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Science

The Changing Definition Of 'Kilogram' 964

Posted by timothy
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."
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The Changing Definition Of 'Kilogram'

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  • Re:Kilogram? (Score:4, Insightful)

    by LX.onesizebigger (323649) on Tuesday May 27, 2003 @12:40AM (#6044724) Homepage

    And since the inf^H^Hmperior^H^Hal system is now defined in terms of the metric system (an inch is 2.54 cm), your strange units change as well.

  • reproducibility (Score:5, Insightful)

    by nthomas (10354) on Tuesday May 27, 2003 @12:50AM (#6044793)
    Although it was mentioned in the article, I think it should be emphasized that the SI definition of the kilogram, unlike their definitions of the meter and second, cannot be reproduced -- or rather, reproduced exactly. This is quite important, as it is neccessary for the standards governing body in each country to have a very precise reference weight of their own.

    Since there is only one reference object for the kilogram, everything else is just a copy -- and even if it is a first generation copy, errors are bound to creep in.

    The redefinition of the kg is long overdue, mad props to the scientists working on this.

  • by toxic666 (529648) on Tuesday May 27, 2003 @01:12AM (#6044929)
    It seems they should just reference the kilogram to a standard, such as x,xxx,xxx,xxx Si (28 isotope) atoms. This would eliminate the complications trying to build a standard, duplicate it and correct for earth's gravitational variations at the time and place of checking physical reference mass (not weight, to which the article alluded). Keep in mind the kilogram is a measure of mass, and not weight. That is why maintaining a physical standard requires correcting for gravity at the location's, time, elevation, tide, (add geophysical conditions, ad nauseum) of measurement.

    If we are maintaing a physical chunk of alloy as the standard, it's time to decide on a more precise measurement, like we did with the meter long ago.
  • by Compenguin (175952) on Tuesday May 27, 2003 @01:26AM (#6045003)
    What i've never understood is since the kilogram is the base unit why didn't they just call it the gram?
  • Re:Kilogram? (Score:3, Insightful)

    by UniverseIsADoughnut (170909) on Tuesday May 27, 2003 @02:01AM (#6045183)
    " Except machinists. All people working use metric, but when you have to build something you wind up in inches and mills."

    while this is still pretty true, many shops are going more metric as the amount of metric jobs increases. More and more machining tools are coming in metric to (largely to the CNC increase which is perfect for making transition). But since the change rate for machines is so slow it's more of the machiniest being behind then being the way it is. No one is expecting a shop to toss all their new 20 years old , bridgeports and and metric ones overnight. But from the designer side I can say it really pisses us off when a machinest goes "wahh?" to a metric design.
  • by atcroft (123896) on Tuesday May 27, 2003 @02:03AM (#6045187)

    Why not just use definitions that can already be made, such as 1/12.0107(8)-th the mass of one Avogadro's constant of a sample of 100% pure carbon-12? or 1/132.90545(2)-th the mass of one Avogadro's constant of a sample of 100% pure cesium-133 (which is its only naturally occurring isotope)? Or base it from the half the energy of the gamma ray generated by the annihilation of a positron-electron pair having no energy from acceleration, or something similar? Yes, it is a bit problematic that most of the physical features it could be based on now seem difficult to measure in a lab, because they relate back to something on the atomic scale, and the counting of objects at that scale or in such a number to be useful daily is difficult. At least, though, it would then be reproducible.

    Having read the NIST article [nist.gov] referenced [slashdot.org] by another respondent earlier, I can agree with their reasons for considering the adoption of another, more accessible standard. One of the cornerstones of science is the ability to reproduce results. Perhaps it is overdue that the unit of mass (kilogram) join its other basic breathern, the units of time (second) and length (meter), in being based not upon one physical sample, but upon a physical quantity that is reproducible and available to laboratories world-wide.

    Reference for constants: The NIST Reference on Constants, Units, and Uncertainty [nist.gov]

  • Re:Kilogram? (Score:5, Insightful)

    by UniverseIsADoughnut (170909) on Tuesday May 27, 2003 @02:07AM (#6045202)
    " 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."

    Bingo! this is why the US has been working on the process so long. Granted the push hasn't been very great but it's happening. If you're a country of a few million and only are the size of a small new england state, the change is pretty cheap and easy. When your huge, there is a massive infastructure change cost. and trying to re-wire 300 million peoples brains to a new way takes a lot more work.

    I think places like Europe were also helped by war. They had to rebuild and start new with so much. So it was a perfect time to start fresh. The US is a pile of legacy ways. And nothing happens to change them.

    With that said I wish we would try harder to convert. Get a dual system going now and run it for 20 years. let people adjust. Teach school in 95% SI ( only enough english units stuff so the comprehend them).
  • by Sigurd_Fafnersbane (674740) on Tuesday May 27, 2003 @02:28AM (#6045291)

    1 liter of water will have the mass of 1kg assuming it is at the right temperature and preassure. Using water you would need a pressure/temparature reference rather than a mass reference. I think the french guys were right that a slab of metal is easier to maintain than some meassure of pressure without having a mass reference.

    Mass is a proporty of matter independant of gravity. Gravity affects matter with mass but an object will have the same mass also in micro-gravity. It is kind of equivalent of the charge of an electron. The charge is independant of any surrounding electric field.

  • Re:Kilogram? (Score:5, Insightful)

    by SN74S181 (581549) on Tuesday May 27, 2003 @02:57AM (#6045416)
    Perhaps we don't have the coercive state apparatus necessary to 'make' the metric system the primary system that we use.

    Me, I have been doing a lot of woodworking lately. It's convenient to use a unit (the Foot) that divides easily into subunits that are multiples of both 3 and 4, without having to get all mired in floating point arithmetic.

    But we have this metric flamefest every time the metric system comes up on Slashdot, and the same crap comes up every time.

    I'm just happy that pointy-head metric zealots don't seem to have much pull in the real world of regular people. Keep on ranting, dudes.
  • Re:Kilogram? (Score:5, Insightful)

    by radish (98371) on Tuesday May 27, 2003 @03:11AM (#6045471) Homepage
    You do know the metric system is many hundred of years old don't you? In fact it's older than your country. The point is the US has had 200 years and they haven't even started the process. There's nothing saying you can't run in parallel - the UK has been doing so for years. It's absurd to say you have to rip out all the imperial pipes and replace them - you just have to keep 2 sets of tools around until those old pipes get replaced naturally. It really isn't hard, it's just the US can't be bothered.
  • by vrt3 (62368) on Tuesday May 27, 2003 @04:01AM (#6045657) Homepage
    If the volt was based on the kilogram (and therefore a relationship exists between the two), and now volt is based on frequency, isn't it possible and wouldn't it be a good idea to base the kilogram on the volt? Then we don't need those perfect references anymore.
  • by Anonymous Coward on Tuesday May 27, 2003 @05:22AM (#6045938)
    Are we that stubborn

    Well, I think the explanation is that while I'm not that stubborn, my parents are that stubborn. I'm 31, and I remember the whole "let's adopt the metric system" push in the 1970's. One day, we drove to the Mobil station to get some gas, and suddenly everything was labelled in liters. I was like, "ho-hum, it's liters, so what?", but my parents -- and every other adult at the station as well as every other adult my parents mentioned it to -- were up in arms about it. Why? They had no idea what a liter was. I, on the other hand, had heard over and over again in science class what a liter was, and measured things in terms of liters even at that young age. That's because at that time there was a big push to educate kids about the metric system. (As an aside, there darn well better still be a push to educate kids about the metric system; otherwise, we'll never get any further and plus my education is partially wasted if we never convert. As it is, I'll be OK with the US converting in a few decades when I'm considered an old guy.)

    To this day, things are little better with my parents. Paraphrase of a phone conversation with my mom:

    • (Mom) How big is a liter?
    • (me) Well, you know when you buy a two-liter of Coke at the grocery store?
    • (Mom) Yeah...
    • (me) A liter is half as big as that.
    • (Mom) What?
    • (me) Coke is measured in liters. You can get a two-liter or a three-liter. The two-liter has just what it says in it -- two liters.
    • (Mom) That doesn't help me.

    Now, my mom is not a stupid person. But, she was never exposed to the metric system, and as long as we don't switch, she doesn't have much motivation to learn, because she has other things to do with her time. And, fact is, it takes quite some time to really get comfortable with a system of units.

    For example, take a pencil and paper and draw a line that's approximately one inch long. Easy, right? Now do the same for a centimeter. Still pretty easy, although if you're an American, you might be less confident about your estimate. But ask an American older than, say, 35 to do that, and there's a good chance that they're not going to have any feel for it at all. They might even know what the centi- prefix means and that a meter is a little longer than a yard, but translating that into something concrete is a pain. And, to be realistic, everyday life has a lot of guesstimating quantities, not actually measuring them. People don't carry around rulers, but if someone asks them whether the piece of chalk they're holding is too small, they'll respond instantly with, "Yeah, it's not even an inch long!" For a measurement system to be useful to the common man, they have to not just know something about it but also have a feel for it. And, by the way, even though I've taken chemistry classes and used celsius for measuring temperature, I couldn't tell you whether 25 degress C is a comfortable temperature outside or not. Well, I could, but I'd have to run it through f=1.8*c+32 in my head first.

    Oh by the way, for my little bit of flamebait this evening, I'd like to point out that some European countries don't really use just the metric system either. Ask an Englishman for a recipe sometime.

  • Silly artifacts (Score:3, Insightful)

    by jabber01 (225154) on Tuesday May 27, 2003 @10:18AM (#6047513)
    IIRC, the kilogram is the last basic unit of measure still expressed in terms of an artifact, as opposed to though an observable phenomenon + mathematics.

    IM(H)O, we need to do away with this, because artifacts exist in only one place. They can be stolen, damaged, or suffer from flaws and natural processes like the one we're seeing right now.

    Of course, the flip side of having everything in terms of observable phenomena creates the problem of measurement, and making tools sensitive enough to do that work. Philosophically, the problem goes circular here, for how do you make a set of calibration weights for a scale, if you have to measure things to the atom first...

    But in practice, there is no problem, because the measurement technology exists, and we're talking about the "standard" or "reference" units here.

    Imagine having to calibrate a scale on Mars, or Alpha Centauri. Getting that artifact to the "job site", to make sure the scale is true, would be a bit of a chore.

    A kilogram should be expressed not in terms of the number of atoms in a particular crystal, but rather in terms of the mass of X moles of standard substance Y.

    We can assume (if we can not, then all else is a lie) that a particular isotope of a particular element will have the same mass eveywhere in the Universe. We know the number of atoms in a mole. Problem solved.
  • Re:Kilogram? (Score:4, Insightful)

    by Dun Malg (230075) on Tuesday May 27, 2003 @10:35AM (#6047692) Homepage
    You do know the metric system is many hundred of years old don't you? In fact it's older than your country.

    As the poster below me said, you are quite wrong. [unc.edu]

    The point is the US has had 200 years and they haven't even started the process. There's nothing saying you can't run in parallel - the UK has been doing so for years. It's absurd to say you have to rip out all the imperial pipes and replace them - you just have to keep 2 sets of tools around until those old pipes get replaced naturally.

    No, you really don't get it at all. As it happens, most people who have tools ALREADY have the two sets of tools. What makes switching difficult is having two sets of PARTS. It's all well and good to say "from now on all parts/raw materials will be measured using the metric system", but what does one do about, say, electrical conduit fittings? There is an UNGODLY amount of installed bass there which is already in inches and adding on to it would require a complicated system of adapters and a complete recalculation of wire capacity. Name any other construction trade and you run into the same thing. How do you add on to an inches-and-feet house with metric lumber? What size metric ducting do I buy to add to a 12-inch heating plenum? Not saying that it can't be done, but there's a lot more to it than "keep[ing] 2 sets of tools".

  • by A55M0NKEY (554964) on Tuesday May 27, 2003 @10:42AM (#6047743) Homepage Journal
    People don't want to change because most everything around here still uses ye olde english system.

    For instance: I won't get a car that has big kilometer numbers and little mile numbers until the speed limit signs have the speed in kph listed in large type with the speed in mph small type at the bottom. I won't get a tape measure that measures in meters/cm until lumber is sold in convienient metric lengths. The building codes should all be in metric too if they are not already. When the gas and milk are sold in litres, I'll have a better intuition as to how much one liter is ( soda is sold in 1 and 2 litre increments so I kinda do already )

    I must have both metric and inch type wrenches because it is completely random as to which type will fit, even on the same item. Who knows if the 50yd line will ever become the 50m line in football or if the game might subtly change by using the slightly longer meter. If they stopped selling TVs with 27 inch screens and used centimeters, people would learn to like centimeters.

    The only way we're ever going to switch is if the government mandates that all the measurements of products are given in metric in larger type than their equivalent in ye olde inches/feet/furlong system, and that the government must use that system exclusively on all signs / documents etc.

  • by Chris Y Taylor (455585) on Tuesday May 27, 2003 @11:44AM (#6048390) Homepage
    "At least in my classes, engineers are still strongly inclined towards imperial units. "

    As an American engineer I prefer Imperial units. The problem with metric is that all the conversion factors are 10.

    If I do a "sanity check" on a metric calculation and find that it is about 10 times too big then I know I probably have a conversion error... somewhere. If I do a "sanity check" on an Imperial calculation and find it is a little over 10 times too big, then I immediately know to start by checking my inches to feet conversions. If it were about 3 times I would first check feet to yards; a little more than 5000 times suggests a problem in feet to miles. The odd conversion factors actually help me recognize and diagnose math errors. Of course, it used to be much harder to multiply by 5280 than it was to multiply by 1000 back when the metric systen was developed, but thanks to the invention of the calculator the difference in effort is now very minor.

    And Imperial is obviously the system of units that God intended to be used for rocket science because it allows (with a little cheating) you to have specific impulse measured in Seconds.

    The ONLY reason I can see to use Metric rather than Imperial is to get rid of that darn gee-sub-cee.

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