
#1
May308, 08:57 AM

P: 151

Just stumbled on this in Wiki and wondered.
The kilogram or kilogramme (symbol: kg) is the base unit of mass in the International System of Units (known also by its Frenchlanguage initials “SI”). The kilogram is defined as being equal to the mass of the International Prototype Kilogram (IPK; known also by its Frenchlanguage name Le Grand K), which is almost exactly equal to the mass of one liter of water.[1] It is the only SI base unit with an SI prefix as part of its name. It is also the only SI unit that is still defined in relation to an artifact rather than to a fundamental physical property that can be reproduced in different laboratories. Certainly a lot of diets are. :) I found this bit further down the page interesting. Ultimately, the watt balance would define the kilogram in terms of the Planck constant, which is a measure that relates the energy of photons to their frequency. The Planck constant would be fixed, where h = 6.62606896 × 10–34 J·s (from the 2006 CODATA value of 6.62606896(33) × 10–34 J·s) and the kilogram would be defined as “the mass of a body at rest whose equivalent energy equals the energy of photons whose frequencies sum to 1.356392733 × 1050 Hz.”[29] 



#2
May308, 11:12 AM

Mentor
P: 11,984

Standards are based on the most accurate measurement technology available. That is why, for example, the definition of the meter was changed from a wavelength standard to a definition based on the speed of light and the second. Technology is such that time can be measured much more accurately than distance, hence the redefinition of the meter in terms of time.
The fact that the kg is based on a prototype reflects the difficulty in using some fundamental standard, such as 1 mole of Carbon12 atoms. How do we count 1 mole of C12 atoms? By the way, efforts are underway to redefine the kg in terms of a silicon atoms. http://www.iop.org/EJ/abstract/00261394/40/6/008 



#3
May308, 11:51 AM

Sci Advisor
PF Gold
P: 2,196

The definition of the the kilogram won't change any time soon. The definitions used in the SI can only be changed at a conference which is held in Paris every 510 years or so (I don't think there is a fixed interval); the next meeting is pretty soon (2009?) and since nothing will happen then it will take at least another ten years or so before anything changes (there are, however, a few definitions that WILL be changed at the next meeting).
An Si artifact might be ultimately be better than the current artifact but most metrologists are not happy about just changing from one artifact to another. Ultimately we would like to have a "natural" standard but so far no one has come up with a method that works well enough. There are two Watt balances in the world (one in the US and one here in the UK) but the latest (preliminary) values from them unfortunately do not agree (and the disagreement is actually larger than the error bars). Moreover, the people working on them are finding it increasingly hard to get funding; meaning it is very unlikely that the work will result in a new standard. There is also a lot of work on new methods for cleaning the artifact in Paris; most of the drift in the kg is likely to be from dirt accumulating on its surface. 



#4
May308, 12:09 PM

P: 567

the Kilogram
What was undesirable about the original definition of the gram and kilogram as the mass of a certain volume of water at STP? Doesn't the precision of the meter now make volumes also precise?




#5
May308, 01:41 PM

P: 2,265

BTW about the meter, a useful reference is Time Line for the Definition of the Meter by switching from a "wavelength" standard to a "speed of light" standard, what they really did was switch from a _{86}Kr standard for length and a _{133}Cs standard for time, to using the same _{133}Cs for both length and time. since the speed of propagation of radiation for _{133}Cs and _{86}Kr is the same, the fact that there was measurement variance in the measure of c before 1983 really simply reflects a measurement error in the relative wavelengths or frequencies of _{133}Cs and _{86}Kr (which are 9.19263177x10^{9} Hz and 4.94886516194x10^{14} Hz respectively). after 1960, when they first moved the meter definition away from the BIPM meter prototype, any variance in the measure of the speed of light really meant a variation in the relative measure of those two frequencies. 



#6
May308, 02:35 PM

Sci Advisor
PF Gold
P: 2,196

Moreover, it is expensive and as I wrote above it seems to be hard to get funding. The Watt balance in the UK (NPL) was actually due to close down a few months ago, the project has been extended by a few months but once they have a new (and hopefully better) value they will have to close down permanently; there is no money for a new generation (and NPL is leaving the building where the current balance is located, there are only a couple of experiments left there). 



#7
May308, 03:46 PM

PF Gold
P: 370





#8
May308, 04:44 PM

Sci Advisor
PF Gold
P: 2,196

Nothing that will have any practical implications unless you happen to work at an NMI (although most people like to idea of a neater SI).
As far as I understand the Kelvin (by fixing Boltzmanns constant) and the Mole (by fixing Avogadros constant) are on track to be redefined. Steps have also been taken to redefine the ampere in terms of the elementary charge; but so far no experiment (electron pumps,counters etc) is sensitive enough, so that will have to wait, at least until the next conference (but it will happen eventually) 



#9
May308, 05:05 PM

Mentor
P: 11,984

I finally found references to working on the new siliconbased kg standard.
"Redefining the kilogram" article in Popular Science: http://www.popsci.com/scitech/articl...iningkilogram The next challenge to making this the standard is counting the number of silicon atoms that make up 1 kg. Google "Avogadro project" for more info: http://www.google.com/search?hl=en&q...22&btnG=Search 


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