I just discovered a film produced by the National Physical Laboratory (NPL) Film Unit in the 1950s explaining the principles behind the first accurate atomic clock. The clock was designed by Louis Essen and built at the National Physical Laboratory in 1955.
This film gives fascinating insight into the physics that run atomic clocks like those behind the WWV and CHU time stations. Better yet, the science is easy to swallow with the traditional “BBC” style voice narration. Enjoy!