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

Regarding the title, one could argue that no one in Britain knows what an inch is now, because they use meters and centimeters. But that's not what this is about. Any measurement must have standards. While measurements are now defined by physics and can be accurately recreated, those standards were once physical. So what happens when the standard prototype is destroyed? That happened when the British Parliament building burned down in 1834. Tom Scott tells the story, and a short history of measuring standards.