The whole piece is worth your time to read, but here's the essence of brain wrap:
> Matt also shows me how to work the platter system. Forget everything about film being stored in upright reels, the classic two-humped silhouette of the old-school projector. In the modern multiplex, film is stored off to the side of the projector on horizontal platters. This saves theaters time by eliminating the hassle of rewinding. A basic reel-to-reel system can only move film from the outside of one reel to the inside of another, reversing its order in the process. In a platter system, film is pulled up from the inside of one platter, run through the projector, and collected on the inside of another platter, ready to be shown again. Of course, every solution carries along its own problems, and the problem with spooling film from the inside can be best illustrated by wrapping a string around your finger, then yanking on it. Instead of unwrapping neatly, the coils of string just get pulled tighter around the center. Similarly, a projector’s endless hunger for film can pull the reel into thousands of tight, black ringlets. If these twists ever enter the projector, you have a gigantic mess on your hands.
> The machine responsible for preventing this disaster is called the brain, a squat cylinder that sits at the center of the sending platter. Film moves from the inside of the reel, through the brain, and out to the projector. When the brain’s arm is up, the projector pulls film as normal, but now the film coils around the brain instead of around itself. When the tension from this wrap builds up, it pulls the brain-arm down, telling the platter to rotate in reverse and unwrap the excess film. The brain can malfunction in many ways, but the most common is called a “brain wrap.” This happens when the brain fails to sense the tension building up around it and allows the film to wrap forever. Friction builds up and the rate of film going to the projector slows, or tries to slow. Projectors must take in film at a rate of 24 frames per second, and they will bring the entire booth crashing down around them before accepting anything slower. The film may slow down, but the sprockets keep their own time, and their teeth will shred any film that resists. Usually, the film just breaks, but sometimes, Matt tells me, the friction can build up so high that it burns instead.
http://www.hypocritereader.com/11/brain-wrap
The whole piece is worth your time to read, but here's the essence of brain wrap:
> Matt also shows me how to work the platter system. Forget everything about film being stored in upright reels, the classic two-humped silhouette of the old-school projector. In the modern multiplex, film is stored off to the side of the projector on horizontal platters. This saves theaters time by eliminating the hassle of rewinding. A basic reel-to-reel system can only move film from the outside of one reel to the inside of another, reversing its order in the process. In a platter system, film is pulled up from the inside of one platter, run through the projector, and collected on the inside of another platter, ready to be shown again. Of course, every solution carries along its own problems, and the problem with spooling film from the inside can be best illustrated by wrapping a string around your finger, then yanking on it. Instead of unwrapping neatly, the coils of string just get pulled tighter around the center. Similarly, a projector’s endless hunger for film can pull the reel into thousands of tight, black ringlets. If these twists ever enter the projector, you have a gigantic mess on your hands.
> The machine responsible for preventing this disaster is called the brain, a squat cylinder that sits at the center of the sending platter. Film moves from the inside of the reel, through the brain, and out to the projector. When the brain’s arm is up, the projector pulls film as normal, but now the film coils around the brain instead of around itself. When the tension from this wrap builds up, it pulls the brain-arm down, telling the platter to rotate in reverse and unwrap the excess film. The brain can malfunction in many ways, but the most common is called a “brain wrap.” This happens when the brain fails to sense the tension building up around it and allows the film to wrap forever. Friction builds up and the rate of film going to the projector slows, or tries to slow. Projectors must take in film at a rate of 24 frames per second, and they will bring the entire booth crashing down around them before accepting anything slower. The film may slow down, but the sprockets keep their own time, and their teeth will shred any film that resists. Usually, the film just breaks, but sometimes, Matt tells me, the friction can build up so high that it burns instead.