Have you ever wondered who decides how much a kilogram weighs?  The answer is the General Conference on Weights and Measures, which meets every few years.  In November 2018, it officially changed the definition of the kilogram.

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For over a century, there has been a single cylinder of platinum-iridium locked away in Paris called the “International Prototype Kilogram,” or IPK for short.  Not only does it weigh a kilogram—it defines the kilogram.  If anything doesn’t weigh the same as the IPK, then it isn’t a kilogram.

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But in 1989, disaster struck: Scientists discovered that the IPK was 50 micrograms lighter than its own replicas.  Since the IPK was the very definition of a kilogram, all of the replicas were now the wrong mass.  Understandably, this was a huge issue.  The kilogram was now a different weight than it had been for a hundred years!  Scientists needed a standard for mass that would not change.

It’s electric

Since a human-made object could change over time, scientists searched the natural world for a new definition of the kilogram.  Finally, they found an answer. The key is a tool called a “Kibble Balance.”

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In short, a Kibble Balance uses electricity and magnetism to measure mass.  Imagine a scale someone would keep in their bathroom.  But instead of saying how many pounds someone weighs, the scale tells how much electric current it has to run through a coil of wire in a magnetic field in order to hold them up.  Starting in May 2019, the definition of the kilogram will be in terms of electric current.  The IPK will be officially out of commission.

A new chapter

For the first time in history, all of the official units of measurement (distance, mass, time, luminosity, electric current, temperature, and substance) will be defined in terms of natural constants.  Gone are the days of needing to lock away a cylinder in a vault in Paris.

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In the words of the U.S. National Institute of Standards and Technology, this is “a turning point for humanity.”