It's the End of the Kilogram as We Know It (And I Feel Fine)

By Patrick Morgan | January 25, 2011 12:32 pm

The speed of light may define a meter and atomic clocks may define a second, but a century-old cube of metal still defines the kilogram–at least, until scientists give this antique lump a 21st-century makeover.

As far as the kilogram is concerned, the year is still 1879, but scientists are meeting in London this week to discuss a change for this humble unit. As the Guardian reports:

“The kilogram is still defined as the mass of a piece of platinum which, when I was director of the International Bureau of Weights and Measures, I had in a safe in my lab,” said Terry Quinn, an organiser of today’s meeting. “It’s a cylinder of platinum-iridium about 39mm high, 39mm in diameter, cast by Johnson Matthey in Hatton Garden in 1879, delivered to the International Committee on Weights and Measures in Sevres shortly afterwards, polished and adjusted to be made equal in mass to the mass of the old French kilogram of the archives which dates from the time of the French Revolution.”

The problem with this dusty old cube is that it isn’t immune to aging: since 1879, it has shed 50 micrograms. In other words, if the President of the United States at the time–Rutherford B. Hayes–were to time travel to the present, he’d officially have more mass because it takes more of today’s kilograms to equal yesteryear’s kilograms–all because our standard is a vulnerable physical object. Adding to the confusion, other units–the newton, for example–rely on the kilogram and multiply the amorphous state of seemingly standard measurements.

Scientists, though, think they have an answer. As MSNBC reports:

“International consensus has been achieved, that in the near future the kilogram shall be redefined, based on a fixed value of the Planck constant,” Michael Stock, a physicist at the International Bureau of Weights and Measurements (BIPM), said in a statement. Stock said researchers have been conducting experiments that establish a link between mass and the Planck constant by comparing measurements of electrical and mechanical power.

The Planck constant reflects the size of quanta, the smallest particles of physical matter, in quantum physics. To connect the quantum world with our everyday world, scientists have been conducting experiments with a device called the watt balance, which can be used to relate the mass of an object to the electrical energy required to move it.

In the coming year, the General Conference on Weights and Measures will meet to vote on changing the kilogram’s definition–until then, we’re still stuck with a shard of metal stored in France.

Related Content:
DISCOVER: A Kilogram Ain’t What It Used To Be
DISCOVER: The Kilogram Isn’t What It Used To Be–It’s Lighter
Bad Astronomy: How do you pronounce kilometer?
Bad Astronomy: What Happens If the Universal Constants Aren’t Constant?

Image: flickr / s2art

  • Daniel

    Why doesn’t the article address the more interesting question: WHY does a solid block of metal lose weight?

  • Rene

    Isn’t a kilogram the mass of one liter of distilled liquid water? And a liter is a volume defined by linear distances measured in meters that is in turn defined by the speed of light?

  • Georg

    the kilogramm being one liter (dm³) of water at a certain temperature
    was the original idea. But they soon realized that this is not accurate enough.
    Later Mohr (inventor of Titration Method) bought a kg-copy in Paris to
    calibrate his 1-ltr-Flasks. So in practice it was then the other way.
    Because I collect old lab equipment, I will write to Sevres, because I hope to
    get the original kilogram as a gift when discarded :=)

  • Torbjörn Larsson, OM

    Daniel, why is that interesting when we will get a fix?

    But please, knock yourself out:

    “Beyond the simple wear that check standards can experience, the mass of even the carefully stored national prototypes can drift relative to the IPK for a variety of reasons, some known and some unknown. Since the IPK and its replicas are stored in air (albeit under two or more nested bell jars), they gain mass through adsorption of atmospheric contamination onto their surfaces. Accordingly, they are cleaned …

    What is known specifically about the IPK is that it exhibits a short-term instability of about 30 µg over a period of about a month in its after-cleaned mass.[18] The precise reason for this short-term instability is not understood but is thought to entail surface effects: microscopic differences between the prototypes’ polished surfaces, possibly aggravated by hydrogen absorption due to catalysis of the volatile organic compounds that slowly deposit onto the prototypes as well as the hydrocarbon-based solvents used to clean them.[17][19]

    Scientists are seeing far greater variability in the prototypes than previously believed. The increasing divergence in the masses of the world’s prototypes and the short-term instability in the IPK has prompted research …”

  • inamorty

    Quanta are not particles but the minimum amount of any physical entity involved in an interaction.

  • Matt B.

    Inamorty, photons are quanta of light.

  • Richard Dinning

    I too was taught, about 1958 in Grade nine, that one gram was one millilitre of pure water at 4 degrees celsius. Since a millilitre is one cubic centimetre one thousand of them or one litre of that same pure 4C water would weigh one kilogram.

    That was set up originally that way so that the entire metric system (SI) would be based on a single unit the Metre. That metre was supposed to be 1/10,000,000 of the prime meridian thru Paris. (Of course if he had measured that correctly Metres and Fathoms would be almost interchangeable.)

    Since a metre is no longer defined by a platinum rod but by a certain number of wave lengths of a certain light frequency great accuracy is possible.


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