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July, 1930] REVOLVING LENS WHEEL PROJECTOR 35
known that much damage may be done to film during the rewinding operation, and the reason is apparent from an examination of the wound film when the take-up reel is removed from the lower magazine.
The ordinary take-up drive is a constant torque affair and is adjusted so that the initial winding on the take-up reel will not be sufficiently tight to cause damage to the film perforations on the lower or "hold back" sprocket. With this adjustment of the driving torque, the tension on the film strip, during the initial take-up when the wound diameter is small, will be considerably more than is necessary to produce a solidly wound mass, but, as the diameter of the wound mass increases, the film tension decreases, and, generally speaking, the outer layers of film, to a depth of one to three inches depending on the torque adjustment, are wound rather loosely. When the reel containing the film, which has been wound at a progressively decreasing tension, is put on the rewinder and the end of the film is attached to the empty reel and the motor is started with a jerk, it is not difficult to understand that film will be drawn from the reel before the reel begins to rotate, and there will be a progressively decreasing amount of slippage between film layers, from the outer layer down to that inner layer which has been wound at a sufficient tension to balance the pull of the power-driven reel on the rewinder. If the brake on the reel being rewound has not been released, the conditions will be aggravated and the slippage between film layers will go deeper into the reel. The film, in passing through a projector, collects dust, dirt, bits of emulsion, and other grit, which adhere to its surfaces, and when, in the rewinding operation, these surfaces slip over each other under pressure, the conditions are ideal for the most effective abrading. The film damage which shows up as "rain" and is most prevalent toward the end of a reel, is generally caused in this manner.
It is interesting to observe that the advent of sound-on-film prints has drawn more attention to the take-up conditions, and the improvement thus far made has taken the form of new type reels with larger diameter hubs and of more sturdy construction. These new reels, incidentally, cost several times as much as the better grade reels of a few years ago.
Our projector is equipped with a take-up control which may be adjusted to give a practically constant winding tension on the film for all diameters of the wound mass, and it will operate equally well