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The Changing Definition Of 'Kilogram'

Posted by timothy on from the jenny-craig-kilograms dept.

DrLudicrous writes "The NYTimes is reporting that the platinum-iridium standard mass for the kilogram is shedding at an appreciable rate -- at least compared to other reference masses. The Pt-Ir cylinder is kept in France, and measured annually, and the slight discrepancy is important because the kg is an SI base unit- thus other quantities such as the Volt are based on it. A new standard is being sought- the two frontrunners are counting the number of atoms in a perfectly spherical single crystal of silicon, and another technique uses a device known as the Watt balance."

6 of 964 comments (clear)

  1. Re:Kilogram? by el-spectre · · Score: 5, Informative

    It's not a matter of dark ages, it's a matter of infrastructure... while not the largest country in the world (the US is probably third or fourth, I'm not sure), we have by far the most technological infrastructure. It is not feasible to change all that in a short period of time.

    A friend is in construction, and guestimates that it will take over 100 years to replace all failing/obsolete tech with the versions in metric equivalents. It just does not make any economic sense to replace a set of, say, water pipes with the metric standard if the current ones will last 20 years. It'll have to be a gradual thing.

    Just to be difficult, though, I'd mention that most construction is done in 'tenths of feet', even the surveying equipment is marked this way. Has nothing to do with the metric system, it just makes the math easier...

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    "Faith: Belief without evidence in what is told by one who speaks without knowledge, of things without parallel." - A.B.
  2. Volt is no longer defined by Kilogram by Dhrakar · · Score: 5, Informative

    The posted article, while interesting, is wrong about the volt being based on the Kilogram. Since about 1990, the volt is defined to be the voltage applied to a Josephson junction that produces a frequency of 483,597.9 GHz. This new standard was implemented in order to get away from relying on 'artifact' standards (such as the Kg cylinder). One quick source page on Josephson junctions (which completely revolutionized the field of Metrology back when I was a calibration tech in the AF) is:
    http://hyperphysics.phy-astr.gsu.edu/hbase/solids/ squid.html
    If I recall correctly, the eventual goal of the international standards organization was to find ways to define everything in terms of frequency/time since we can measure time so accurately/precisely.

  3. Re:Counting Si by dhovis · · Score: 5, Informative

    Vacancies are not necessarily a problem. As you say, vacancies are entropically favored, but there is also a formation energy associated with a vacancy. So thermodynamics tells us there will be a balance between the energy required to create a vacancy with the entropy gained by creating one.

    Thus, there is an equilibrium number of vacancies in any crystal. As long as you know what the equilibrium value is for a given temperature and you maintain that temperature, then you will also know how many vacant sites you will have on the crystal lattice. I don't have any of my texts handy, but I'm sure someone can chime in with the numbers for silicon.

    To sum up. All crystals will have vacancies because vacancies are thermodynamically favored. However, the number of vacancies will tend towards an equilibrium value which allows them to be accounted for.

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    The internet is the greatest source of biased information in the history of mankind.

  4. Re:I Agree - We should go metric by The+Cydonian · · Score: 5, Informative

    Fundamentalist Islamic country without any telephones?

    Can I have some of whatever your smoking please?

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    More than mere navel gazing.
  5. Re:Counting Si by neodymium · · Score: 5, Informative

    If you really would try to build such a crystal, vacanies could very well be the problem. As you said, there is an equilibrium value of defects in any crystal. This equilibrium value is temperature dependant with a exp(-Eform/kT) law, where Eform is the formation enthalpy. High temperature means high rate of defects.

    Si single crystals are usually prepared at very high temperatures out of molten Silicon (1414C, Czochralsky method). Essentially, this will lead to a freezing of the defect structure at temperatures close to the melting point, because the lattice reorientation kinetics (point diffusion) also are thermally activated.

    You would have to temper the crystal for _very_ long times at temperatures of i.e. 300C to get a thermal equilibrium of defects at this temperature. These times could be >>years !

  6. So is the US by pommiekiwifruit · · Score: 5, Informative
    IIRC the pound (or most versions of it anyway) is defined in terms of the kilogram these days, rather than the average weight of a grain of barley. Otherwise they would have problems if the US genetically modified barley to include jellyfish genes or whatever the seed companies reckon you should be forced to eat (and unless you grow your own food, you are eating it).

    And the metre is defined properly these days (as is the second) in terms of wavelengths of radiation.