Author: Emily Conover / Source: Science News
Out with the old — kilogram, that is.
Scientists will soon ditch a specialized hunk of metal that defines the mass of a kilogram. Oddly enough, every measurement of mass made anywhere on Earth is tied back to this one cylindrical object. Known as “Le Grand K,” the cylinder, cast in 1879, is kept carefully sequestered in a secure, controlled environment outside Paris.
On November 16, at a session of the 26th General Conference on Weights and Measures in Versailles, France, representatives of countries from around the world voted to kick that convoluted system to the curb, enacting a plan to redefine several units of measurement, including the kilogram (SN: 11/12/16, p. 24).
For a small cadre of scientists called metrologists — those who specialize in the science of measurement — it’s a big day. “It’s about as excited as you’re going to see metrologists get,” says David Newell of the National Institute of Standards and Technology in Gaithersburg, Md. He has spent much of his career working toward the change. “I can’t believe we’re finally getting it done.”
On May 20, 2019, Le Grand K will lose its special status, and the mass of a kilogram will be defined by a fundamental constant of nature known as the Planck constant. At the same time, other mainstays of the metric system will also be revamped: the ampere (the unit of electric current), the kelvin (the unit of temperature) and the mole (the unit for amount of substance).
Now, instead of being based on arbitrary quantities or physical artifacts that might change over time, “all the definitions will be based on what we call the fundamental constants of nature,” says metrologist Estefanía de…
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