Versailles, France—A convocation of delegates representing 60 countries voted today, 16th November 2018 in Versailles to implement the most significant change to the International System of Units (SI) in more than 130 years. For the first time, all measurement units will be defined by natural phenomena rather than by physical artifacts. The event was the 26th General Conference of Weights and Measures and was hosted by the International Bureau of Weights and Measures.

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After decades of groundbreaking scientific work by national measurement institutes (NMIs) from around the world, the delegates have voted to redefine the kilogram and three additional basic SI units: electric current (ampere), temperature (kelvin) and amount of substance (mole). The new definitions will be effective on May 20, 2019, World Metrology Day, which celebrates the establishment of the SI, or metric system, in 1875.