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THE PROJECTOR 301
Because the intermittent sprocket is in motion onefourth of the complete intermittent cycle of action, the movement is a "three to one." The film lies over the aperture three times as long as it is in motion, though since the rotating shutter has two blades of equal width, the time of actual projection is reduced, approximately to half that time. In actual practice the period of projection is really a little more than that
(44) The film is fed down to the aperture by upper sprocket A (Fig. 136) at a regular, unvarying rate of speed, namely, 90 feet per minute, or 18 inches per second. It is taken away by lower sprocket G, at the same rate of speed. Between these two sprockets it is moved intermittently by intermittent sprocket E — its total speed at this point being exactly equal to that of the film above and below sprocket E. (45) Now, this intermittent movement film action is made possible by upper loop D and lower loop H, which function to absorb the stoppage of the film between the steadily moving sprockets A and G. (46) While the film is stationary at aperture C, loop D becomes longer and loop H shorter, and since during each cycle of action by the intermittent sprocket exactly three-quarters of an inch of film is moved down over the aperture, exactly that length of film is fed to upper loop D by sprocket A and taken away from loop H by lower sprocket G. The compensation is equal and the action can continue indefinitely.
Inspect Sprocket Teeth
(47) The constant friction of film against metal wears the metal away. Worn intermittent sprocket teeth not only produce an unsteady picture but tear the film sprocket holes. Therefore the intermittent sprockets should be examined after each fifty hours of operation. Use a magnifying glass to discover the flaws that frequently cannot be seen by the naked eye. If there is any visible undercutting in the form of a small notch at the base of the working side of the teeth: the sprocket should be replaced at once. If no notch shows but the