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16-MM. PROJECTION EQUIPMENT
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sound track on the opposite edg^,of the film. Fig. 1 shows an enlarged section of sound film of this type. The speed of the film is 24 frames or 7.2 inches per second, corresponding to the same number of frames or 18 inches per second in the 35-mm. film. At that reduced speed it is, of course, a great deal more difficult to obtain response at the higher frequencies, because of the reduced length of wave along the film. For example, a frequency of 5000 cycles corresponds to a wavelength of 0.0014 inch with the small film as compared to 0.0035 inch with the 35-mm. film. It has been found possible, however, with care in design and focus of the optical systems, and by providing adequate mechanical filtering for obtaining uniform film velocity, to attain very good quality of reproduction at the reduced speed.
THE PROJECTOR
Fig. 2 is a photograph of the front side of the machine. The two reels are mounted coaxially at the top, and the three feeding sprockets are all mounted on one shaft below. The path of the film is as follows: after coming off the back reel, it passes around the sprocket (a, Fig. 2) nearest the center plate, and thence to the picture gate with a loop, which is skewed one position toward the observer so as to be in line with the middle feed sprocket. An intermittent claw at the bottom of the
picture gate moves the film at that point, after which the film takes the form of a horizontal loop at the bottom of the machine before passing vertically upward through the sound gate. From the sound gate it passes around two rollers, one of which is connected to a free flywheel on the opposite side of the center plate. The friction between the film and the roller drives the flywheel without slippage, and thus the effective mass of the film is increased. The middle sprocket, which pulls the film at this point, is flexibly mounted on its shaft by means of a helical spring inside the sprocket. After passing it, the film takes the form of a skewed loop, shifting one position toward the observer and under the front sprocket, which is a
FIG. 1. 16-mm. sound film.