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On Saturday, September 28, 1889, representatives of eighteen countries gathered in Sèvres, outside Paris.They were there to bring the world under the measure of a single meter known as M and a single kilogram, K — to bless a particular ruler and a particular weight.The conference arbitrarily chose one meter stick and one sample weight from thirty nearly identical copies, declaring: “This prototype of the meter will from now forward represent, at the temperature of melting ice, the metric unit of length. . . .This prototype [of the kilogram] will be considered from now on the unit of mass.” Each delegate strode solemnly to receive his country’s copy of the iridium-platinum X-shaped bar, his nation’s exemplary weight.

Until the sanctioning ceremony, M and K had been but two of many carefully—measured standards. No one among them was yet crowned as the one perfect length or weight. All were certified to be within two ten-thousandths of a meter of all their siblings; this bar might be 1.0001 meters long compared with another, that one 0.9998 meters. But at 1:30 pm that afternoon, the officials loaded the chosen ones—M and K—into a triple-locked vault, M in a sealed, felt-lined brass cylinder and K in a triple bell jar. Standard-bearers then took K for burial, with K always surrounded by its six témoins. These copies of the kilogram were witnesses in the truest sense, chosen to bear witness with their very bodies should anything untoward befall the standard.

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