We've had atomic clocks for so long that we take them for granted, putting them into preamps and looking to replace them with even more accurate nuclear versions. Fifty years ago, though, the first accurate atomic clock was only a few years old, and the UK's National Physical Library made the short film below to explain how it worked. There are plenty of other videos on the YouTube channel as well, including this two-minute one about mapping sound with lasers, but sadly most of them lack the superlatively authoritative narrator of the atomic clock piece.