It used to be that movie theaters were set up with two projectors in each house and projectionists had to swap between those projectors to change the film reels about every 20 minutes or so when they would run out.
35 mm prints today are still shipped on reels of about 20 minutes each, but when they arrive at the theater, a projectionist splices them all together into one giant reel, like the one on the bottom platter in the photo. A two-hour movie weighs about 80 pounds.
The beginning of the movie is at the inside of the print. To feed it out, the projectionist threads the film through a regulator known as the brain. The tension on the brain causes the platter to spin faster when more film is needed and slow down when too much film is already available.
The film then travels from the brain to the projector, runs through the projector so that the audience can see it, and returns to another platter that spools up the entire print as it plays. That platter winds up the film the same way, with the beginning at the inside, so that when the movie is over, the print is ready to go again right away from the second platter. Nothing has to be rewound.
In fact, you can't rewind a 35mm print like this, a fact that sometimes upsets moviegoers when there has been a problem with the film that caused them to miss part of it.
But thanks to platter systems, problems that shut down the projector happen less often than they once did. And projectionists don't have to change the reels anymore.
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