6′50 for a O, blink, a kilo is out the door


On Friday, representatives of more than 60 nations, gathered in Versailles, France, approved a new definition for the kilogram.

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. […]

Though the newly defined kilogram won’t affect your bathroom scale, it will have practical applications in research and industries that depend on meticulous measurement. […]

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.

So, in a 2014 meeting at the BIPM, the metrology community resolved to redefine the kilogram. 

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On April 7, 1795, the gram was decreed in France to be “the absolute weight of a volume of pure water equal to the cube of the hundredth part of the metre, and at the temperature of melting ice.”

Since trade and commerce typically involve items significantly more massive than one gram, and since a mass standard made of water would be inconvenient and unstable, the regulation of commerce necessitated the manufacture of a practical realization of the water-based definition of mass. Accordingly, a provisional mass standard was made as a single-piece, metallic artifact one thousand times as massive as the gram—the kilogram.

At the same time, work was commissioned to precisely determine the mass of a cubic decimetre (one litre) of water. Although the decreed definition of the kilogram specified water at 0 °C—its highly stable temperature point—the French chemist Louis Lefèvre-Gineau and the Italian naturalist Giovanni Fabbroni after several years of research chose to redefine the standard in 1799 to water’s most stable density point: the temperature at which water reaches maximum density, which was measured at the time as 4 °C. They concluded that one cubic decimetre of water at its maximum density was equal to 99.9265% of the target mass of the provisional kilogram standard made four years earlier.

That same year, 1799, an all-platinum kilogram prototype was fabricated with the objective that it would equal, as close as was scientifically feasible for the day, the mass of one cubic decimetre of water at 4 °C. The prototype was presented to the Archives of the Republic in June and on December 10, 1799, the prototype was formally ratified as the kilogramme des Archives (Kilogram of the Archives) and the kilogram was defined as being equal to its mass.

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Only three countries—Burma (Myanmar), Liberia, and the US—have not adopted the International System of Units as their official system of weights and measures.

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