WEBVTT Kind: captions Language: en 1 00:00:00.000 --> 00:00:02.168 Great idea, Tom. Film in front of Parliament. 2 00:00:02.168 --> 00:00:04.335 It's imposing. It's relevant to the story. It'll look great. 3 00:00:04.335 --> 00:00:06.240 It is covered in scaffolding. 4 00:00:06.240 --> 00:00:09.060 When you buy a tape measure,  that tape measure had to be   5 00:00:09.060 --> 00:00:11.850 checked and calibrated with equipment at the tape measure factory. 6 00:00:11.850 --> 00:00:14.160 And that manufacturing equipment had to be calibrated 7 00:00:14.160 --> 00:00:16.980 by other calibration equipment at its factory. 8 00:00:16.980 --> 00:00:19.888 And that equipment had to be calibrated, and so on, and so on, and so on, 9 00:00:19.890 --> 00:00:22.635 until at some point,  that chain has to stop, 10 00:00:22.635 --> 00:00:24.930 there has to be an organisation whose job it is to say: 11 00:00:24.930 --> 00:00:28.623 "you don't need to calibrate any  more. Here is the truth." 12 00:00:28.623 --> 00:00:31.350 These days, that job's done at the National  Physical Laboratory in the UK, 13 00:00:31.350 --> 00:00:34.020 or the National Institute of  Standards and Technology in the US. 14 00:00:34.020 --> 00:00:36.775 I've actually been lucky enough to visit both of those in the past, 15 00:00:36.780 --> 00:00:38.850 so I'll put links to those  videos at the end of this one. 16 00:00:38.850 --> 00:00:42.060 The international standard  for length is the meter, 17 00:00:42.060 --> 00:00:46.080 and it's based on unchanging  physical constants: 18 00:00:46.080 --> 00:00:49.800 a meter is defined as how far  light travels in a vacuum in   19 00:00:49.800 --> 00:00:51.180 a certain fraction of a second. 20 00:00:51.180 --> 00:00:54.095 Is that all in frame? I've got no idea. I can't see my screen. 21 00:00:54.095 --> 00:00:55.530 But anyway, ever since the mid-20th century, 22 00:00:55.530 --> 00:01:00.390 the international agreement has been that an inch is exactly 25.4 millimetres. 23 00:01:00.390 --> 00:01:03.360 Which means yes, the US and imperial  systems of measurement, 24 00:01:03.360 --> 00:01:06.870 inches, feet, yards, miles: they're all based on metric now. 25 00:01:06.870 --> 00:01:09.180 But before all those  modern standards were set, 26 00:01:09.180 --> 00:01:12.030 those systems of measurement  were based on prototypes. 27 00:01:12.030 --> 00:01:14.490 Not 'prototypes' as in  'early test versions', 28 00:01:14.490 --> 00:01:17.490 but as in physical artifacts  which were, by definition, 29 00:01:17.490 --> 00:01:19.410 the length they said they were. 30 00:01:19.410 --> 00:01:22.110 "The metre" was the distance  between two marks on a   31 00:01:22.110 --> 00:01:24.210 platinum-iridium bar stored in Paris. 32 00:01:24.240 --> 00:01:26.250 If those marks somehow changed, 33 00:01:26.250 --> 00:01:29.340 then the definition of the  metre changed with them. 34 00:01:29.340 --> 00:01:32.310 Before we went metric, Britain  had physical standards for the   35 00:01:32.310 --> 00:01:34.410 old imperial units, too,  dating back centuries. 36 00:01:34.410 --> 00:01:37.320 There are references to various  standards all through history,   37 00:01:37.320 --> 00:01:38.670 but as far as I could tell, 38 00:01:38.730 --> 00:01:43.238 the first prototype that could reasonably be called a British national standard 39 00:01:43.238 --> 00:01:46.620 was a brass rod that was created in 1760. 40 00:01:46.620 --> 00:01:50.130 That standard was eventually  set in law: in 1825, the yard, 41 00:01:50.130 --> 00:01:52.590 so three feet, 36 inches, was defined   42 00:01:52.590 --> 00:01:55.590 as being the distance between  the marks on that one object, 43 00:01:55.590 --> 00:01:58.950 stored with the Clerk of the  House of Commons, in Parliament. 44 00:01:58.950 --> 00:02:03.009 Nine years after that law was passed... Parliament burned down. 45 00:02:03.009 --> 00:02:05.409 They were trying to burn old records  that were on wooden "tally-sticks", 46 00:02:05.430 --> 00:02:06.900 and the fire got out of control. 47 00:02:06.900 --> 00:02:10.055 That famous Parliament building, the one that's behind the scaffolding, 48 00:02:10.055 --> 00:02:12.480 it isn't some ancient castle: heck, 49 00:02:12.480 --> 00:02:15.360 the US Capitol Building is  older than what's behind there. 50 00:02:15.360 --> 00:02:17.850 Only a few parts from medieval  times are still standing: 51 00:02:17.850 --> 00:02:20.850 the rest was built during Queen  Victoria's reign, after the fire. 52 00:02:20.850 --> 00:02:23.250 Anyway, all the standards  for imperial units 53 00:02:23.250 --> 00:02:25.170 were also destroyed in that fire. 54 00:02:25.170 --> 00:02:28.350 The ‘standard Yard’  had been partly melted. 55 00:02:28.350 --> 00:02:32.040 Which meant Britain didn't have  any legal standard of measurement. 56 00:02:32.040 --> 00:02:34.980 But that was fine! Because  the law also had a provision   57 00:02:34.980 --> 00:02:38.040 for what to do in that case:  you make a new Standard Yard, 58 00:02:38.040 --> 00:02:42.690 defined by the length of a pendulum that  takes a certain amount of time to swing. 59 00:02:42.690 --> 00:02:44.880 Great idea. More than a century early, 60 00:02:44.880 --> 00:02:49.500 they were defining the measurements by  physical constants! Just one problem: 61 00:02:49.500 --> 00:02:52.020 those definitions were wrong.  Or as the law later put it, 62 00:02:52.020 --> 00:02:55.650 "by the Researches of scientific Men  Doubts were thrown on the Accuracy". 63 00:02:55.650 --> 00:02:58.800 Those Scientific Men were  the Standards Commission, 64 00:02:58.800 --> 00:03:01.110 headed by Astronomer and  Mathematician Sir George Airy. 65 00:03:01.110 --> 00:03:04.770 That commission was in charge of  re-establishing the lost units, 66 00:03:04.770 --> 00:03:06.600 and their final report made it clear: 67 00:03:06.600 --> 00:03:09.900 the government had put a  dodgy experiment into law. 68 00:03:09.900 --> 00:03:12.030 It wasn't accurate enough. So instead, 69 00:03:12.090 --> 00:03:14.670 the standard was reconstructed  based on all the copies and   70 00:03:14.670 --> 00:03:16.890 other versions that had  been made over all the years, 71 00:03:16.890 --> 00:03:19.675 all the objects, not like tape measures, or anything like that, but 72 00:03:19.675 --> 00:03:23.025 the things that were one step  down that chain of calibration. 73 00:03:23.040 --> 00:03:25.950 So by 1855, a new physical yard had  been constructed, made of bronze, 74 00:03:25.950 --> 00:03:28.560 and the law was changed  to define the idea of a   75 00:03:28.560 --> 00:03:32.160 "yard" as the distance between  two gold studs on that new bar. 76 00:03:32.160 --> 00:03:34.770 There were a few other suggestions  in the commission's report: 77 00:03:34.770 --> 00:03:38.820 they strongly argued for the decimalisation of units and currency, 78 00:03:38.820 --> 00:03:41.296 which wasn't successful for a century. 79 00:03:41.310 --> 00:03:44.040 But they also said that there should be a public version of the standards 80 00:03:44.040 --> 00:03:45.570 in every large town: 81 00:03:45.570 --> 00:03:47.490 not because they'd be  perfectly accurate, 82 00:03:47.490 --> 00:03:49.997 but because it would make it really really obvious 83 00:03:50.010 --> 00:03:53.670 if a shopkeeper with a dodgy tape  measure was trying to cheat you. 84 00:03:53.670 --> 00:03:57.240 Those standards still exist  in a few places. In London, 85 00:03:57.240 --> 00:04:00.120 you can find them in Trafalgar Square, in the Guildhall, 86 00:04:00.120 --> 00:04:02.100 and by the gate of  the Royal Observatory. 87 00:04:02.100 --> 00:04:07.410 So if you have bought lunch from, let's say, a certain shop and want to check if 88 00:04:07.410 --> 00:04:11.490 your "footlong sandwich" really  is a foot long, you can do that. 89 00:04:11.490 --> 00:04:12.750 And I did. 90 00:04:12.750 --> 00:04:15.960 So: was the new yard the same  distance as the old one? Well, 91 00:04:15.960 --> 00:04:17.520 it would be difficult to  mess it up too much, 92 00:04:17.520 --> 00:04:19.650 there were a lot of copies  of the original to work from. 93 00:04:19.650 --> 00:04:22.620 But every time you make an  analogue copy of something, 94 00:04:22.620 --> 00:04:24.450 you do lose a little bit of precision. 95 00:04:24.450 --> 00:04:28.800 Before the 1834 fire, the yard was the  length of one particular block of metal; 96 00:04:28.830 --> 00:04:32.040 after 1855, it was the length of  another block. In between: well, 97 00:04:32.040 --> 00:04:33.750 you could take a pretty accurate guess, 98 00:04:33.750 --> 00:04:36.330 but that chain of calibration,  that source of truth, 99 00:04:36.330 --> 00:04:38.760 was broken. For twenty-one years: 100 00:04:38.940 --> 00:04:44.070 no-one knew how long a yard, a foot, or an inch actually was.