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Au revoir to Le Grand K: The chunk of metal that defined one kilogram has been retired

Scientific representatives from more than 60 countries have voted to change the definition of a kilogram.

The kilogram, the ubiquitous measurement instrumental to everyday life, was officially defined over 130 years ago.

Since its inception, the weight of a kilogram has been based on a platinum-based block called “Le Grand K” that is kept in a vault in Sévres, France.

That’s right. The entire unit of measurement is based off a single ingot locked away in France.

If you ever wanted to dispute the weight you put on after Christmas dinner or complain about exorbitant baggage prices, you would have to travel to France to compare your kilo to the official standard.

Now, the mass standard will be based on a fundamental factor in physics known as the Planck’s constant, and not on the little chunk of metal it has been based on for more than a century.

It sounds crazy, but that’s how the measurement is produced. There are exact copies around the world that companies use as reference. From bathroom scales to dumbbells, Le Grand K has been used as the standard since the late 19th century.

Now the way the kilogram is measured will change forever. A historic vote at the General Conference on Weights and Measures in Versailles has decided to get rid of Le Grand K.

The modern plan is to base the world’s system of mass measurement on the Planck constant, an infinitesimally small number that describes the behaviour of photons.

Researchers have spent years developing a new weighing machine called a Kibble balance that will put the old “Le Grand K” out of service after more than a century.

A Kibble (or watt) balance is an electromechanical measuring tool that measures the weight of an item by the power of the electric current and voltage necessary to make a compensating force.

Basically, it’s a lot more scientific than using a special chunk of platinum. The change means that for the first time in history the International Metric system will be based entirely on natural phenomena.

Le Grand K CG Image

A computer-generated image of the International Prototype kilogram – Le Grand K (Image: Wikipedia Commons)

The new definition will improve the current metric system without changing the actual value of any units. That means that unfortunately you won’t be dropping any kilos overnight.

“The vote went through unanimously. It was very emotional,” Stephan Schlamminger, a physicist from the US National Institute of Standards and Technology and a member of the team who has been working on redefining the kilogram for years, told The Guardian.

“They did a roll call of each country. One got a sense of how big metrology is.”

So, what’s with the sudden change? It turns out that the values of matter change through time and can get heavier or lighter depending on things like the number of atoms it absorbs in the air.

That’s what happened to Le Grand K. Scientists are convinced that the former standard now weighs less than what it did when it was officially registered back in 1889.

It goes without saying that a standard kilogram that is no longer a standard kilogram isn’t good for business, chief.

The problem highlights the biggest disadvantage of setting a standard on a material object; a problem that measuring with a Kibble balance addresses.

The updated criteria to define a kilogram will be effective on May 20, 2019. While that baggage dispute will continue for a few more Christmas seasons, the redefinition will make a world of difference for the manufacturing and chemical industries.

Three other standard units were redefined along with the kilogram. The Ampere, the Kelvin and the Mole will convert to using electromagnetic measuring devices.

Alor, au revoir Le Grand K! Vous nous avez bien servi! 

About the author

Filmmaker. 3D artist. Procrastination guru. I spend most of my time doing VFX work for my upcoming film Servicios Públicos, a sci-fi dystopia about robots, overpopulated cities and tyrant states. @iampineros

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