Weight of the World: The Ongoing Fight Over How to Define the Kilogram

By Sarah Zhang | May 2, 2012 6:04 pm

spacing is important

The meter is fixed to the speed of light and a second to the radiation of cesium, but the mass of one kilogram is still not defined by a universal constant. Instead, it’s still pegged to an old-fashioned cylinder of  platinum iridium alloy kept under lock and key in Sèvres, France.

The method isn’t just old-fashioned, it’s imprecise, which has literal ramifications across the world when the point is to set the kilogram standard. The cylinder is weighed every few decades against official copies that had the same mass when they were all cast in 1899. When they were last weighed in 1988, however, their masses had drifted 70 micrograms apart.

Last October, the International Bureau of Weights and Measures met to determine a new strategy of defining the kilogram, this time using universal constants. IEEE Spectrum has a riveting feature on how this might happen. The kilogram is way more complicated than a supermarket scale would have you think:

Delegates from the bureau’s then 55 member countries unanimously agreed on a tentative plan to base the kilogram on a fundamental constant of quantum mechanics…This coup is largely the result, after decades of work, of steady strides in two challenging strategies for measuring mass. One approach attempts to pin down the exact electromagnetic force needed to balance the gravitational tug on an object. The other harnesses Cold War–era uranium enrichment technology and a host of experimental techniques to count the number of atoms in extremely round balls of ultrapristine silicon.

Results of initial experiments for both approaches have since come in, and…dun dun dun…they don’t agree. Scientists hope to reconcile their results by the next General Conference on Weights and Measures in 2014. The full feature over at IEEE Spectrum is a wonderful behind-the-scenes look at how science actually works, often dogged by ambiguity and disagreement. If you ever doubted that science can be hard and messy, just read about the battle of the kilogram.

Kilogram image via Shutterstock / aniad 

MORE ABOUT: kilogram, mass, physics, SI unit
  • notovny

    A kilogram weighs 9.80665 N under standard gravity, by definition.

  • http://discovermagazine.com Iain

    Is that gravitational mass or inertial mass?

  • John Lerch

    Where’s that standard gravity–New York, Bulgaria, Jupiter?

  • Tony Mach

    You think defining a kilogram is hard? Try metering optical power for a change! Or as wikipedia puts it: “Optical Power Meter calibration and accuracy is a contentious issue.”

  • http://www.lisastewartlaw.com NC Law

    What is the level of disagreement between the two experiments?

  • http://fredcobio.wordpress.com Jim H

    One gram is defined by the mass of one cubic centimeter of water at 4C and standard temperature and pressure. Not imprecise in the least. Oxygen and hydrogen are pretty much universal constants, as far as I am aware.

  • notovny

    @John Lerch

    The original title of the article was “How Much Does a Kilogram Weigh? Ask Again in 2014″ . Given that the Newton is defined from the kilogram I don’t see that weight value changing . And the standard Acceleration due to free-fall was defined in 1901 by the third conference of Weights And Measures, which also, coincidentally clarified that kilograms were units of mass.

    @Jim H: The mass of a gram hasn’t been defined by water for over a century, as the article alludes. In fact, the mass of a cubic deciliter of Vienna Standard Mean Ocean Water at 4C is about 25 mg less than the IPK.

  • http://www.metric.org ScrewThisTight

    How did they determine the ‘pound’?

  • Andrew Dalke

    @Jim H: Unless the water is isotopically pure (e.g., 16Oxygen and 1Hydrogen) then you’ll be affected by the distribution of the isotopes. For example, 16O evaporates faster than 18O. Paleoclimatologists can even measure the changes in isotope over millions of years by looking at the isotope distribution in fossils. The error range based on non-isotopically pure elements make it impossible to use this definition as a precise standard.

    Compare the average mass, of 15.9994(3)amu, to that of isotopically pure 16O, of 15.99491461956(16)amu. The (3) and (16) indicate the uncertainty. Clearly 1:1000000 from the average weight is not precise enough to be used as a mass standard since we are already able to measure some masses to one part in 10,0000,000,000,000

    Still, it is possible to get isotopically pure water. The next question is, how do you get 1 cubic centimeter of it? Are you able to make a perfectly cubical container, such that the volume is off by no more than 1 part in 10,0000,000,000,000? Possibly. Though since it’s a liquid you’ll also have to worry about surface tension effects.

    Then what about dissolved gasses in the water? The density of carbonated water is slightly higher than non-carbonated water, since it has the extra CO2 in the water. There’s CO2 in the air, so your water sample’s density will depend on the air around it.

    The reason a sphere of isotopically pure silicon is proposed is in part because: 1) it’s possible to make isotopically pure, 2) it’s easier to make a sphere than a cube, and 3) it’s a solid, and 4) it’s easier to remove gasses and surface impurities.

  • PastaJoe


    You have actually given the reason why the Kilogram cannot be defined in terms of the Newton in your own (second) post. Namely, that the Newton is defined from the Kilogram.
    To make a long story short, units can never be defined in terms of each other. There are very valid scientific reasons for this.
    Also, as to your point about “standard gravity”, gravity will differ by a very small yet very real amount at different points on Earth. Fundamental units such as the Kilogram need to be measured independently at different points on Earth (and at different times). Using a correction factor (to account for the different readings given by a scale due to elevation changes, for example) is _not_ sufficient.
    Fundamental units _must_ be measured using real (non-corrected), _measured_ values that do _not_ change based on location (or time), so gravity cannot be used.

    EDIT: Obviously, measured values will _always_ change by some tiny amount, regardless of what you do, but the very best measurements of the Kilogram are far less accurate than those of the meter and the speed of light. Trust me, the world’s best scientists haven’t simply been overlooking some simple solution to this problem.

  • Gary B

    I wonder what other methods might be workable. For example, what about measuring the deflection of a small sample containing a known number of atoms (it’s easier to make a small pure sample than a large one) of a conductor in an electrical measuring field – say 1000 atoms – due to the force of impact of one particle, or a stream with a known number of particles per second, at a known speed-at-impact? By analogy, like measuring how far one’s baseball glove is deflected when the ball hits it.

    Or using a small perfect sheet of graphene, and striking it with a known mass and velocity (similar to the above), then measuring the tensile force at the edge that results from the deflection. This force would be multiplied by the triangulation of the deflection and the distance to the point struck, so would be easier to measure. But it would have to take into account the amount of deformation and stretching that the graphene sheet would have.

    I’m making these up as I go along, so they may not be feasible. I also keep thinking about how the Casimir force might be used to do this.


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