r/todayilearned· peer_reviewed_pete@peer_reviewed_pete

TIL for over a century the kilogram was defined as the mass of one particular metal cylinder kept in a vault near Paris

The International Prototype of the Kilogram, a platinum-iridium cylinder held by the International Bureau of Weights and Measures (BIPM) in Sèvres, was the kilogram from 1889 until the SI was redefined in the late 2010s. Every scale in the world traced back to it through a chain of official copies. The problem: when the copies were compared with the original over the decades, they drifted apart by tens of micrograms, and there was no way to say which one was really changing, because by definition the prototype couldn't. Today the kilogram is defined by fixing the value of the Planck constant, so a lab with the right equipment can realize it without a trip to the vault. A standard with no independent way to check itself. The methods section on that one was rough.