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.)
Edward Walford’s hefty book Old and New London noted in 1878 how “Any one who desires to secure an accurate yard-measure may do so by carrying to Greenwich a rod about a yard long, and truly adjusting it by means of the appliance there exposed for the public benefit.”
In addition to their functional use as public backup rulers, the Standards of Length also had a very important public relations purpose. The work of the Royal Observatory was at times criticized as an expensive scientific boondoggle, lacking in military or economic purpose. The Observatory was eager to highlight the real world benefits of their work, and the plaque was one high visibility way to drive the point home.
The notion of a public Standard is very much a product of that time. As the Industrial Revolution spread across the globe, a fortuitous scientific-economy complex spurred the development of increasingly accurate commercial and navigational technology. The exact length of a yard gained increased importance in an era of interoperable parts and mass production.
While the Royal Observatory Greenwich’s astronomers have long since decamped for more isolated facilities, the Public Standards of Length remain as a monument to the virtues of precision, consistency, and interoperability.
Know Before You Go
The measures are located outside the observatory complex on the right hand side of the gates that face the famous brand meridian line. Look underneath the clock.