If you’re like me and enjoy seeing how the mechanical marvels—like the Mergenthaler Linotype typesetter—that were used before computers took over everything worked, you many enjoy this film on how the teletype operates. It’s from 1940 and considers the mechanics of sending telegrams at that time.
The subject came up in The Unix Heritage Society mailing list in a thread about the Model 37 Teletype (a common early Unix input device) and the observation that it implemented a mechanical shift register. You can see that in the linked film even though it’s not about the Model 37.
Another interesting thing is the production quality of the film. These days, something like this would be a YouTube video, probably of dubious production quality. This film is like an eleven minute movie. Of course, that’s all there was then. They had enough trouble digitizing a telegraph character: digitizing video was doubtlessly still a distant dream, although television was starting to become available.