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Fig. 5. Endless-loop film magazine.
For example, just before the detent is withdrawn, the lower sprocket has fed all except about a half frame out of the lower loop so that this loop is nearly flat. As a result, the film tension is considerably increased, a greater force is developed to overcome gate friction, and a greater initial acceleration occurs than would result with the semicircular loops. Similarly, the upper loop flattens as the pulldown stroke nears the end and this, together with the reaction of trapped air between the loop and fixed track, helps to decelerate the film. Controf of the cushioning effect is afforded by
proper sizing of the hole leading through the fixed track from under the loop to the outside.
The movable detent D is of such small size and mass that only a relatively small pressure of the spring S is needed to make it follow the cam C. For this reason, the point of the detent does not damage the film if at any time it should fail to enter the perforation and instead rub against the moving film. Test runs have shown consistently that film life with or without the detent in operation is the same.
Figure 4 is from a photograph of this projector showing the pulldown mechanism. Figure 5 shows the projector mounted in the cabinet. Space does not permit a description of all the auxiliary devices associated with this machine but it may be of interest to consider the continuous loop film magazine seen just beneath the projector in Fig. 5.
The magazine is a tall, thin, rectangular box holding film looped back and forth in a random manner. The back of the box is a flat aluminum plate and the front is of plate glass to allow observation of the film. The space between the plates is somewhat greater than the width of the film and at the edges is closed in by narrow strips of wood undercut in their inner faces to avoid contact with the picture area. A gap in the top permits entry of the film as it comes from the projector. At the lefthand bottom corner is a similar gap fitted with a roller underneath which the film passes to the vertical channel and then back to the projector.
The feature of interest is that the loops of film are supported by a current of air flowing upward in the magazine. Were it not for this current, the loops would fall to the bottom where the lowest would tend to be crushed flat and permanently creased. The film would then, of course, be unfit for projection.
The supporting air enters the magazine through holes at the bottom of the back plate and flows upward through the
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May 1954 Journal of the SMPTE Vol. 62