The practice of shipping and projecting motion pictures on 2,000-foot reels, and employing "changeovers" between two projectors, was due to the carbon rods used in projector lamphouses having a lifespan of roughly 22 minutes

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newinsightfulfun

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The projectionist would replace the carbon rod when changing film reels. The two-projector changeover setup largely disappeared in the 1970s with the advent of xenon projector lamps, being replaced with single-projector platter systems, though films would continue to be shipped to cinemas on 2,000-foot reels.

[–]jagworms 2 insightful - 1 fun2 insightful - 0 fun3 insightful - 1 fun -  (0 children)

By that time, the Reel MFG industry had set the standard on how much film to move per batch. Retooling is a bitch. It's funny how many things are the size/shape they are because of things that no longer matter, but people are still used to measuring them in that way, because those are the UNITS of the industry.