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Science

Ampere Could Be Redefined After Experiments Track Single Electrons Crossing Chip 299

Posted by timothy
from the micro-management dept.
ananyo writes "Physicists have tracked electrons crossing a semiconductor chip one at a time — an experiment that should at last enable a rational definition of the ampere, the unit of electrical current. At present, an ampere is defined as the amount of charge flowing per second through two infinitely long wires one meter apart, such that the wires attract each other with a force of 2×10^-7 newtons per meter of length. That definition, adopted in 1948 and based on a thought experiment that can at best be approximated in the laboratory, is clumsy — almost as much of an embarrassment as the definition of the kilogram, which relies on the fluctuating mass of a 125-year-old platinum-and-iridium cylinder stored at the International Bureau of Weights and Measures in Paris. The new approach, described in a paper posted onto the arXiv server on 19 December, would redefine the amp on the basis of e, a physical constant representing the charge of an electron."
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Ampere Could Be Redefined After Experiments Track Single Electrons Crossing Chip

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  • why would the weight of the platinum/iridium slug fluctuate? I could imagine the size fluctuating, but not the weight.

    • by Chris Mattern (191822) on Tuesday January 14, 2014 @01:59PM (#45953583)

      why would the weight of the platinum/iridium slug fluctuate?

      Because a few atoms of the slug can sublimate into the surrounding atmosphere, even at room temperature. And because a few atoms of the surrounding atmosphere can adhere to the slug. And yes, at the precision we're talking about here, it makes a difference.

      • by rahvin112 (446269)

        Platinum and Iridium sublimate? Are you serious? The reason they choose platinum is because of it's durability. When you scratch platinum you don't lose material, you just create a furrow in the metal and all the mass is retained. Iridium I believe is the same, and it's a platinum slug with an Iridium coating.

        Now collecting a few atoms of air I can see happening, but maybe that's what the Iridium coating is for. Or maybe that is why they store the slug in a temperature controlled vault, IIRC either in a low

        • It's not iridium coated, it's iridium-alloyed - adds hardness.

          The problem here is declaring something involving atoms impossible, and well, on the atomic scale nothing is. It sits in lit room at room temperature - there is a whole hell of a lot of EM falling on it, and so every now and again somewhere on it you might exceed the work function of the metal and sputter it. You've also got to deal with not just air sticking to it, but being absorbed into it's bulk - atmospheric humidity would be a big concern s

        • by sjames (1099)

          That's great in theory, but in practice, there is a master kilogram and several reference kilograms actually used for standardizing measurement. They keep drifting apart somehow.

          That somehow is either the evil machinations of the faery world or it is sublimation and adhesion.

        • by Rich0 (548339) on Tuesday January 14, 2014 @11:13PM (#45961097) Homepage

          Platinum and Iridium sublimate? Are you serious?

          Sure it can. It just does it VERY slowly. The solid state is greatly preferred for these metals at room temperature, but at any temperature and pressure solid, liquid, and gas are all in equilibrium. When you're talking about thermodynamics and statistical mechanics, about the only absolute is zero, and that is a state that doesn't physically exist anywhere.

          Then throw in quantum mechanics. There is probably some small but finite probability that I'll appear in your living room before I finish typ

    • by forrestt (267374)

      I can only imagine it fluctuating as a result of corrosion. For example, if it were iron the act of rusting would alter the weight.

    • It's hard to consistantly and accurately measure weight when the force of gravity constantly changes, add to the fact that there may be radioactive decay of trace elements, oxidation of metals, Dust/erosion, sublimation of trace components), it's easy to understand how using a physical object to consistantly measure a weight, would fluxuate. when your "constants" are actually "variables" it's really hard to nail down constants...

      • The kilogram [wikipedia.org] is a measure of mass [wikipedia.org], not weight [wikipedia.org].

        • by oodaloop (1229816)
          No shit. And measuring the mass of the slug in question involves weighing it. Or do you have another method involved for determining the mass of an object used as the constant for measuring mass?
          • Re: (Score:3, Informative)

            by Phreakiture (547094)

            Or do you have another method involved for determining the mass of an object used as the constant for measuring mass?

            Placing two masses on a balance is the usual method . . . . and it is gravity-independent. Gravity is necessary, of course, but it only needs to be constant across the two platters of the balance.

          • by gardyloo (512791)

            1) Measure its acceleration when subjected to a known force.
            2) Measure the period of a system where it's part of the inertial loading, and the restoring characteristics are known (or can be decoupled post-experiment).

            • by tepples (727027)
              First define a known force. Right now the unit of force is defined in terms of a kilogram.
              • by hchaos (683337)

                First define a known force. Right now the unit of force is defined in terms of a kilogram.

                That's easy! Gravity, of course!

          • by quenda (644621)

            A simple beam-balance scale can measure mass unaffected by gravity variations. You just need a reference mass.

          • by Immerman (2627577) on Tuesday January 14, 2014 @02:43PM (#45954415)

            Sure, the same way they "weigh" things in freefall - measuring the radial forces necessary to keep it moving in a fixed circular path at a given speed. You can even vary the speed to get multiple measurements to reduce error. That may be as simple as a scale in a centrifuge, but does not depend on any way on potentially fluctuating gravitational field. It also incidentally directly measures inertial mass, rather than gravitational mass, which *apparently* is always present in precisely proportional amounts, but which we currently have no accepted theoretical reason to believe is a fundamental equivalence.

          • by Minwee (522556)

            No shit. And measuring the mass of the slug in question involves weighing it. Or do you have another method involved for determining the mass of an object used as the constant for measuring mass?

            Comparing it to another mass using a balance makes any variation in the force of gravity irrelevant. You could travel to Mercury or Jupiter and still make accurate measures of mass using the same scale. Of course, that brings us right back to needing something to compare your mass _to_, and that something is eventually a cylinder made up of platinum and iridium stored in a vault just outside of Paris.

      • The parent's point about gravity being variable really needs to be modded up!

        To drive the point home ...

        http://en.wikipedia.org/wiki/File:Earth-G-force.png [wikipedia.org]

        As you get closer to the earth the force of gravity increases as expected however at the Gutenburg discountinuity the force of gravity then goes down to zero at the core.

    • by firex726 (1188453)

      Wear... Even at a microscopic level the it can still suffer wear as a result of otherwise imperceptible movement and is also why it's designed as it is.
      Since one of the most common SI units is based on however much this thing weights it's important that it be left as intact as possible.

      http://en.wikipedia.org/wiki/Kilogram#Stability_of_the_international_prototype_kilogram [wikipedia.org]

      > K4 was originally delivered with an official mass of 1 kg75 g in 1889, but as of 1989 was officially calibrated at 1 kg106 g and ten

      • by Muad'Dave (255648)

        K4 was originally delivered with an official mass of 1 kg75ug in 1889, but as of 1989 was officially calibrated at 1 kg106ug and ten years later was 1 kg116ug. Over a period of 110 years, K4 lost 41ug relative to the IPK.

        Your mu's got eaten by slashdot. Those should all be micrograms. Naturally μ doesn't work.

      • by msauve (701917)
        s/ g / microgram /

        Preview is your friend, /. doesn't support Unicode.
    • by gstoddart (321705)

      Mostly I think it's dirt and other crud.

      Essentially, the reference object isn't free from contamination or interacting with its environment.

      We don't have a mechanism which allows us to define it in a more rigorous manner which can be reproduced, so we've got this hunk of of whatever it is sitting on a shelf which is defined as the reference kilogram. (OK, it's not just sitting on a shelf, but close)

      Not what you'd call an objective standard. More of an approximation which is official and which the other of

      • by Immerman (2627577)

        >Unlike an atomic clock where you can precisely define the unit of time in wavelengths of whatevers, there's no way to define the kilogram in a more specific way.

        Really? Seems to me it would be easy enough to say 1g = 1/12 the mass of one mole of Carbon-12, or your element of choice - preferably something common, stable, and monoisotopic for convenience (http://en.wikipedia.org/wiki/Monoisotopic_element). Obviously we'll want to choose our atom carefully since per-nucleon mass is different fro every el

    • Re: (Score:2, Interesting)

      by Anonymous Coward

      That's part of the problem. Scientists aren't exactly sure. Shortly after it was made, several copies were made in 1889 that were verified to be the exact same mass. Over the years, the mass of the original and its copies have slightly drifted. The copies appear to have grown heavier, while the original has grown lighter. But even that's hard to determine for sure, since we can only be sure of the *difference* of the masses, not their absolute mass, because absolute mass is defined in terms of these ki

  • Condescend much? (Score:3, Interesting)

    by Oligonicella (659917) on Tuesday January 14, 2014 @01:58PM (#45953565)
    "almost as much of an embarrassment"

    You would have done better with the technologies at hand at the time how?
    • You would have done better with the technologies at hand at the time how?

      You would have practiced science using methodologies nearly a century out of date when? See that's the thing about science -- it's supposed to change in response to new data. And it usually does, except for some of our basic units of measurement, which remain stubbornly stuck in the past. That's why it's an embarassment. The whooshing sound you heard is the point sailing over your head.

      • And it usually does, except for some of our basic units of measurement, which remain stubbornly stuck in the past. That's why it's an embarassment. The whooshing sound you heard is the point sailing over your head.

        Umm, I'm pretty sure scientists have been working on the kilogram problem for some time. Your use of "stubbornly" implies that there's some sort of resistance to a redefinition. But I don't think there's any evidence that that's the case.

        Here's a NYT article [washington.edu] from 2003 detailing the then-current attempts at redefinition. I'm sure there are older things out there detailing the scientific efforts to work on this problem, too... this was literally one of the top three hits in an internet search.

        Anyhow,

      • by Baloroth (2370816)

        You would have done better with the technologies at hand at the time how?

        You would have practiced science using methodologies nearly a century out of date when? See that's the thing about science -- it's supposed to change in response to new data. And it usually does, except for some of our basic units of measurement, which remain stubbornly stuck in the past. That's why it's an embarassment. The whooshing sound you heard is the point sailing over your head.

        You have a better idea for a mass unit? The scientific community would absolutely love to hear about it, because every other idea either doesn't work at all, or isn't practically measurable (sure, you could define it as x number of atoms of y substance, but good luck counting out a few trillion atoms with precision every time you want to make an accurate measurement).

        Most of the basic units of measurement in science have in fact been redefined when someone came up with a better system. Mass, however, it tu

    • Re:Condescend much? (Score:5, Interesting)

      by UnknownSoldier (67820) on Tuesday January 14, 2014 @03:23PM (#45955259)

      I'll probably get down modded but it is not just the technologies but the basic definitions of the SI system are pretty fucked up.

      There are numerous problems, the primary being:

      * the seven SI base units are not independent
      e.g. the Amp depends on the definition of the kilogram ?!?!

      http://www1.bipm.org/en/si/si_constants.html#figure [bipm.org]

      Quoting Dr. Xavier with my emphasis added:

      "If for instance, one had to change the definition of the Kg unit, we see that the fundamental units candela, mole, Amp and Kelvin would change as well. .. So one cannot say there are seven fundamental SI units if these units are not independent of each other. The other big fault is the obvious redundancy of units. Although not very well known to all of us, at least two of the seven base units of the SI system are officially known to be redundant, namely the mole and the candela. These two units have been dragging along, ending up in the SI system for no reason other than historic ones. "

      * http://www.blazelabs.com/f-u-suconv.asp [blazelabs.com]

  • by Infiniti2000 (1720222) on Tuesday January 14, 2014 @01:59PM (#45953579)
    A kilogram is straightforwardly defined as 2.20462 pounds. Simple enough.
    • by gman003 (1693318)

      And as we all know, the pound is defined as 7000 grains, which are simply defined as the mass of a grain of wheat.

    • How is the pound defined? Thought I'd look it up , but all I can find is Wikipedia [wikipedia.org] that says a pound is defined as "(b) the pound shall be 0.45359237 kilogram exactly".

      The definition for a pound of force [wikipedia.org] was even less helpful as it's related to an Avoirdupois pound [wikipedia.org], which is defined as:

      The avoirdupois pound, also known as the wool pound, first came into general use c. 1300. It was initially equal to 6992 troy grains. The pound avoirdupois was divided into 16 ounces. During the reign of Queen Elizabeth, the avoirdupois pound was redefined as 7,000 troy grains. Since then, the grain has often been an integral part of the avoirdupois system. By 1758, two Elizabethan Exchequer standard weights for the avoirdupois pound existed, and when measured in troy grains they were found to be of 7,002 grains and 6,999 grains.

    • A kilogram is straightforwardly defined as 2.20462 pounds. Simple enough.

      Yeah right! There's over a dozen different definitions for the "pound". You're citing the intuitively named international avoirdupois pound designation. Unfortunately, your own definition is over a century out of date! The Mendenhall Order of 1893 defined it as 2.20462 ... but the following year, someone got their hands on a British kilogram and it was redefined to be 2.20462234.

      And where-fore did the previous pound measurement come from, before it was normalized to the kilogram? Why, the weight of 120 Arab

  • Bah, I say (Score:3, Interesting)

    by dmatos (232892) on Tuesday January 14, 2014 @02:07PM (#45953765)

    The Ampere was only chosen as an SI fundamental unit because it was easier to measure than a Coulomb. To me, an Ampere will always be 1 Coulomb per second.

    And since the electric charge is 1.602E-19 Coulombs, we can just invert that number to find the number of electric charges (ie, electrons) in a Coulomb.

    • Here's what I don't understand, whats so wrong about defining the Ampere in terms of coulombs per second? Or in terms of anything else for that matter? I guess I fail to see whats so bad about having the Ampere as a derived unit.

      • by gstoddart (321705)

        Here's what I don't understand, whats so wrong about defining the Ampere in terms of coulombs per second?

        Really? Because this sounds like it's a pretty darned hand-waving definition to me.

        At present, an ampere is defined as the amount of charge flowing per second through two infinitely long wires one meter apart, such that the wires attract each other with a force of 2x10^-7 newtons per meter of length. That definition, adopted in 1948 and based on a thought experiment that can at best be approximated in t

      • > I guess I fail to see whats so bad about having the Ampere as a derived unit.

        The point of Physics is to simply everything down into fundamental units. Adding a derived unit just "clutters" that goal.

    • Re:Bah, I say (Score:4, Informative)

      by Chris Mattern (191822) on Tuesday January 14, 2014 @02:24PM (#45954083)

      And since the electric charge is 1.602E-19 Coulombs, we can just invert that number to find the number of electric charges (ie, electrons) in a Coulomb.

      Well, yes. But the point here isn't shuffling around the units. The point here is to increase the accuracy at which the elementary charge is known, which would be necessary whether you're defining the Ampere in terms of the charge or the Coulomb in terms in the charge. Currently, we know the elementary charge to ten decimal places. That's not good enough, so that's what this is about--finding out that figure to greater accuracy so it can be used as a universal measurement standard. For comparison, the definition of the second is accurate to 15 decimal places.

  • by bobbied (2522392) on Tuesday January 14, 2014 @02:13PM (#45953881)

    The fine article is incorrect. How an Ampere is defined does not change.

    What may change is how you can measure current in the lab using other known standards because it's really hard to count electrons. Or perhaps the way a Coulomb is defined may change but the Ampere will not change.

    One Ampere will remain defined as One Coulomb per second.

  • And ohms law still works out. ... hang on, that's already a nice easy calculation to define and Ampre. I = E/R

  • by TheloniousToady (3343045) on Tuesday January 14, 2014 @02:31PM (#45954207)

    How about we change "At present, an ampere is defined as" to "Currently, an ampere is defined as"?

    • I guess whoever marked the parent as "Flamebait" didn't get the "Currently" joke. The use of "a modest [editorial] proposal" in the title was a hint, but I guess that wasn't enough. :-)

    • by EmagGeek (574360)

      What idiot moderated this as "Flamebait" when it is clearly "Funny?"

      Who the hell is in charge of this place?

      • by gstoddart (321705)

        Who the hell is in charge of this place?

        Nobody is in charge.

        Dice owns it. The 'editors' are barely literate, can't spot dupes, and are in charge of periodically adding stories. There's a team of incompetent coders furiously working on the beta site they occasionally force me to see.

        Once you hit the moderation system ... it's monkeys flinging poo. ;-)

      • Glad you got it, thanks for your support. I'll keep trying, though clearly some part of the population here suffers from HDD, Humor Deficit Disorder, so I guess we gotta live with that.

  • redefine the amp on the basis of e, a physical constant representing the charge of an electron.

    Until some smarty pants physicist comes along and determines that e or the charge of an electron changes depending on [pick something, this is physics after all]

  • Huh? You don't have two infinite lengths of wire lying around? They're fairly easy to get; you just take one infinitely long wire and cut it in half.

    More seriously though, what's wrong with just reversing the definition (1 A := 1C/s rather than 1C := 1As), and then redefining 1 Coulomb as a precise number of electrons?

    • (Addendum: wait, never mind, that's basically their proposal with an additional step via Coulomb.)

  • by wbr1 (2538558)
    I am shocked.
    My puny brain doesn't have the power to Coulomb through this information.
    However, I am sure my fellow slashdotters can tell me Watt is going on.
    Maybe a car analogy using the Chevrolet Volt?
    If Joule help me, I can make it worth your while.
  • by Ngarrang (1023425) on Tuesday January 14, 2014 @04:02PM (#45955999) Journal

    http://www.youtube.com/watch?v=ZMByI4s-D-Y [youtube.com]

    This youtube video answered most of my questions about the kilogram.

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