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PRINCIPLES OF CINEMATOGRAPHY
CURVED STATIONARY ^ § POINTING GATE
Fig. 146. — Location of film gate and sprocket teeth in "stationary-gate printers".
occur in the finish of the metal into which small particles of film base will become lodged. Once any such surface becomes scratched the small particles of film base will continue to accumulate and to form into small pockets of an exceptionally sharp character which will cause an ever increasing scratch to any films which are subsequently passed through the machine.
In the design shown in Figure 146 four main sources of this trouble are present. Firstly, stripper plates, shown at 'P' and 'Q* are located just past that point on the sprocket wheels 'B' and *D' where the film moves out of engagement from the sprocket teeth and, since the film will bear lightly on the surface of these plates, longitudinal scratches may be caused at these points after a considerable amount of film has been printed. Secondly, the pressure of contact between the curved gate and the film will, after considerably less film has been printed, give rise to further longitudinal scratches the reason for this is that the pressure at this point is much greater than at the stripper plates. Finally, the contact between the film and the edge-guides mounted within the sides of the gate is at a much higher tension and scratches at these points may develop very rapidly. Although damage to the film edge may not appear to be so serious it may cause a film to be printed with considerable flutter, due to the varying load between the edge-guides and the rough film edge, or may cause the track to become misplaced with respect to the exposing aperture. It is not uncommon for springs, used to guide the