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How and why we redefined the kilogram

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The kilogram used to be a metal object, but now it’s a constant of nature

An ounce is an ounce, and a kilogram is a kilogram, but how are these units actually measured? How do your scales know how much a pound or a gram weighs?

Since 1889, the kilogram’s definition has been based on a metal artifact stored under lock and key in a vault near Paris. It’s called the International Prototype Kilogram, or Le Grand K, and it is the world standard. What it weighs, the kilogram weighs. No more, no less.

Le Grand K was created (along with the rest of the metric system) during the French Revolution when scientists and revolutionaries were united in their desire to remake the world. This meant reforming the French language, the calendar, and the country’s weights and measures. The idea was to make these units accessible to every citizen and consistent across different nations. It was a utopian project.

Read more: The kilogram is dead; long live the kilogram

The original International Prototype Kilogram was cast in 1889 and is kept in a trio of vacuum-sealed bell jars in a vault near Paris.
Photo: BIPM

But in November, scientists voted to redefine the kilogram, replacing the International Prototype Kilogram with a definition based on a constant of nature. They did this partly because the kilogram artifact was losing weight, which was causing trouble with international calibration, but also because they wanted to fulfill the mission laid out by those 18th century revolutionaries.

By defining the kilogram using a constant of nature, they are freeing it from physical constraints. You’ll still need some pretty complex machinery to actually measure the new kilogram, but, theoretically, anyone can do it. The kilogram is now truly accessible to all.