A typical movie projector has two rotating reels, or powered spools for unwinding and winding film. For horizontally oriented projectors, the feed reel is usually on the right side, just above the lamp and lens. For vertically oriented movie projectors, the feed reel is usually on top. The take-up reel is an empty reel located opposite the feed reel. As implied by its name, the take-up reel ''takes up'' the parts of the film emerging from the projector and winds them for safekeeping. At the end of a screening, a projectionist can return the film on the take-up reel to the feed reel using a movie projector's rewind function.
When film stock enters a projector during operation, the drive mechanism takes over. A typical movie projector drive mechanism uses a series of sets of interlocking, helical gears outfitted with sprockets to move the film stock through the unit along the film path at a steady rate. The sprockets match the size and spacing of holes on a specific gauge of film, from small gauge formats like 8 mm and super 8 to oversize formats like 70 mm. The vast majority of movie projectors work for only one gauge of film. However, consumer model "dual 8" projectors have two sets of gears -- one for standard 8 mm film and one for super 8 mm film.
Invented by engineers working under Woodville Latham, the Latham Loop is a concave area of the film path that precedes the projector lamp and reduces tension on the film. By allowing a slack loop of film to form, the Latham loop translates the intermittent movement of the projector's electric motor into steady movement across the film gate. This guarantees that the film speed of a projector will remain constant.
The optical elements of a projector include the lamp, parabolic mirror, shutter and lens. The projector holds each film frame in place momentarily as it passes the gate. The shutter opens and light from the lamp bounces off of the mirror and through the film and lens, projecting an image. In most movie projectors, the lamp and drive mechanism are on separate switches so the projectionist can turn the lamp on and off as the projector itself continues to run.