The Latham Loop

The take-up reel at the bottom is pulling the filmstrip down at a continuous speed. Meanwhile, the intermittent-movement control device (the drive shaft mechanism to the left of the lower reel—see Figure 1.32) is retarding the filmstrip’s movement several times each second. Obviously, pressure will be exerted on the filmstrip whenever the reel is pulling it forward and the control device is holding it in place. The solution is to loop the filmstrip at two points: above and below the gate (which is located behind the lens, between the two reels). When the film stops in the gate, the upper loop (as here) gets larger while the lower loop grows taut. When the filmstrip is advanced through the gate, the lower loop enlarges by taking up the slack from the upper loop.

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