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Old Kilo Loses Weight With Age!

October 10, 2011 Leave a comment Go to comments

Photo: Christopher Griffith; kilogram models by Jim Zivic

“The official US kilogram — the physical prototype against which all weights in the United States are calibrated — cannot be touched by human hands except in rare circumstances. Sealed beneath a bell jar and locked behind three heavy doors in a laboratory 60 feet under the headquarters of the National Institute of Standards and Technology 20 miles outside Washington, DC, the shiny metal cylinder is, in many ways, better protected than the president…

The American prototype is one of some four dozen such national standards around the world, and each of those, in turn, is accountable to an even higher authority: a regal artifact called the international prototype kilogram. Familiarly known as Le Grand K and held in a vault just outside of Paris under three bell jars, it dates back to the 1880s, when it was forged by the British metallurgist George Matthey from an alloy of nine-tenths platinum and one-tenth iridium.

Aside from a yearly ceremonial peek inside its vault, which can be unlocked only with three keys held by three different officials, the prototype goes unmolested for decades. Yet every 40 years or so, protocol requires that it be washed with alcohol, dried with a chamois cloth, given a steam bath, allowed to air dry, and then weighed against the freshly scrubbed national standards, all transported to France.

It is also compared to six témoins (witnesses), nominally identical cylinders that are stored in the vault alongside the prototype. The instruments used to make these comparisons are phenomenally precise, capable of measuring differences of 0.0000001 percent, or one part in 1 billion.

But comparisons since the 1940s have revealed a troublesome drift. Relative to the témoins and to the national standards, Le Grand K has been losing weight — or, by the definition of mass under the metric system, the rest of the universe has been getting fatter. The most recent comparison, in 1988, found a discrepancy as large as five-hundredths of a milligram, a bit less than the weight of a dust speck, between Le Grand K and its official underlings.”

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