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A WEIGHT, LIFTED

The weight formerly known as the kilogram is dead

A photograph of the International Prototype Kilogram inside of its double-glass-domed case.
AP Photo/Christophe Ena
The weight formerly known as kilogram.
By Natasha Frost
Published Last updated This article is more than 2 years old.

As of today (Nov. 16), the kilogram is dead. The International Prototype “king” Kilogram, held in sanctitude in Paris, has been stripped of its title; a new system for deciding the weight of a kilogram, based on electric current, now takes its crown.

One by one, delegates at the annual General Conference on Weights and Measures stood to voice their vote. They unanimously decided to define one kilogram as the amount of mass needed to balance a scale being pulled by an electromagnet using a specific amount of electricity. Weight is now defined using a fundamental constant in nature, instead of a hunk of metal in a vault.

The change has no measurable effect. Under the new definition of a kilogram the International Prototype weighs precisely as much as before. Instead of calculating weight by comparing everything to that metal cylinder, scientists now define the weight based on electrical force. That force is measured using Planck’s constant, effectively the smallest amount of energy possible. If the International Prototype was damaged, lost, or destroyed, the world would still be able to standardize weights.

It’s a historic day worth remembering. This weighty topic can finally be celebrated with appropriately measured excitement.

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