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Science

NIST Wants An Electronic Kilogram 270

Posted by Hemos from the standards-are-gud dept.
Dearing writes "According to the Global Engineering Journal, NIST, those not-so-standard standards people, want to give up the hunk of metal they've been calling a kilogram, even though it never weighs the same twice. In it's place, an electronic kilogram could act as the permanent standard."
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NIST Wants An Electronic Kilogram More

NIST Wants An Electronic Kilogram

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  • The ultimate diet hack (Score:5, Funny)

    by smnolde ( 209197 ) on Friday August 31, 2001 @09:57AM (#2239045) Homepage
    We all now lost 10% of our weight. I just hacked NIST's computers and changed the reference.

    Why aren't I thin now? I must hack the electronic tape measure next.

    $10 if you want me to make you taller, too.

  • Damn kids... (Score:5, Funny)

    by chinton ( 151403 ) <chinton001-slash ... m ['gma' in gap]> on Friday August 31, 2001 @09:59AM (#2239055) Journal
    Why when I was a lad, all we had was a platinum-iridium cylinder, and we liked it. But you damn kids today, with yer newfangled electronic kilogram, why I oughta...

    • Re:Damn kids... (Score:4, Funny)

      by tbone1 ( 309237 ) on Friday August 31, 2001 @10:33AM (#2239240) Homepage
      #include <python/yorkshire.h>

      Well, we had it toof. We didn't have any of this metric rubbish. We used the stone-furlong-fortnight measurement system. Everyone had to lug around a big stone and a flatulent racehorse for two weeks just to measure something. A platinum-iridium cylinder? Luxury!

      • You had a system? Oh, what we'd 'a given for a system of measurement. All we had was "a lot" or "a little", and we had plenty little a that, and everyone would disagree about whether he'd 'a been cheating his neighbors, givin' 'em a little muck when the deal was for a lot.

  • What? (Score:4, Interesting)

    by Reality Master 101 ( 179095 ) <RealityMaster101 @ g m ail.com> on Friday August 31, 2001 @10:02AM (#2239070) Homepage Journal

    They balance it against gravity to measure it? Wouldn't that be really, really inaccurate, since gravity varies by altitude, local density variations, etc? Did I misunderstand what I just read?

    Sheeze, why not just define it as 1.498e20 atoms of carbon (or whatever number), and be done with it.

    • Re:What? (Score:4, Funny)

      by sphealey ( 2855 ) on Friday August 31, 2001 @10:04AM (#2239080)
      "Sheeze, why not just define it as 1.498e20 atoms of carbon (or whatever number), and be done with it."

      No problemo - as soon as you figure out a practical method for counting out those atoms on the floor of your typical machine shop. 'Oops - dropped another one. Someone blow the oil off it NO NOT THAT HARD - damm, out the window'.

      sPh

      • Well, clearly it must be harder than I think, since they would have done it that way a long time ago if it were easy.

        But still... wouldn't you know the volume that so many atoms of a material would take if you melt it and form it under certain conditions (e.g., zero pressure)? Once you had a block, you could cut it to exact dimensions using lasers. Presumably we have very accurate way of measuring the meter, so we ought to be able to do very precise cuts.

        • how would you move the laser? Using stepper motors? You would need a stepper capable of moving an atom at a time.

          • You would need a stepper capable of moving an atom at a time.



            We've got them: how do you think they move a STM tip? They're piezoelectric.

            There's other problems with this though: the chances of cutting a clean, macroscopic edge are basically nil.

            Eric

    • Gravity has nothing to do with it. A Kilogram is a measure of mass, not weight.

    • Re:What? (Score:2, Informative)

      by kingdon ( 220100 )

      "They balance it against gravity to measure it?"

      The article linked to makes it sound that way, but if you have an electronic measurement for force (which is what the "electronic kilogram" is - see the excellent page [nist.gov] posted by aktbar), and you have other standards for the meter (certain number of wavelengths of a certain light) and second (certain vibration of some molecule, I'd have to look up the details), then F=ma (force = mass times acceleration) lets you derive the kilogram. (in units terms, Newtons = kilograms times (meters per second squared) - if you have newtons, meters, and seconds, then you get kilograms).

      "Sheeze, why not just define it as 1.498e20 atoms of carbon"

      This approach has also been investigated. See the Avogadro Group [nist.gov] or an article [nist.gov] summarizing it. These things boil down to what you can measure more accurately.

    • I visited NIST and had it explained (Score:5, Insightful)

      by dragons_flight ( 515217 ) on Friday August 31, 2001 @10:51AM (#2239323) Homepage
      IIRC the idea is to convert the standard of mass to a number of electrons accelerated by some well known voltage.

      The electrons since they are moving, produce a magnetic field which pushes against a well known reference magnetic field (which can be measured without concern for mass). This magnetic repulsion is used to balance a 1 kg reference mass against gravity.

      Since gravity produces acceleration independant of mass (ma=F=mg => a=g), it's also possible to measure the local gravity to a high precision by means of the acceleration with needing to know something's mass.

      Thus we have a way define mass in terms of a number of electrons (and a geometry of the path they take, technically) and other measured quantities which don't use mass in their standards.

      You could say mass is so many atoms of some reference substance, but how do you measure it? Since you can't first weigh it and extrapolate from there. Similarly volume would depend on temperature, structural arrangement, and other things. The people at NIST claim this provides a more easily reproducible method of defining mass. (Of course I'd rather just stick with the electronic scale or balance pan since these tend to be accurate enough for me.)
    • That would be defining 'mass', not 'weight'.
    • "They balance it against gravity to measure it? Wouldn't that be really, really inaccurate, since gravity varies by altitude, local density variations, etc? Did I misunderstand what I just read?"

      We're not talking about spring scales and such, it's a beam balance. It compares the weight of whatever you're trying to find the weight of on one side to the weight of the official kilogram on the other. The idea is that, if the beam is level, the weight of the kilogram mass on one side is the same as the stuff you're measuring on the other, so the masses must be the same.

      Sure, the gravitic attraction on the two samples will be slightly different over the length of the beam (say 8"), but the difference is disgustingly negligible.
    • Because we can measure the relative mass of macroscopic objects to something like 20 significant figures, whereas we can only determine Avogadro's number (which is the number for converting macroscopic masses to atomic quantities) to 10 or so, IIRC.

      At least, that was the explanation back when I took first-year physics. Presumably the accuracy of Avogradro's number will improve over time, but an extra 10 sig figs is a challenge . . .

  • Just a different was of measuring it (Score:3, Informative)

    by Mononoke ( 88668 ) on Friday August 31, 2001 @10:03AM (#2239078) Homepage Journal
    It measures, with great precision, how much current passes through a wire coil in a strong magnetic field to balance the pull of gravity on
    a one-kilogram mass standard[emphasis mine]
    Doesn't look like they've replaced the hunk of metal, they've replaced the balance scale.

    The late Dr. Dick Deslattes said something like, "If we ever have to communicate from afar with ET aliens, we could explain all our science standards in terms they would understand, but we'd have to throw them the mass standard to explain that."
    Wouldn't we have to throw them a dictionary first?

    • Doesn't look like they've replaced the hunk of metal, they've replaced the balance scale.

      No, they are just trying to make sure that the new mathematically "electrically" defined kilogram is as close as possible to the current kilogram.

      The same way they redefined a second based on a certain number of rotations of a cesium atom (or something like that) and redefined a meter in terms of light-seconds. They got the new definitions as close as possible to their old values.

      This is nothing more than doing essentially the same thing with the meter, however more difficult.
    • What they can do with this is precisely measure the weight of the kilogram hunk of metal, and define the kilogram to be the mass of anything that gives the same result. They're basically replacing the balance with the special cylinder on one pan with a one-pan scale that will stay calibrated.

      I'm not entirely clear on how they intend to deal with the mass vs weight issue, though. If the experiment has to be done in Earth's gravity at that particular spot, we'd have to throw the aliens the whole planet to explain anything...
    • They're using the current(physical) standard to establish the new(elctronic) standard.

      The new standard is going to be "the ammount of mass properly balanced by XXX volts and YYY amperes in the referenced system." That ammount is expected to be more consistent than "the ammount of mass needed to properly balance that hunk of metal we have in the basement."

      The current (physical) standard changes from time to time due to dust, wear(from cleaning), etc.

      • The new standard is going to be "the ammount of mass properly balanced by XXX volts and YYY amperes in the referenced system."

        SI standards based on absolute numbers (as opposed to chunks of metal) include the second (9192631770 ticks of a cesium atom) and the meter (the distance light travels in 1/299792458 second). But you can't define kilogram in terms of volt or ampere because they're already based on the kilogram. A volt is one watt per ampere. A watt will raise a 1N weight at 1m/s, while a newton will accelerate a 1kg mass at 1m/s^2. An ampere is the current in two parallel wires 1m apart that produces 2e-7N per meter of length. Therefore, defining a kilogram in terms of a volt or ampere would be circular (unless NIST skillfully arranges the equation to solve for kg); NIST must define its new version of the kilogram in terms of the second and meter.

        Sources include NIST's current definitions [nist.gov].





    • The late Dr. Dick Deslattes said something like, "If we ever have to communicate from afar with ET aliens, we could explain all our science standards in terms they would understand, but we'd have to throw them the mass standard to explain that."

      Wouldn't we have to throw them a dictionary first?



      It would be charitable to let them know what its mass was first so it didn't destroy (or be destroyed by) whatever they were catching it with...

  • Why have we been working with actual physical models to provide standards for so long? Didn't there used to be a bar representing the "exact" length of a meter before they switched it over to the distance light can travel in 0.xxx seconds?

    If you're going to have standards, at least base them on constants, like the aforementioned speed of light for distance, the mass of certain molecules for mass, and...hmmm...can anyone think of anything for time? (I don't know what they currently use) Keep in mind that speed-of-light is taken.

  • well duh (Score:2, Informative)

    by Overphiend ( 227888 )
    hunk of metal they've been calling a kilogram, even though it never weighs the same twice.

    A gram is not a measurment of weight.

  • Of course it never weighs the same... (Score:5, Insightful)

    by Robber Baron ( 112304 ) on Friday August 31, 2001 @10:05AM (#2239086) Homepage
    A kilogram is a unit of mass not weight. Weight is dependant on gravity. Mass is not.
    • A kilogram is a unit of mass not weight. Weight is dependant on gravity. Mass is not.

      True enough, but if you want to determine the mass of a small object, how do you do it? Odds are really good you're going to weigh it in some manner, and divide by 9.8 m/s^2.

      According to NIST [nist.gov], they've got a variance on the order of 3% per century in the observed mass (probably measured by weight) of the standard kilo brick.

      Wow! I thought the recent news of observations that show that the fine structure constant or the speed of light may be minutely changing as the universe ages [yahoo.com] were pretty far out, but to think that mass is this variable...

      • Re:Of course it never weighs the same... (Score:4, Informative)

        by mmontour ( 2208 ) <mail@mmontour.net> on Friday August 31, 2001 @10:51AM (#2239326)
        According to NIST [nist.gov], they've got a variance on the order of 3% per century in the observed mass (probably measured by weight) of the
        standard kilo brick.

        3 percent??? Do you have any idea how HUGE a variation that would be in a primary standard? Maybe if they polished it with a belt sander before every measurement...

        The link you provided says:

        [...]are causing the mass of the kilogram to vary by about 3 parts in 108 per century relative to sister prototypes.

        Now I'm not certain about this, but I'd wager that the "108" is actually a "10 to the 8th power" that got mangled somewhere in the conversion to HTML. If so, it would represent a more plausible 0.000003% per century variation.
      • ...if you want to determine the mass of a small object, how do you do it? Odds are really good you're going to weigh it in some manner, and divide by 9.8 m/s^2.

        Actaully, most high-schools have this new high-tech thing that actually measures mass. It's called a beam balance. You have some known quantity on one side, and your unknown on the other, then you compare the two. Really revolutionary!

        Also, 9.8 m/s^2 is only at sea-level. Raise your hand if you live a sea-level? That's what I thought.

  • "As electronic measurement equipment gets better, we can apply it to the experiment to get better results, which may lead to even better equipment, which we can apply to the project, and so on..."

    => as Quake gets better, I can apply it to my hardware, wich may lead to a better quake, and so on

    I think I understand ! I'am a potention Nobel Prize, Just as he's a Potential Quake geek !
  • From the artice: It measures, with great precision, how much current passes through a wire coil in a strong magnetic field to balance the pull of gravity on a one-kilogram mass standard, and how much voltage is generated by moving the coil. Separate systems in the laboratory determine reference levels for voltage and gravity.


    Call me a computer scientist, but isn't there something recursive about defining the prototype kilogram with gravity and then measuring gravity at the same time? Or is there something I don't understand about gravity?
    • Call me a computer scientist, but isn't there something recursive about defining the prototype kilogram with gravity and then measuring gravity at the same time?

      There are ways of measuring gravity without a known mass standard. For example, you could measure the acceleration of an object falling freely in a vacuum chamber (only need length and time measurements).

      However, it does seem strange to me that they'd base the kilogram standard on something as indirect as the local gravitational field.

      • For example, you could measure the acceleration of an object falling freely in a vacuum chamber (only need length and time measurements).


        Only if you know your exact distance from the earth's center (as well as the earth's mass). How do you determine that? To measure G you need to use a torsion balance, and that requires a definition of kg (since G is defined in terms of N*m^2/kg^2).

        • Only if you know your exact distance from the earth's center (as well as the earth's mass). How do you determine that? To measure G you need to use a torsion balance, and that requires a definition of kg (since G is defined in terms of N*m^2/kg^2).

          From the description of the device, it relates the mass of the standard kilogram to an electromagnetic force, by balancing that electromagnetic force against the gravitational force on the mass (== the weight of the mass).

          Therefore, the only relevant parameter is the one relating the mass of an object to the gravitational force on that object. This is "little g", the local gravitational acceleration, and it can be measured directly with only a length and a time standard. You don't need to know "big G" or anything about the earth.

          The gravitational force on an object is given by:
          Force(N) = mass(kg) * g(N/kg)

          (note that 1 N/kg == 1 m/(s^2))

    • Re:Something's fishy (Score:3, Interesting)

      by gilroy ( 155262 )
      Blockquoth the poster:

      Or is there something I don't understand about gravity?

      If there isn't, then you're way smarter than most practicing physicists... gravity is hard.

  • You could define a kilogram as the amount of water in a cubic decimeter. Or, you could define it as the mass of 6.02 * 10^23 protons, or any number of other ways. I don't understand how this measurement they intend to make with the device they have will be any more accurate or easier to deal with.

    You could even define it as the energy in some huge number of photons of a particular wavelength. :-)

    • Re:Why not define in terms of other standards? (Score:5, Informative)

      by abde ( 136025 ) <apoonawa-blog@yaho o . com> on Friday August 31, 2001 @10:25AM (#2239196) Homepage
      that wouldn't work - after all, then you've just scaled the problem down to ask "whats the weight of a proton"

      remember that the base physical units have to be directly related not to theor, but to empirical observation. That's the difference between "units" and "physical quantities"

      MASS is a physical quantity. "kilogram" is a "unit" of that quantity. defining it in terms of the "mass of a proton" makes no sense because thats essentially a *circular* argument.

      if you;re gonna construct a vast edifice of science, the foundation better be damn rigorous! this isnt just semantics, its essential, the way that we have to be absolutely sure that 2 + 2 = 4 (which can be derived from the Completeness property of the Real number Set). A good reference for basic units and quantities is here [amazon.com].

      • the way that we have to be absolutely sure that 2 + 2 = 4 (which can be derived from the Completeness property of the Real number Set).

        Wait! 2 + 2 = 3, not 4

        Bleem lives on :)

        -Todd

      • MASS is a physical quantity. "kilogram" is a "unit" of that quantity. defining it in terms of the "mass of a proton" makes no sense because thats essentially a *circular* argument.

        if you;re gonna construct a vast edifice of science, the foundation better be damn rigorous! this isnt just semantics, its essential, the way that we have to be absolutely sure that 2 + 2 = 4 (which can be derived from the Completeness property of the Real number Set). A good reference for basic units and quantities is here [amazon.com].

        So, this means that you think the mass of a proton is going to change?

        Using atomic fountains it's actually feasible to get well known number of protons together in a group that's possible to accurately measure the mass of. I think that basing it on something like this is an excellent way to define it. Simple, easy to intuitively grasp, and doesn't involve any hairy adjustments for gravity.

    • You could define a kilogram as the amount of water in a cubic decimeter.

      Do you know how many states of water there are? Not to mention somewhere you are going to have to define a pressure, a temperature etc. You don't exactly want to end up with a circular reference in there....

      Now, at CSIRO [csiro.au], they are researching into using a super spherical ball of silicon, about 8 cm across, and weighing 1kg. It is spherical to an accuracy of 8nm, and was built by the same glass grinders that build lenses for our precision instruments and telescopes. We have shipped one or two overseas (and have one or two in .au), so that people around the world can test 'em.

      Pretty cool in all - I watched the guy pick it up with cottonwool, in the same room that I was in - no contaminant free clothes, either - it is pretty robust. It is all part of an international effort to produce new standards of mass etc - the platinum bar in Paris is getting a bit old. IIRC - CSIRO are researching another method, but can't remeber what it was....

      You could even define it as the energy in some huge number of photons of a particular wavelength. :-)

      Hmmm - which unfortunately comes back to a density of photons, and a length cubed, which unforteunalty comes back to that damn platinum bar in Paris. IIRC - it has a chip in the corner of it too - Ooops. I just dropped your metre - my, how you have just grown!

    • The problem is a practical one; they are trying to find an easier way to take something, and determine if it is a 'kilogram' or not.

      Balancing it against a reference weight is one way.

      What they are saying is, doing this electronically is more accurate and easier, using magnetic fields and measured currents and such rather than a classical balance.

      The definition of a kilogram of mass isn't going to change. They're just finding a better way to measure it.
  • The late Dr. Dick Deslattes said something like, "If we ever have to communicate from afar with ET aliens, we could explain all our science standards in terms they would understand, but we'd have to throw them the mass standard to explain that."

    Yeah, because an advanced species capable of space travel would never understand something so complex as to think of a standard measurement of a physical item that isn't affected by gravity.
    • The point is, once we had worked out a "dictionary", we could explain to them the experiments they could do to produce, for example, exactly one ampere. Then they could see how many of their units of current are contained in one ampere, and Voila!, suddenly each species can calibrate the electronics experiments of the other. This means that we could actually compare data, physical theory, etc.


      Of course, you'd need more than the amp. There are a bunch (seven?) of fundamental constants, out of which all other units can be made. You'd need them all, and to communicate them, you'd need experiments that determine their value. So far, we have no such procedure for mass... so we couldn't calibrate the masses in the experiments.


      It's not that the aliens are stupid. It's that we don't yet know how to communicate the value of a mass independent of a reference mass.



      Oh, and by the way: The late doctor never said the aliens had space travel. Indeed, he implied the opposite: If we had to communicate from afar... using, I suppose, EM waves.

  • More information from NIST itself... (Score:5, Informative)

    by aktbar ( 22510 ) on Friday August 31, 2001 @10:11AM (#2239120)
    (one of) NIST's own web page(s) on this is at http://www.eeel.nist.gov/811/elec-kilo.html [nist.gov]. There's a lot more technical detail there than at the link given in the article.

    This really does make sense to replace the artifact with something independent -- they have a bunch of "voodoo" every time they measure the current kilo to try to get the same answer.

  • Why? (Score:2, Insightful)

    by jcronen ( 325664 )
    Realistically, a metal cylinder is impractical. You can't throw it on a balance and determine if your kilogram of Cheezy-Poofs actually weighs a kilogram, because you'd not only ruin the kilogram from disturbing it in its precious environment, but you'd get yellow-cheese-dust on it.

    In reality, I'm sure we could at least replace it with a theoretical definition that's more accurate than the cylinder. Even though the current definition of the meter is physical, in practice it's difficult to measure (the distance light travels in 1/299,792,458 of a second).

    So, define the kilogram as the amount of mass that one liter of pure water contains at 4C. We already know the definitions of the meter exactly (defined by scientists, thanks very much), electric field permittivity (\epsilon_0), magnetic field permeability (\mu_0), the speed of light (c), etc.

    With all these constants defined exactly, it just seems like there would be a better way...

    • So, define the kilogram as the amount of mass that one liter of pure water contains at 4C.

      At what pressure? Your unit of pressure will need to be based on your unit of mass.

  • So now if I want to buy a kilo of cocaine, some dealer's gonna have to walk up to some two story tall machine and poor in the coke until it's 1 kilogram?

    This should be GREAT for police officers. Just look for two story tall electronic kilogram machines to bust all the coke dealers.

  • why, exactly, a kilogram [...] never weighs the same twice?
    • why, exactly, a kilogram [...] never weighs the same twice? MouseR

      Presumably, because of engineering imperfections in the (mechanical) measurement devices, and perhaps also due to local variations in gravity, caused by tectonic forces, tides, etc.

      The bottom line is: weighing a physical chunk of metal is as poor a standard as measuring the length of a chunk of metal. We do better if we can relate these standards to invariant values derived from basic physics.

    • The simple answer: The environment.

      Its not just a matter of determining a fixed quantity of material. The machinery used to determine the measure is also affected by its environment. A room that increases its temperature by 1 degree is going to cause the spring to stretch that much further (or coil to conduct X more electrons).

      Also, the Earth does not exert gravity at the exact same force at all points on the globe at all times. Gravity is "currently" one of the forces involved in the measure of weight.

      Finally, the speed of the Earth's motion could produce some relativistic effects on the measure (although I'm not sure it would apply in this case).
  • They can send it to me. It would make a nice paperweight and conversation piece, don't you think? And not just because of the value of the precious metals.

  • EKG? (Score:2)

    by Nater ( 15229 )
    ..an electronic kilogram...

    Sorry, the acronym EKG is already taken. Please try again.
  • ...just like economists talk about the 1974 dollar as being a standard to refer to (when discussing values from different years, to account for inflation) I guess scientists will start having to use "dated kilograms".

    For example: "Pluto masses 1.203e12 Kg? Is that in 1993 or 2001 kilograms?"

  • Oh no, not again... (Score:5, Interesting)

    by isomeme ( 177414 ) <cdberry@gmail.com> on Friday August 31, 2001 @11:57AM (#2239611) Journal
    From the postscript to the article:

    We'd like to know how you feel about an electronic standard for weight.

    That's odd, I don't understand how this question relates to an article about an electronic standard for mass. And before you flame me for nitpicking, let me remind you that Mars has some very expensive upper-atmospheric dust right now thanks to imprecise communication about units of force. Ordinary people can blithely confuse mass and weight without causing problems. Engineers can't, and this article appears in an engineering publication. When are we going to learn to be more precise about this sort of thing?

  • derive measures from physics constants (Score:3, Informative)

    by peter303 ( 12292 ) on Friday August 31, 2001 @12:03PM (#2239642)
    The ultimate goal is to derive all measures from
    the fundamental constants of physics.
    The two most popular are "c" the special of light
    and "h" Planck's quantum of action.

    A recent Physics today suggests a using
    E=mc^2 and E=hv, where v is a frequency.
    Frequencies are the most accurately measurable
    item in the universe, at a current accuracy of
    one part in 10^19. So the proposal is to choose
    a "kilogram frequency" that precisely defines
    the kilogram. There is already a "meter frequency"
    that precisely defines the meter length in terms
    of light velocity. And a "second frequency"
    which some frequency count close to an astronomical
    second.

    The least well-known constant is the gravitational
    constant, measured only to four decimal places.
    The probably is instrumental error, because
    everything pulls on everything else.
    At least twice in the past decade someone has
    proposed changing the law of gravitation because
    of funny measurements, but every time an
    experiment error was found. The constant "G"
    doesn't fit into many physics equations,
    so it isn't as easy to bootstrap equations
    as for the other constants and measurement units.

  • not to be redundant, but (Score:3, Informative)

    by option8 ( 16509 ) on Friday August 31, 2001 @12:36PM (#2239788) Homepage
    "NIST, those not-so-standard standards people, want to give up the hunk of metal they've been calling a kilogram, even though it never weighs the same twice."

    of course it weighs different every time, it's a standard kilogram, which is a measure of mass. the weight of the Kg will differ as gravity differs - which is a fun little trick having to do with the mass of the earth and the nearby celestial bodies.

    the whole point of the new measuring device is (basically) to more accurately measure the force of gravity on the standard mass - by doing some magic with a magnet keeping the whole thing in balance. this is really just getting at a better measurement of gravity than anything else.

    the crux of the situation is that the only standard for a kilogram is the actual lump of platinum itself. other things, like the standard second, are based on fun stuff like exactly how many times a cesium atom vibrates at a particular temperature. it might be fun to try and define a kilogram as Exactly This Many platinum atoms and be done with it, but that's kinda tricky for the moment.

    it might be a better "standard" to accelerate the "standard" mass at a "standard" rate and measure the forces. say, by swinging the thing around in a calibrated centrifuge at whatever we're calling one Gee. then you can get to the bottom of the whole "weight" issue (in terms of newtons, i suppose).

    besides, unless the standard mass is made of something that's decaying (radioactively - it's not like they'd make the thing out of, say, beef), it'll be pretty much the same mass for quite some time. it's just those nitpickety scientists at the NIST (on which i read a very interesting article recently, i believe in National Geographic Magazine) who want it to be defined in terms of something that will never change

    and secondly, since when is the NIST "not-so-standard"? they are the national frickin' institution for the damned things, so they should be an authority on the subject...

    • [the standard mass will be] pretty much the same mass for quite some time.

      Suppose it isn't. Suppose an anti-metric-system terrorist manages to shave a chunk off the standard kilogram and swallow it. What then?

      The basic idea of making a reproducable standard is a really good idea. Right now, the US has a standard kilogram that is a careful copy of the master standard kilogram, but how can we be completely sure it's an accurate copy?

      I'd be interested, once they get the new standard sorted out, to have them check the US standard kilogram and see to how many decimal places it is accurate.

      steveha

  • Buncha science sissies (Score:3, Funny)

    by rho ( 6063 ) on Friday August 31, 2001 @01:00PM (#2239956) Journal

    There's a guy on the corner what can measure out quarter and half ounces with amazing consistancy... dunno why they have to go to all that trouble, when they could hire this guy cheaper.

    Betcha if scientists were wont to shoot NIST people if their measurement vehicle was wonky because NIST's dumbell was off, you'd see some pretty accurate measuring going on over there...

  • Standard kilogram (Score:2, Insightful)

    by jea6 ( 117959 )
    The mass of the platinum-iridium bar is always 1 kilogram. If you don't beleive me, compare its mass to that of the standard-bearer platinum-iridium bar. If it is the same, then the mass is one kilogram.

    This reminds me of when a previous physical-object based basic measurement standard was updated, the meter. Instead of being "one/ten-millionth of the distance from the equator to the north pole along a meridian through Paris" (http://www.surveyhistory.org/the_standard_meter.h tm), the measure was eventually (after a couple of rounds of revisions) standardized to be the distance light travels, in a vacuum, in 1/299,792,458 seconds with time measured by a cesium-133 atomic clock.

    When people asked if it was the measurement that was being changed, the answer was no, just the precision and accuracy that we can replicate the measurement.

    Same goes here, the sea-level weight of 1 kg of mass is not being changed, just its precision and accuracy.

    • Following up on this, given that we know the relationship between mass and energy (E = mc^2) for rest mass, doesn't the problem for a definition of mass become a problem for the definition of energy (given that the value 'c' is fixed)?

      So, define the energy of a photon from the above caesium clock and you've then defined the kilogram.

      ...or am I missing something?

  • The whole point. (Score:3, Insightful)

    by Lumpy ( 12016 ) on Friday August 31, 2001 @02:29PM (#2240473) Homepage
    sure we can define a Kg as X number of X element's atoms. but we cannot use that standard. (How exactly do you count out X number of atoms?) what they are trying to do is make a standard that is actually useable. The standard for time and length are actually useable. the standard for volume is actually useable. the standard for mass is not useable and has needed a replacement for decades.
  • These are the wanks who own the government contract to extort hundreds of dollars from anyone wishing a copy of a standard developed by the government.

    They get rich selling the public its own property.

    They can go pound sand.

    --Blair
    "NIST, on the other hand, is a national treasure."

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