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Au revoir au Grand K

The International Bureau of Weights and Measures voted earlier today to ditch the platinum-iridium prototype kilogram in favor of a value of mass based on Planck's constant. The Post explains:

Since the 19th century, scientists have based their definition of the fundamental unit of mass on a physical object — a shining platinum iridium cylinder stored in a locked vault in the bowels of the International Bureau of Weights and Measures (BIPM) in Sevres, France. A kilogram was equal to the heft of this aging hunk of metal, and this cylinder, by definition, weighed exactly a kilogram. If the cylinder changed, even a little bit, then the entire global system of measurement had to change, too.

With Friday’s vote, scientists redefined the kilogram for the 21st century by tying it to a fundamental feature of the universe — a small, strange figure from quantum physics known as Planck’s constant, which describes the smallest possible unit of energy.

In 1875, the signing of the Treaty of the Metre made the system official. Two platinum and iridium prototypes — a meter-length bar and a kilogram-mass cylinder — were forged to serve as the standard units for the whole world. The BIPM distributed copies of each prototype to the signatory nations; the century-old U.S. national kilogram still sits in a glass case in a locked room down the hall from Pratt’s lab.

But the kilogram prototype, known as “Le Grand K,” was made by humans and is subject to all our limitations. It is inaccessible — the safe containing the cylinder can be opened only by three custodians carrying three separate keys, an event that has happened fewer than a dozen times in the object’s 139-year history. And it is inconsistent — when Le Grand K was examined in the 1980s, it weighed several micrograms less than it was supposed to. This meant that anyone who made products based on the standards had to reissue their weights. Manufacturers were furious. Lawmakers were called. Metrologists, people who study measurements, were accused of incompetence.

The article doesn't say what the Bureau will do with the kilogram prototype.

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