AaronRaff sez, "My grandpa worked up in the booth as a movie projectionist for over 25 years in Queens, NY. In this video he explains how projectionists would rig up a bell to ring when reels were reaching their end so the projectionist could prepare for a reel change. He said these mechanisms were responsible for a lot of those vertical scratches you'd sometimes see in movies. You'd get scratches when the ball bearing that runs along the film's surface would get insufficiently lubricated and create long running scratches across the surface of the film."
Projectionist trick: Reel alarm bell
(Thanks, AaronRaff!)
