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Published on January 16, 2019

Wacky World of Rubber: Goodbye to Le Grand K

Deep in a vault just outside of Paris, a hunk of metal sits vacuum-sealed under three jars. This was arguably the most important piece of metal in the world.

Now, it's just another rock.

Officially known as the International Prototype Kilogram, or "Le Grand K," this piece of metal has been the universal definition of a kilogram since it was forged 1889. The cylinder of platinum-iridium alloy has served as the benchmark for all other weights are compared.

Le Grand K was so important that it was only removed from its enclosure once every 40 years so that its mass can be checked against sister copies housed in other locations across the globe. The process is cumbersome because even hydrocarbons on fingertips or moisture in the air could contaminate the original's surface, which would change the world's definition of the kilo.

That is exactly why scientists have become concerned. Obviously, the mass of a kilogram cannot change even if its benchmark did. And despite their best efforts to keep Le Grand K preserved, it has been losing weight. At its last weigh in 20 years ago, it was found to be 0.05 milligrams lighter than its replicas.

Experts had varying theories, that its replicas were gaining weight since they were handled more often or that Le Grand K was outgassing—air gradually escaping the metal. Granted the difference amounted to basically a grain of sand, but when we're talking about every single scale worldwide, even the smallest change could lead to a massive (pun intended) problem.

The world needed a better way or else a kilogram in Europe wouldn't be the same as the U.S., Asia, South America or Africa. With several different definitions, different countries would need to adjust prices for, say, metric tons of rubber depending on what regional scale is used to determine the amount. A company in the U.S. weighs it and charges Price A, but once shipped and checked in Europe it turns out it was only worth Price B. This is starting to sound like something a James Bond villain would think up.

Scientists had to do something, so they turned to…science, of course. In November, hundreds of scientists gathered for the 26th General Conference on Weights and Measures and voted to replace Le Grand K with a universal formula that defines the kilogram with the quantum laws of nature. Instead of being linked to the mass of one object, the kilogram will be defined by the Planck constant—the ratio of quantum energy a frequency of light can carry to that same frequency.

But while the pressure is off Le Grand K, it's still not getting a vacation. Scientists will keep it and its six official copies locked up in the same conditions and continue to study just how their masses decay over time. With 130 years of data in the books, why stop now?

Though maybe it will get out a little more now that it's not literally carrying the weight of the world.

Chris Sweeney definitely gets more vacation days than Le Grand K. Follow him on Twitter @CSweeneyRPN.

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