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Part IV of discussion of basic mechanics of projector and camera
WHY MOVIES MOVE
I N THE preceding three articles in this series, we have covered the parts essen
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this series, we h
tial to the production of the cinematographic illusion of "moving" pictures, the intermittent and the shutter. To make machines for the production and exhibition of movies practicable, a few additional items are required to complete the mechanism.
We have demonstrated that the film moves intermittently through the gate. To provide a reasonable screen time, film is wound on a reel containing several hundred feet. Let us see what would happen were the film fed directly to the gate.
To the left of Fig. 1, the condition with the film just taut is shown. The intermittent next pulls one frame through the gate, and it will be obvious that this pull will be transmitted through the film to the reel, causing it to rotate. Now a full reel has considerable inertia so that it will continue to rotate after the manner of a flywheel until friction on its spindle brings it to rest. Although the film will continue to run through the gate, the feed reel will at first run faster than required, producing a free loop of loose film as shown in the second stage. As the reel gradually comes to rest, the loose film will be drawn into the gate until it is again taut as it was in the first position. The next movement of the intermittent will exert a tug on the film, and the whole cycle will repeat itself. At each jerk, the intermittent must overcome the inertia of the reel of film. This action imposes a severe strain on both film and intermittent mechanism which cannot be tolerated.
The trouble is cured by the insertion of a sprocket which runs at constant speed, between the feed reel and the gate. This draws the film from the feed reel at an even speed, without strain, and a small loop of film is left between the sprocket and the gate to accommodate the intermittent movement. The inertia of this short length of film is negligible and no strain is imposed upon the intermittent.
Similarly, to prevent drag from the takeup reel tending to pull the film too far through the gate and so upset the proper engagement of the claws, a takeup sprocket is interposed between the gate and takeup reel. The normal design for a cinematograph machine therefore takes the form shown in Fig. 2.
Unlike the feed reel which runs freely under the gentle pull from the film passing over the feed sprocket, the takeup reel must be driven. A glance at Fig. 3 will show how, as the film winds on, the effective diameter of the reel "core" increases. In consequence, a form of infinitely variable speed drive must be applied, first, to drive it fast, then slower and slower as [Continued on page 73]
• Fig. 1, showing the effect that the inertia of the reel would have if feed and takeup sprockets were not provided. Fig 2, normal design for projector film supply and takeup. Fig. 3, diameter of film "core" increases as film winds on takeup reel; so, variable speed drive must be applied. Fig. 4, basic standard design of projector dictated by mechanical requirements.
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