFAIK kilogram *was* originally defined as the weight of 1 liter of water,
but for whatever reason they changed it to the weight of a specific object
(I really can't understand why).

  According to wikipedia, one liter of pure water at 4 degrees celsius (the
standard temperature for measuring SI units) is 0.9999720 kilograms.

  If they changed the definition of kilogram back to its original form, the
change would be less than 0.003% from the currently accepted value. Would
that be a huge catastrophe?

-- 
                                                          - Warp

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Warp wrote:
>   According to wikipedia, one liter of pure water at 4 degrees celsius (the
> standard temperature for measuring SI units) is 0.9999720 kilograms.

It may be that it's too difficult to measure "pure water" at "exactly 4 
degrees celsius" for modern measurement precision.

Plus, of course, if you make *everything* circular, then you have nothing. 
If a kilogram is the weight (well, mass) of a liter of water, and a liter is 
1000 cubic centimeters, and a centimeter is the length of one gram of carbon 
atoms lined up (or some such) then the whole thing falls down.

Given that a cm is defined in terms of the speed of light, and time is 
defined in terms of a cesium atom at rest at 0K, I guess we already have 
unusable base metrics, tho.

>   If they changed the definition of kilogram back to its original form, the
> change would be less than 0.003% from the currently accepted value. Would
> that be a huge catastrophe?

Only for scientists working on 15 digits of precision in their scientific 
experiments. Kind of like "would it really make a difference if we skipped a 
leap-second every 3 to 5 years?"

-- 
   Darren New, San Diego CA, USA (PST)
   I ordered stamps from Zazzle that read "Place Stamp Here".

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>> ...and here I was thinking that 12g of C12 was *defined as* one mole of 
>> C12 atoms...
> 
>   Hence the definition of 'mole' depends on the definition of 'kilogram',
> not the other way around.

Ah, I see.

Couldn't they just reverse it? I mean, just say that 1 Kg = the mass of 
XXX C12 atoms?

-- 
http://blog.orphi.me.uk/
http://www.zazzle.com/MathematicalOrchid*

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Darren New <dne### [at] sanrrcom> wrote:
> Warp wrote:
> >   According to wikipedia, one liter of pure water at 4 degrees celsius (the
> > standard temperature for measuring SI units) is 0.9999720 kilograms.

> 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).

> Plus, of course, if you make *everything* circular, then you have nothing. 
> If a kilogram is the weight (well, mass) of a liter of water, and a liter is 
> 1000 cubic centimeters, and a centimeter is the length of one gram of carbon 
> atoms lined up (or some such) then the whole thing falls down.

  But a centimeter is not defined like that. It's defined from the speed of
light in vacuum. There's no circle.

> Given that a cm is defined in terms of the speed of light, and time is 
> defined in terms of a cesium atom at rest at 0K, I guess we already have 
> unusable base metrics, tho.

  Unusable base metrics?

> >   If they changed the definition of kilogram back to its original form, the
> > change would be less than 0.003% from the currently accepted value. Would
> > that be a huge catastrophe?

> Only for scientists working on 15 digits of precision in their scientific 
> experiments. Kind of like "would it really make a difference if we skipped a 
> leap-second every 3 to 5 years?"

  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.

-- 
                                                          - Warp

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Warp wrote:
>   But a centimeter is not defined like that. It's defined from the speed of
> light in vacuum. There's no circle.

Yes, I saw that.

>   Unusable base metrics?

0K is unreachable even in theory.

>   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.

True. I don't know how they work it now.

-- 
   Darren New, San Diego CA, USA (PST)
   I ordered stamps from Zazzle that read "Place Stamp Here".

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On 12-10-2009 18:27, Warp wrote:
> Neeum Zawan <m.n### [at] ieeeorg> wrote:
>>         Perhaps they could _change_ the definition to that, but I'm willing to 
>> bet that it's off by a bit from the current definition.
> 
>   AFAIK kilogram *was* originally defined as the weight of 1 liter of water,
> but for whatever reason they changed it to the weight of a specific object
> (I really can't understand why).

Standards are defined on whatever can be most accurately measured. So if 
you can measure distance and light speed more accurate than time you 
define time as a the interval it takes light to cross a certain length. 
If you increase the accuracy of your clock, you might want to define a 
length as velocity times time, etc. At some point the accuracy of 
quasars was approaching our best clocks, if they would have passed that 
we would now have time defined in heaven.
The standard kilogram can be more accurately measured than a liter of 
water. When we are able to count a specific number of atoms of a known 
isotope and weight those with sufficient accuracy that will probably 
replace the standard kilogram. ATM we are not that far, but only just, 
as this article explains.

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Invisible schrieb:

> ...and here I was thinking that 12g of C12 was *defined as* one mole of 
> C12 atoms...

No, the /mole/ is defined on this basis.

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Orchid XP v8 schrieb:
>>> ...and here I was thinking that 12g of C12 was *defined as* one mole 
>>> of C12 atoms...
>>
>>   Hence the definition of 'mole' depends on the definition of 'kilogram',
>> not the other way around.
> 
> Ah, I see.
> 
> Couldn't they just reverse it? I mean, just say that 1 Kg = the mass of 
> XXX C12 atoms?

That's one attempt to it (except that they appear to intend to use 
silicon instead of carbon).

However, at present (or at least until very recently) measuring the mass 
of the Grand K can still be done with greater precision than counting 
atoms, so they don't know /exactly/ how many atoms make up a mole.

Definitions of SI units change over time depending on how they can be 
measured with highest precision.

For instance, the SI unit of distance (1 metre) was, like the kilogram, 
initially (*) defined based on a physical entity - the "International 
Prototype Metre". (* Actually, even before, for a few years the metre 
was based on other properties, including being a 10,000-th of the 
half-meridian through Paris, but those didn't last long.)

Later, scientists managed to measure wavelengths at precisions that 
exceeded that of the Prototype Metre, and therefore it was decided in 
1960 to redefine the metre as the vacuum wavelength of a particular 
emission of krypton-86.

Some two decades later, both time and the vacuum speed of light could be 
measured even better than wavelengths, and the metre was redefined in 
1983 as the distance travelled by light in vacuum within a certain 
period of time.

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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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