# The kilogram is being redefined as a fundamental constant, not just a chunk of metal

Scientists from more than 60 countries agreed to change the definition of the kilogram during a conference on Friday in Versailles, France.

**The history:** The kilogram was originally defined as being equivalent to the mass of a liter of water. Since 1889 scientists have based its definition on a physical object: a platinum alloy cylinder known informally as “Le Grand K,” which was kept in a safe in Paris. An obvious drawback was that if the cylinder changed, so did the entire global system of measurement. When it was measured in 1980s, it weighed several micrograms less than it was supposed to, forcing manufacturers who made products based on the standards to reissue their weights.

**What happens now?** A kilogram will now be measured in terms of the amount of electricity needed to counteract the weight. The quantity that relates weight to electrical current is named Planck’s constant, after the German physicist Max Planck, and denoted by the symbol *h*. A super-accurate set of scales, called a Kibble balance, can measure mass to an accuracy of about one part per billion by using a fixed value of *h*, which scientists have already measured extremely precisely.

**The new system’s advantages:** Rather than measuring all kilograms against one single object—Le Grand K—the new definition means anyone with a Kibble balance can check their weights. And this means that rather than being based on an arbitrary, physical object that can change over time, definitions are based on the fundamental constants of nature.

*Correction: An earlier version of this post incorrectly described the use of the Kibble balance.*