The Kilogram: Just a Little Off

For the last hundred years or so, a metal cylinder about the size of a souvenir shot glass has been the standard against which all kilograms were measured. Take a look at this Wikipedia provided computer generated image of the kilogram.

The REAL kilogram, the International Prototype Kilogram (IPK) is kept in the International Bureau of Weights and Measures near Paris. Several official clones of this kilogram are kept in various locations around the globe. Every high school and college science lab has sets of weights and measures that are supposed to be exact duplicates of the IPK.

Therein lies the tale. When the various official copies are compared to the official IPK, the weights don’t match up after a period of time. Scientists don’t know whether the official measure is getting heavier or lighter, but something is off. The measures are off by about 50 micrograms. (Goodness knows what discrepancies would arise if the IPK were compared to the one kilo weight at your local high school.)

A microgram is about one millionth of a gram. It takes 1000 grams to make a kilogram, so that means a microgram is a billionth of a kilogram. That may not sound like much to you and me, but scientists are picky about things like that. When they say precise, they mean precise.

Ann Althouse wrote a three sentence post about the kilogram issue at The kilogram. Really, *the* kilogram. This is all she said:

It’s a particular cylinder. Don’t sneeze on it. Be careful washing it. Dislodge a molecule and you throw off all the weights in the world.

She got 20 rather lengthy comments on her three sentence post. The precise measure of a kilogram is a weighty matter to some people.

An NPR story, This Kilogram Has A Weight-Loss Problem which you can listen to in about 5 minutes, explains what some scientists are doing to redifine the kilogram, not as a physical object but as a numerical constant. They’re use a device called a watt balance, which measures electrical and magnetic forces, to attempt a numeric definition of the kilogram.

Badgermama, not one to let the question of the kilogram’s weight problem go unnoticed, got pumped reading Wikipedia. In The elusive kilogram! she describes other ways science is trying to redefine the authentic K.

The kilogram is the only unit not defined off a physical constant – it’s defined from this particular object, the 130-year-old International Prototype Kilogram or IPK. And a whole bunch of other metric units are defined using mass, like newtons, pascals, joules, amperes, couloumbs, volts, teslas, webers, candelas, lumens, and lux. (The plural is not “luxes”. I looked it up.) It was created and then defined as the standard. But some replicas of it were created, like the Kilogram of the Archives, and over time they have diverged from each other. The story of what they’re all made of, and how they’re periodically compared and verified, is pretty cool. And sort of insane. Is that a whole bunch of people’s life work? Making sure that we know how wrong our kilograms might be? Eeeeeee! That’s so hot!!!!!!

And so are multiple bell jars over a brass-looking pedestal thingie! It’s like The International Geek Thingamajig on a Steampunk Cake Stand of Awesome!

bell jar encasing kilogram

Burrow deeply into the kilogram article and you will get to the proposed alternatives that would tie the kilogram to a constant. Atom-counting approaches (I liked the Avogadro project, which would use a silicon sphere); Ion accumulation; and the rather sexy sounding watt balance method: the electronic kilogram!

The watt balance is finding a new home in Ottawa, where conditions are hoped to work toward more precision in defining the kilogram. The Ottawa Citizen reports Canada joins quest to set new kilo: Formula to replace inexact old ingot.

It may take several years before the “inexact old ingot” is replaced by a measure science finds acceptable. In the meantime, do your best not to polish too many molecules off the local neighborhood kilogram.

Cross posted at BlogHer.

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