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New Definition Of The Kilogram Coming Soon

January 24, 2011

Scientists are close to establishing a non-physical definition of the kilogram–the last remaining base unit in the International System of Units (SI) still defined by an existing object–after it was revealed that the prototype used to establish the measurement is shrinking.

That prototype, which is a cylinder of platinum-iridium housed at the International Bureau of Measurements (BIPM) in France, has gotten approximately 50 micrograms (0.0000017 ounces) smaller over the past 100 or so years, according to Reuters reporter Caroline Copley.

While that is a minute amount–”the size of a small grain of sand,” according to Copley–the Reuters reporter notes that the “stability of the kilogram is crucial as it forms the basis from which many other units are derived. Thus, experts are gathering at the British national academy of science, the Royal Society, in order to iron out a new definition based on quantum physics.

“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 Sunday.

The Planck constant, named in honor of quantum theory co-founder Max Planck, was first described as the proportionality constant between the energy of a photon and the frequency of its associated electromagnetic wave. Its origins can be traced back to 18th Century France, according to Copley, and it is one of the smallest constants used in physics.

The reason for the platinum-iridium cylinder’s decay is unclear, according to the Daily Mail. While the new standard is coming, according to AFP, “Researchers caution there is still some way to go before their mission is complete.”

However, if it is successful, “it would lead to the end of the useful life of the last manufactured object on which fundamental units of measure depend,” the French news agency reported on January 23.

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