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Atlas Obscura Public Standards of Length

This century-and-a-half old brass placard outside the Royal Observatory in Greenwich was originally installed for Brits eager for a way to officially verify their measuring sticks. Located alongside the Shepherd 24-hour Slave Clock and the green Meridian Laser, an inquisitive visitor can hypothetically calibrate a lab full of scientific instruments using nothing more than this public display.

The Public Standards of Length at Greenwich measures the official lengths of the yard, foot, and inch. It was first placed in 1866, and the placard received a “recalibration” every 10 years by the Standard Department of the Board of Trade. The Standards were determined with an obsessive degree of craftsmanship and were designed to be used to their fullest accuracy at 15.5° Celsius. (Any warmer or colder and the rulers’ thermal expansion and contraction gives off a misleading measurement.)

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