Become a fan of Slashdot on Facebook


Forgot your password?

Experts Suggest Replacing Definition of Kilogram 844

fenimor writes "The kilogram is the only one of the seven basic units of the international measurement system defined by a physical artifact rather than a natural phenomenon. International team of scientists suggest replacing the kilogram artifact -- a cylinder of platinum-iridium alloy about the size of a plum --with a definition based on one of two unchanging natural phenomena, either a quantity of light or the mass of a fixed number of atoms. They propose to adopt either one of two definitions for the kilogram by selecting a specific value for either the Planck constant or the Avogadro number."
This discussion has been archived. No new comments can be posted.

Experts Suggest Replacing Definition of Kilogram

Comments Filter:
  • How about ... (Score:3, Interesting)

    by canwaf ( 240401 ) on Saturday February 26, 2005 @07:56PM (#11790212) Homepage Journal
    1 litre of H2O at ATP?
  • Hmm... (Score:4, Interesting)

    by ProudClod ( 752352 ) on Saturday February 26, 2005 @08:00PM (#11790258)
    Planck's constant would be a very elegant solution - it being the smallest possible quantity of energy, and of course, energy == mass * c^2
  • by pboyum ( 139114 ) on Saturday February 26, 2005 @08:01PM (#11790264) Homepage
    Picture of the International prototype kilogram: pe.jpg []
  • by alw53 ( 702722 ) on Saturday February 26, 2005 @08:15PM (#11790400)
    What happens when the speed of light changes? [].
  • Re:How about ... (Score:5, Interesting)

    by InfiniteWisdom ( 530090 ) on Saturday February 26, 2005 @08:18PM (#11790418) Homepage
    The point is to have something that you can define just by counting some phenomenon or natural objects. For example a second is defined as:
    "the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium-133 atom."

    and a metre is defines as:
    The metre is the length of the path travelled by light in vacuum during a time interval of 1/299 792 458 of a second.

    Unfortunately, there hasn't been a good way to count the number of atoms with any kind of precision, so that has precluded a good definition of the kilogram so far. Maybe now the physicists can actually count atoms accurately enough.

    One could define it as the mass of some number of H2O molecules, but maybe its easier a measure a quantity of light or to count some larger atoms.
  • Re:I suggest (Score:5, Interesting)

    by tibike77 ( 611880 ) <tibikegamez@yahoo. c o m> on Saturday February 26, 2005 @08:23PM (#11790477) Journal
    Not only that, but it is also the only "basic measuring unit" that's not even a measuring unit, but a multiple of another one that's not considered "basic". At least in name. Kilo-gram. Get it ?

    Actually, if they happend to re-define it based on Avogadro's number, they might as well just say the GRAM is the new "basic unit" and the kilogram is just 10^3 grams.
    Why ?
    Because Avogadro's number is JUST an artifact of the definition of the (kilo)gram, not a fundamental constant - it's (been originally) defined as the number of atoms in 12 grams (or, whatever, 0.012 kilogram) of Carbon-12.
    Talk about circular references then...

    Now, basing the definition of the kilogram (might I suggest they also change that basic to gram instead of kilogram... please) on Planck's constant somehow would be a MUCH better ideea. However, the value of that constant [i.e. 6.6260693111111 * 10^-34 and so on] makes it pretty wierd to work with unless you multiply it with 9 [to get exactly 5.96346238 * 10^-33 which makes more sense somehow]. And even then it won't satisfy some people, as I'll bet you'll hear that 0.111111 and so on *9 does not equal 1 :p
    Not only that, but Planck's costant was ALSO measured "accurately" using the kilogram unit as reference.

    Ok, this actually does give me a headache.
  • Re:Pressure (Score:2, Interesting)

    by rgmoore ( 133276 ) * <> on Saturday February 26, 2005 @08:26PM (#11790500) Homepage

    So define it as a different volume of liquid water at water's triple point. The triple point specifies a temperature and pressure based on the physical properties of water, which eliminates your objection. The Kelvin temperature scale is already defined using the triple point of water, so there shouldn't be any problem using it in another basic definition. A more practical problem is that we simply don't have the technology to build a container whose volume is as precise as we'd like the standard for the kilogram to be.

  • by tehshen ( 794722 ) <> on Saturday February 26, 2005 @08:31PM (#11790534)
    My friend and I tried to use metric time once. It is a lot harder than it looks (7.50 o'clock news anyone? Church mass at 3.75?) Metric time is useless if not enough numbers divide into ten.
    Attempts of a JavaScript clock are here [] if anyone is interested.
  • Re:Pressure (Score:2, Interesting)

    by XanC ( 644172 ) on Saturday February 26, 2005 @08:33PM (#11790555)
    You could do that, but then you've got pressure as one of the fundamental units, and kilograms defined in terms of that. (Quite disruptive.) And then you would have the same trouble defining pressure that we're having defining mass.
  • Re:artifact (Score:5, Interesting)

    by JaxWeb ( 715417 ) on Saturday February 26, 2005 @08:34PM (#11790557) Homepage Journal
    It is well known there are 7 base units.

    "Candela essentially measures the same things as watts."

    But watts are not a base unit. A watt is the same "Joules per second", and Joules is also not a base unit, but is defined as a Newton Metre. But a Newton isn't a base unit, it is defined as a Kilogram Metre per second per second. So:

    Newton = kg.m.s^-2
    Joule = Nm = kg.m.s^-2.m = kg.m^2.s^-2
    So a watt is in-fact a kg.m^2.s^-3 , or "Kilogram metre squared per second per second per second" - hence changed the kilogram will change the watt, despite them seeming unrelated!

    A mole isn't the same as mass at all. It is more to do with things on an atomic level. It's really used in chemistry - I've personally never used it outside of a chemistry exam (or coursework). It is sort of just a number, but it actually isn't.

    Kelvin is a fundamental base unit too. Momentum is defined as "Newton Seconds", and so (remembering the definition of a Newton) kg.m.s^-1. Kelvin's measures temperature, which is a measure of kinetic energy, so I can see where you are coming from. You're just wrong.

    Ampere is too. Helpfully, from it you can define other helpful things like volts. A volt, for your interest, is defined as kg m^2 s^-3 A^-1 , or "Kilogram metre squared per second per second per second per amp". And so yet another thing this change would affect.

    It's all very interesting.
  • by BigusDickus ( 160858 ) on Saturday February 26, 2005 @08:42PM (#11790626)
    "The kilogram is the only one of the seven basic units of the international measurement system defined by a physical artifact rather than a natural phenomenon."

    Not entirely true.

    The metric system originally was developed in France around the time of the French Revolution. The idea was that measurements used at the time had "royalist" origins. The foot was the length of some king's foot, the yard was the length from some kings nose to his outstretched thumb, and so on. The metric system was meant to purge these and replace them with measurements derived from nature.

    The meter was originally defined as 1/10,000,000 of the distance from the equator to the north pole along the Paris meridian. The liter was defined as the volume of a cube 10 centimeters on each side (1 liter = 1000 cubic cm). Finally, the kilogram was defined as the weight of 1 liter of water.

    Due to inaccuracies creeping in under various circumstances and the development of better measuring equipment, the current definitions were substituted for these original ones.

    The metric clock they also came up with (10 hours per day, 100 minutes per hour, 100 seconds per minute) obviously never caught on.
  • by tod_miller ( 792541 ) on Saturday February 26, 2005 @08:52PM (#11790708) Journal
    They are cleaning the definition, not the value.

    A new kilogram with equal an old kilogram. This will only make a difference to the history books and those who actually want to make thier own 'kilogram'.

    I can imagine how many 'net savvy drug runners are looking at this and thinking, 'shit, I have snorted too much coke, does this affect my business? whats a planck? oh man, Avocado constant? [sic]

    I say since the kilogram was an arbitrary measurement (in any definition) then why try and make it more formalised? I realised that celcius fit nicely with pure water at sea level freezing and boiling, and other measures have thier own basis (has the definitions have changed). Take my friend the meter. I always use the old skool definitions for rules of thumb.

    Year Definition
    1793 1 / 10 000 000 of the distance from the pole to the equator.
    1795 Provisional meter bar constructed in brass.
    1799 Definitive prototype meter bars constructed in platinum.
    1889 International prototype meter bar in platinum-iridium, cross-section X.
    1906 1 000 000 / 0.643 846 96 wavelengths in air of the red line of the cadmium spectrum.
    1960 1 650 763.73 wavelengths in vacuum of the radiation corresponding to the transition between levels 2p10 and 5d5 of the krypton-86 atom.
    1983 Length traveled by light in vacuum during 1 / 299 792 458 of a second.

    So you see, a meter was the same in all these cases, but they just wanted to act clever.

    The thing is, after world war 3, which measure will be easiest to revert to for a meter? trying to find scientist who can measure "Length traveled by light in vacuum during 1 / 299 792 458 of a second." or just comparing a brass stick with a length of wood while trying to build something using pre-existing specs (that you are relying on to build a post WW3 bridge). ;-)
  • by spectasaurus ( 415658 ) on Saturday February 26, 2005 @09:17PM (#11790941)
    Better question. Why is it in 3 vacuum jars?
  • Re:Anyone Else? (Score:3, Interesting)

    by Tim C ( 15259 ) on Saturday February 26, 2005 @09:51PM (#11791221)
    That's true, but that doesn't answer the question. They didn't pick a lump and say "whatever the mass is of this will be the SI unit of mass!".

    The question is, all the other units are "base" units, while the SI unit of mass has a (kilo) prefix; how come?
  • by gotr00t ( 563828 ) on Saturday February 26, 2005 @09:59PM (#11791266) Journal
    Before laser technology, the meter was defined as the distance between two markings on a bar of platinum-iridium kept in Paris. It was after Michelson invented his interferometer that the meter was redefined as 1,650,763.73 wavelengths of a certain orange line in the spectrum of krypton-86. This was later redefined in the 80's to be in terms of C, the speed of light.

    As technology to measure substances to great precision increases, its about time the kilogram got a redefinition as well, one not based on a single object.

  • Re:Just wait. (Score:3, Interesting)

    by Mycroft_VIII ( 572950 ) on Saturday February 26, 2005 @10:09PM (#11791349) Journal
    That actually depends on how you define 'english'. If you define it simply as the language spoken in England, them by definition anyone there speaks it rather well.
    However some mean a specific form of english, and in this particular conext (how well Americans speak it) they're often refering to how closely to the english of the time when Europeans, especially British, first settled the americas in the 1500-1600's. In that case it's my understanding the honors go to a subsection of America (or at least did about a decade ago when I saw this discussed on nova or some such). There are villages and such in the appallation(sp?) mountains that have been sufficiently isolated that thier language use has drifted less than in the rest of the english speaking world.

  • Why not gravity? (Score:3, Interesting)

    by clambake ( 37702 ) on Saturday February 26, 2005 @11:17PM (#11791756) Homepage
    Why not define it in terms of gravity? i.e. 1kg of mass is equal to the mass of a perfect sphere of platinum that can accelerate from rest another equally sized perfect sphere of platinium placed 1 metere away by X m/s?
  • Re:How about ... (Score:4, Interesting)

    by Geoffreyerffoeg ( 729040 ) on Saturday February 26, 2005 @11:19PM (#11791772)
    Don't you mean 1 L H2O at STP?

    Adenosine triphosphate doesn't have much bearing on the mass of a quantity of water, even though it does provide biochemical energy to the physicists who're measuring it. :-)
  • Re:Earth Theory! (Score:3, Interesting)

    by jericho4.0 ( 565125 ) on Saturday February 26, 2005 @11:58PM (#11791991)
    That worked for a while, but the inaccuracies intoduced by getting exactly 1000 cm^3 water at exactly 289K (plus the correct air pressure) were limiting the measurments we could take.

    Still, the intuitiveness of it is nice.

  • by ozbird ( 127571 ) on Sunday February 27, 2005 @12:18AM (#11792090)
    To prevent contamination (moisture, hydrocarbons etc.) that may affect its mass. Even so, the Kilogram Prototype has lost some mass (50 micrograms as of 2003) for reasons unknown.

    There is an international effort to come up with a new standard extremely precise silicon spheres - the cool thing about them is it is apparently impossible to tell if it is stationary or spinning unless you have a reference point on the surface (e.g. a speck of dust.)
  • Re:I suggest (Score:2, Interesting)

    by Random832 ( 694525 ) on Sunday February 27, 2005 @12:22AM (#11792102)
    why is it then pro-metric americans i talk to deny that anyone in metric countries use anything but kilograms when i try to cite these?
  • Re:My thoughts (Score:3, Interesting)

    by Hollins ( 83264 ) on Sunday February 27, 2005 @12:58AM (#11792249) Homepage
    Yet check out the definition of an ampere [], which involves "parallel conductors of infinite length, of negligible circular cross-section, ...".

    This would seem to constitute a physical reference that is impossible to generate.
  • Re:Hmm... (Score:2, Interesting)

    by ph43drus ( 12754 ) on Sunday February 27, 2005 @02:07AM (#11792497)
    I like it when you define hbar = c = 1 (Plank's Constant over 2*pi, instead of being in energy/Hz, it's in energy*sec/radian). Once you've done this, because of E = mc^2, mass has units of energy, and then time and distance both have units of 1/energy. Neat, eh? It's the natural unit system of the universe. It's pretty inconvenient unless you do particle physics, however.

    Basically, the idea is once you've got c, hbar, and the three basic unit types (mass, length, time), you've got a complete system of units. c and hbar then turn into conversion factors which can be set arbitrarily to get a system of units--this is what actually makes the Plank's constant elegant, by nailing it down as an exact number, we've completely defined our system of units (c is already a defined quantity). Then, more accurate measurements of hbar are not measurements, so much as refinements of the definitions of kilograms, seconds and meters.

  • by amliebsch ( 724858 ) on Sunday February 27, 2005 @02:42AM (#11792650) Journal
    The only reason that the metric system is better is because it uses base-10 to convert between units

    THIS IS EXACTLY WRONG. In fact, this is why the imperial system is still in use - because it is largely base-12 in nature. Base-12 is far superior to base-10. The only advantage to SI metric system is that it is CONSISTENT in its conversions, where the imperial system is not. But using Base-10 for those conversions is a major headache, especially for bakers, carpenters, and anybody who has to frequently divide by 2, 3, or 4. The BEST system would be a metric/SI system that uses base-12. But I'm sure the Base-10 bigots will find reasons to disagree.

  • by CBDSteve ( 716562 ) on Sunday February 27, 2005 @09:59AM (#11793632) Homepage
    Do you realise that the definitions of Fl Oz, Pints and Gallons are different in America and Britain? []

    A British Pint is .568 of a litre, whereas the American one is 0.473 (so the definition of a gallon is different).

    Fluid Ounces (fl oz.), too - there's 20 in a UK pint, and 16 in an American pint.

    I think the American version is actually the original one - us Brits changed our measurements some time after the Revolutionary War, while the US kept them the same.
  • Re:I suggest (Score:3, Interesting)

    by danila ( 69889 ) on Sunday February 27, 2005 @10:03AM (#11793647) Homepage
    There is nothing in SI that prevents humans from using it. In Russia the "pollitra" (literally half-a-litre) is a very common unit of volume. So common, in fact, that, when buying vodka, people would often buy two pollitras instead of 1 litre. :) Also, units such as "nol-dva" (zero-two) and "nol-tri" (zero-three) are very usable to refer to 0.2l and 0.3l volumes respectively.

    Similarly, the common size of a land plot is "shest sotok" (six hundreds), meaning, of course, 600 square metres. And most people seem to be entirely capable of saying how much meat they want using SI units. You don't need a Ph.D. to ask for "300 grams" or "700 grams" of meat.

    Ditto for everything else. There is no rational reason whatsoever not to use SI units and metric system in general. All it takes is a little determination from the government to mandate its use where it matters and people will adapt very soon.

    I personally don't give a shit about ignorant Americans or Brits who are/were so intent on using non-metric systems. It's their damn fault and it's their damn problem. They lose productivity, not me.
  • by Crouchy ( 7129 ) on Sunday February 27, 2005 @11:30AM (#11793963)
    Bellow link suggests that when the kilogram was devised it was based on 1 litre of Pure water. As a side note, when I bake I simply change ml to grams when weighing water (close enough for cooking). 2040&dekey=kilogram&gwp=8&curtab=2040_1 []

  • by hankaholic ( 32239 ) on Sunday February 27, 2005 @12:05PM (#11794099)

    But using Base-10 for those conversions is a major headache, especially for bakers, carpenters, and anybody who has to frequently divide by 2, 3, or 4.

    Baking is always cited as a reason for using imperial units. I enjoy making bread on a regular basis, and occasionally other baked tasty things, and let me tell you the Imperial system is virtually worthless when dealing with flour and to a lesser extent sugar and salt.

    Any good bread recipe targeting the average American baker will usually state something to the effect of "3 to 4 1/2 cups of flour". That's not because the person writing the recipe didn't feel like specifying greater accuracy -- it's because volume is a horrible way of measuring flour.

    To properly measure a cup of flour involves fluffing up the flour, gently filling your measure, and sweeping the excess off of the top. This is a pain and a mess -- you try effectively fluffing flour without creating a dust cloud, keeping in mind that this will often be done in a kitchen and that a cloud of flour is quite easy to ignite. Even when consistent measuring techniques are used the amount of flour in a given volume can vary considerably due to other factors, including the mill of the flour itself.

    Generally, the experienced bread maker will start with an amount of flour that they know to be less than what they need, and work in extra flour as needed. This works with breads meant to be chewy, but as working the dough makes it more elastic this is less than ideal for more delicate items such as biscuits.

    Things are further complicated when using volume to measure salt or sugar -- flake size can vary significantly, and the amount of variation in a tablespoon of salt (especially kosher salt, which has a generally large flake size and is easier to work with in the kitchen) can make a notable difference in the final flavor. Equal weights of sugars provide equal sweetening, but a cup of white sugar, brown sugar, and confectioner's sugar might weight 200, 220, and 120 grams respectively. Again flake size within the individual types of sugar varies from manufacturer to manufacturer. In addition to changing the texture and flavor of the product sugar affects the rise time and quality when yeasts are involved. Too much, and the product will expand too much, ruining the texture. Dough for breads is often worked, left to rise for flavor development, and then shaped into its final form and allowed to rise again. Too little sugar, and the dough's first rise may be its last, leaving you with a surprisingly dense and hard dough brick after baking.

    By purchasing a quality digital kitchen scale, the baker can place the container to be filled on the scale, "zero out" the scale (which tells the scale to treat the reading it is currently getting as its "zero mass" point), and fill the container to the desired amount. This leads to easier conversion between various types of sugars, salts, and flours, including unintentional conversions when your particular brand of kosher salt has a larger flake size than that used by the person who wrote the recipe.

    I just don't follow the justification that bakers have to commonly divide by 2, 3, or anything for that matter. Few people find, say, 1/3 cup by starting with a cup and dividing it into three parts. Even something such as filling a one-cup measure exactly halfway with flour or moleasses is quite difficult!

    I'd say that it's just as easy given the proper tools, and more accurate in terms of the actual amount of the ingredient involved, to measure 50 grams of sugar as it is to measure 1/4 cup. In either case, nobody is dividing anything, simply using the proper measuring device. In the case of the recipe calling for 40 grams of sugar, though, the SI system has a clear advantage. The SI baker would add to the scale until it read 40g. How about 40 grams of sugar (80% of 1/4 cup) in terms of standard Imperial measuring devices?

    Google calculator gives "80% of ((1/4) US cup) = 4

Perfection is acheived only on the point of collapse. - C. N. Parkinson