Catch up on stories from the past week (and beyond) at the Slashdot story archive

 


Forgot your password?
Close
typodupeerror
×
Science Technology

Graphene and Quantum Hall Effect Could Help Redefine Metrics 92

Posted by timothy from the no-paywall-and-that-rocks dept.
eldavojohn writes "The National Physical Laboratory has published research in Nature that could lead to redefining two of our most commonly used metrics. There's been a lot of trouble stemming from defining an exact Kilogram as some lump of platinum-iridium sitting in a glass case somewhere, so the proposal was put forth to study the quantum hall effect with different materials. Enter the Nobel prize winning, super strong, silicon usurping graphene. NPL now says you can add quantum resistance metrology to the list of graphene's many conquests as they say the quantum hall effect in graphene is 'very robust and easy to measure.' With this at their disposal, the Kilogram may be redefined in terms of h, the Planck constant, and the Ampere may be redefined in terms of e, the electron charge (alias Elementary charge or the charge of a proton). You can find the full paper here."
This discussion has been archived. No new comments can be posted.

Graphene and Quantum Hall Effect Could Help Redefine Metrics More

Graphene and Quantum Hall Effect Could Help Redefine Metrics

Comments Filter:
  • ...that the ampere was already defined in terms of the charge of the electron.

    • Re:I guess I always assumed... (Score:4, Informative)

      by Goaway ( 82658 ) on Thursday September 29, 2011 @10:26AM (#37553598) Homepage

      The definition of a unit must be physically instantiable. That is, you have to be able to use the definition to build a device or artifact that can be used to calibrate a meter for said unit. Otherwise, the unit is useless.

      This means that some units still have cumbersome and strange definitions, as we do not have the technology to use the obvious definitions to calibrate measurement devices.

      • But the lump of metal is located "somewhere" i.e. outside the U.S. So it can not be used. Now, we have an american definition of "kilogram" and kilograms have become a real unit, ready to be used in the real world. As soon, as someone can claim intellectual property, that is.

        • Re: (Score:2)

          by Goaway ( 82658 )

          But the lump of metal is located "somewhere" i.e. outside the U.S. So it can not be used.

          Oh, you gravely underestimate the amount of work that goes into this system.

          Every country has their own copy of the weight. Every now and then, they very, very carefully bring their own weight to Paris, and calibrate it against the weight that sits there. Then they equally carefully bring it back. Once it is back, more copies are manufactured locally, and sent out to institutions and industry who need it to calibrate their own equipment.

          And so in the end, through many intermediate steps, your kitchen scales

          • And so in the end, through many intermediate steps, your kitchen scales are calibrated against the single kilogram in Paris.

            The compounding error, it burns... it burns!

            Heh. Not that it's really a terrible way of doing things. Just glad to see there's enough confidence in an experimentally reproduceable metric to replace the 'lump of metal defined to be 1kg' model.

            • And so in the end, through many intermediate steps, your kitchen scales are calibrated against the single kilogram in Paris.

              The compounding error, it burns... it burns!

              Oh, definitely. Any attempt to equate physicalities through intermediaries must be assumed to incorporate compounding error.

              But a balance is very, very simple; it has fewer internal sources of error than a voltmeter, for instance. You validate a balance the same way you validate a level; it's absolutely dead simple and requires no tools oth

              • On of my cow-orkers was stunned when I told him how you initially set an atomic clock. (Strip away the jargon, and you're just referencing against Flamsteed's stick - when the stick has no shadow, it's noon.)

                So this only works at the equator, eh? We need to find a better metric, or Ecuador and the other equatorial nations will hold this over us like the Sword of Damocles!

                • Oh, it's worse than that. Flamsteed's observatory is near Greenwich, England... Universal Time Co-ordinate Zero.

    • The SI base unit is the Ampere. The Coulomb is a derived unit (Ampere-seconds).

      Which is definitely silly - the base unit is charge flow, and the derived unit is charge?

      • Agreed, the charge unit (preferably one defined via the elementary charge) should be the base.

        • Farady forever!

          (a Faraday is the charge of a mol of electrons, or 96,485.30899 Coulombs)

          • That kind of conflicts and would be easily mistaken for the F (farad), the unit for capacitance. Also, in physics you generally don't name two units after the same person.

            • Except that they already have, sort of, they call it a constant http://en.wikipedia.org/wiki/Faraday_constant>

      • Re: (Score:3)

        by khallow ( 566160 )

        Which is definitely silly - the base unit is charge flow, and the derived unit is charge?

        Not really. Simple algebra can easily convert from one unit to another. And the second is far better defined than the Coulomb is. So there's no measurable error introduced by using the Ampere as the base unit rather than the Coulomb.

      • Re: (Score:3)

        by fizzup ( 788545 )

        If you spent the time needed for two laboratory exercises, one to prove that you had created a circuit with an Ampere of current and another to prove that you had amassed a Coulomb of charge, then you would understand why the base unit is Ampere, not Coulomb.

      • Re: (Score:2)

        by Goaway ( 82658 )

        Which is definitely silly - the base unit is charge flow, and the derived unit is charge?

        This is because we are better at measuring the effects of charge flow than of charge, and thus it is easier to find a unit definition based on it. Practicality.

  • Graphene - the smartest material known to Man! (Score:4, Funny)

    by lxs ( 131946 ) on Thursday September 29, 2011 @10:25AM (#37553588)

    It even won a Nobel prize.

  • SECOND POST! Both kg and A may be redefined in terms of h and e.

  • When was the last time someone redefined a pound?

    • Re: (Score:1)

      by Anonymous Coward
      Just about every second. [exchangerateusd.com]

      • Well, to be fair, while there are multiple definitions of "pound" there are also multiple definitions of "meter".

        Although in terms of what's in the article, I don't think "gram" can ever be used in other contexts but "amp" is a contraction of more than just "ampere".

        • Re: (Score:2)

          by rossdee ( 243626 )

          'there are also multiple definitions of "meter"'

          Fortunately the System International unit of length is the metre

        • Actually, English is pretty much a compilation of lots of languages, and "gram" does have lots of meanings, most associated with writing. It does allow one to consider adding (to microgram, kilogram and other aggregate units) the following...
          pangram - typical weight of a chimp.
          lipogram - typical excess weight of a human. (Useful as you only get heavier if you eat more than other fatties.)
          seismogram - typical weight of a tectonic plate
          urogram - something that weighs piss all
          and so on. It's all quite simple.

    • Re: (Score:3, Informative)

      by Alpha Whisky ( 1264174 )

      1963 when the UK parliament adopted the international definition (from 1959) of the pound as 0.45359237 kilogrammes. Ironically for you, it will change again if the definition of the kilogramme changes as per the article.

    • Re: (Score:2)

      by artor3 ( 1344997 )

      Why bother? Pounds are only used in day to day life, where the sorts of precision being discussed here would go to waste. You may as well ask why a schoolkid's calculator only holds a dozen digits of pi.

    • Re: (Score:1)

      by Anonymous Coward

      Stop showing your ignorance. "Pound" has multiple definitions already, and has a schizophrenia about if it is a measure of force or mass. In ye olden physics, pound is used for force and the slug is the unit of mass.

      For the "mass" concept, the commonly adopted definition is in fact 0.45359237 kilograms. In other words, it's defined in terms of SI.

      Similarly, your beloved "inch" is defined as exactly 25.4 mm these days.

      You can try defending the imperial system in a number of ways, but picking consistency

    • Re: (Score:2)

      by Jonner ( 189691 )

      When was the last time someone redefined a pound?

      Which pound? There are pound force and pound mass. There are at least US and Imperial varieties. At least modern pound mass [wikipedia.org] is generally defined in terms of the kilogram, so if it changes, so does the pound.

  • 1) Can bitcoins be minted from graphene?

    2) How does this affect the Packt constant?

  • 3 orders of magnitude better than the lump (Score:5, Informative)

    by Covalent ( 1001277 ) on Thursday September 29, 2011 @10:53AM (#37554022)
    Right now the accuracy with which the kilogram can be measured is about 1 part per 1E8. The paper mentions a noise of around 1 part per 1.6E11. That's over 1,000 times better. That certainly suggests that this method will be sufficiently "better" to be used as the new standard.

    I, for one, welcome our incredibly accurate overlords.

    • Re: (Score:3)

      by Jamu ( 852752 )
      It also doesn't depend on an artifact ("the lump"): It will be possible to determine one kilogram from the definition only.

      • Re: (Score:2)

        by devent ( 1627873 )

        I don't know why AC was modded down. A kg is whatever you define as a kg. It could be the weight of my refrigerator, the weight of 1000 gold atoms, or as current "The kilogram is defined as being equal to the mass of the International Prototype Kilogram[1] (IPK),[Note 3] which is almost exactly equal to the mass of _one liter of water_.".

        So now we define some other property as the kg. The important thing is, that you can measure the "lump" accurate.

        • I don't know why AC was modded down. A kg is whatever you define as a kg. It could be the weight of my refrigerator

          Yeah, but if the kg is defined as the mass of your refrigerator, then I can't arrive at the kg using just the definition. I also need your refrigerator. I can't build a refrigerator of my own and use that to calibrate my scale, because without access to your refrigerator mine is going to mass differently than yours, and the unit is defined in terms of yours and yours alone.

          Whereas when the definition is based on a physical property of the universe, anyone anywhere can recreate the unit and calibrate thei

    • Redefining the kilogram with the Planck constant doesn't help with accuracy, since the Planck constant itself is known to the precision of 5e-8 only. So, in effect, the determination to the accuracy of 1e-8 isn't a major improvement. Just think of 1e-8 in terms of the prototype kilogram. 1e-8 means that the mass of the prototype (1000 g) can be fixed to within 0.01 mg, which is really a lot. This is an amount that can be measured by hand, without using any fancy and expensive machine. Our university departm

    • It means precisely what it is intended to mean.

      • As a financail advisor recently pointed out.

        There is no trade imbalance with China. They give us tons of wortgless goods and we give them tons of eorthless dollars.

        Seems fair wen you think about it.

        • But they are loaning us the dollars we are paying them with, and charging interest. As long as the interest rate they charge exceeds inflation, they'll still get paid.

        • Its a guess, but I think I know where your keyboard was imported from...

  • I'm glad to see that we can finally dump that silly imperial system and get to a set of eminently sensible standards and measures that aren't obscure and/or arbitrary.

    Because when I want to buy meat, I certainly first think "how will this pile of hamburger relate to the Planck constant?"

  • How much I pay for my baggage at the airport?
  • Eventually Scotty will come up with a way around it at Kirk's behest.
  • Isotropically pure diamonds, either all C-12 or all C-13, have 50% higher thermal conductivity then isotropically mixed diamonds. http://en.wikipedia.org/wiki/Isotopically_pure_diamond [wikipedia.org] When using graphene for this kind of measurement, do they also use a single isotope of carbon? Does it make any difference if the carbon used in not isotropically pure?
  • One Horsepower is the power of a big white dead horse kept at the headquarters of the International Bureau of Weights and Measures in Paris

    • Unfortunately, its value is slowly losing its accuracy from its use in the recalibration of the standard unit for redundant futility.

  • I hope the Plank Constant is not found to vary over the life of the universe, as alpha has been conjectured to.

    I am a little surprised that the several spheres of silicon scattered around the world hadn't already redefined kg standard. I saw one of those balls 10 years ago, and understood then that the work was almost complete - the deviations from a perfect sphere were negligible, radius well determined, and purity excellent.

    I'm also a little surprised that these versions of the kg standard need exist at

    • No, "water" is not, even assuming its "pure." What hydrogen and oxygen isotope ratios?
      What temperature and pressure? 1 gm/ml is an approximation, water's density is typically
      a little less than one in everyday conditions.

Slashdot Top Deals

One man's constant is another man's variable. -- A.J. Perlis

Close