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For 21 Years, No-One In Britain Knew How Long An Inch Was

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Great idea, Tom. Film in front of Parliament. It’s imposing. It’s relevant to the story. It’ll look great. It is covered in scaffolding. When you buy a tape measure, that tape measure had to be  checked and calibrated with equipment at the tape measure factory. And that manufacturing equipment had to be calibrated by other calibration equipment at its factory. And that equipment had to be calibrated, and so on, and so on, and so on, until at some point, that chain has to stop, there has to be an organisation whose job it is to say: “you don’t need to calibrate any more. Here is the truth.” These days, that job’s done at the National Physical Laboratory in the UK, or the National Institute of Standards and Technology in the US.

I’ve actually been lucky enough to visit both of those in the past, so I’ll put links to those videos at the end of this one. The international standard for length is the meter, and it’s based on unchanging physical constants: a meter is defined as how far light travels in a vacuum in  a certain fraction of a second. Is that all in frame? I’ve got no idea. I can’t see my screen. But anyway, ever since the mid-20th century, the international agreement has been that an inch is exactly 25.4 millimetres. Which means yes, the US and imperial systems of measurement, inches, feet, yards, miles: they’re all based on metric now. But before all those modern standards were set, those systems of measurement were based on prototypes. Not ‘prototypes’ as in ‘early test versions’, but as in physical artifacts which were, by definition, the length they said they were. “The metre” was the distance between two marks on a  platinum-iridium bar stored in Paris. If those marks somehow changed, then the definition of the metre changed with them.

Before we went metric, Britain had physical standards for the  old imperial units, too, dating back centuries. There are references to various standards all through history,  but as far as I could tell, the first prototype that could reasonably be called a British national standard was a brass rod that was created in 1760. That standard was eventually set in law: in 1825, the yard, so three feet, 36 inches, was defined  as being the distance between the marks on that one object, stored with the Clerk of the House of Commons, in Parliament. Nine years after that law was passed… Parliament burned down. They were trying to burn old records that were on wooden “tally-sticks”, and the fire got out of control. That famous Parliament building, the one that’s behind the scaffolding, it isn’t some ancient castle: heck, the US Capitol Building is older than what’s behind there.

Only a few parts from medieval times are still standing: the rest was built during Queen Victoria’s reign, after the fire. Anyway, all the standards for imperial units were also destroyed in that fire. The ‘standard Yard’ had been partly melted. Which meant Britain didn’t have any legal standard of measurement. But that was fine! Because the law also had a provision  for what to do in that case: you make a new Standard Yard, defined by the length of a pendulum that takes a certain amount of time to swing. Great idea. More than a century early, they were defining the measurements by physical constants! Just one problem: those definitions were wrong. Or as the law later put it, “by the Researches of scientific Men Doubts were thrown on the Accuracy”. Those Scientific Men were the Standards Commission, headed by Astronomer and Mathematician Sir George Airy. That commission was in charge of re-establishing the lost units, and their final report made it clear: the government had put a dodgy experiment into law.

It wasn’t accurate enough. So instead, the standard was reconstructed based on all the copies and  other versions that had been made over all the years, all the objects, not like tape measures, or anything like that, but the things that were one step down that chain of calibration. So by 1855, a new physical yard had been constructed, made of bronze, and the law was changed to define the idea of a  “yard” as the distance between two gold studs on that new bar. There were a few other suggestions in the commission’s report: they strongly argued for the decimalisation of units and currency, which wasn’t successful for a century. But they also said that there should be a public version of the standards in every large town: not because they’d be perfectly accurate, but because it would make it really really obvious if a shopkeeper with a dodgy tape measure was trying to cheat you.

Those standards still exist in a few places. In London, you can find them in Trafalgar Square, in the Guildhall, and by the gate of the Royal Observatory. So if you have bought lunch from, let’s say, a certain shop and want to check if your “footlong sandwich” really is a foot long, you can do that. And I did. So: was the new yard the same distance as the old one? Well, it would be difficult to mess it up too much, there were a lot of copies of the original to work from. But every time you make an analogue copy of something, you do lose a little bit of precision. Before the 1834 fire, the yard was the length of one particular block of metal; after 1855, it was the length of another block. In between: well, you could take a pretty accurate guess, but that chain of calibration, that source of truth, was broken.

For twenty-one years: no-one knew how long a yard, a foot, or an inch actually was..

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