Movie Makers (Jan-Dec 1943)

473 WHY MOVIES MOVE Part il of discussion of basic cine mechanics G. A. G A U L D , ACL HAVING discussed cinematograph shutters and, let us hope, given a satisfactory explanation of the differing requirements of camera and projector, we are now in a position to look into the more complicated problems associated with the intermittent film movement through the gate. We have already stated that a good movement does not in any sense "jerk" the film frame by frame through the gate. No film or mechanism would stand up to such treatment for long, because film has weight and consequently inertia. The inertia is naturally more pronounced in the case of 35mm. film than it is in the lighter substandard gauges, but it cannot be entirely ignored, even in 8mm. film. To avoid strain of any kind, the film must be moved from rest with a uniform acceleration up to a maximum speed, then retarded uniformly until it again comes to rest, with the next frame in position in the gate. The retarding movement is equally necessary because, with such a movement, the minimum pressure is required on the film in the gate to prevent its overrunning. This movement, in turn, eases the strain on film and mechanism during the period of acceleration when both gate friction and film inertia must be overcome. Sprocket holes are punched in one or both sides of the film strip, at equal distances corresponding to the distances between successive frames. All film movement is accomplished by means of these sprocket holes. It may be taken for granted that the holes are punched in the film with the utmost precision by the film manufacturer, so that, given a reasonably accurate mechanism, a perfectly "steady" picture will be obtained. Most intermittents belong to the "claw" family. A prong is made to enter a sprocket hole at the top of its stroke; it then travels downward, pulling the film down through the gate a distance of one frame, and withdraws, passing back and upward, clear of the film to repeat the cycle. To avoid any displacement of the film when it is in a stationary position in the gate, it is clear that the claw must enter and leave the sprocket holes in the film without drag or shock of any kind. Furthermore, the claw should follow the gate path ; that is to say, there must be no in and out movement across the film as it travels downward. This latter condition is not always precisely fulfilled, STROKE although it is possible to obtain extremely simple mechanisms which comply very closely with the ideal. Such mechanisms are usually compact and of light weight. It may be argued that the wearing action of a slight in and out movement across the edge of the film at the sprocket holes can be ignored in a camera through which a film usually passes but once, as it is only during that run that the film is subject to the action of the claw. The light claw minimizes vibration, and its simplicity reduces the cost of production of the camera. These advantages do offset the disadvantages of the inaccurate movement to some extent, but the defect cannot be tolerated in a projector through which a single film may be run several hundred times. Claws have been mentioned first because claw mechanisms in various forms are employed almost universally for substandard work. Yet another mechanism, the Maltese cross, is invariably used for 35mm. projection. The mechanism is particularly interesting, and an investigation into the reason for its absence from the substandard field justifies the digression. In the first place, the Maltese cross mechanism, as used in almost every motion picture theatre in the world, is fundamentally the same as the mechanism employed in the earliest oinematographs of Robert Paul in England and Edison in America. (Being [Continued on page 490] * Figs. 1 to 4, the Maltese cross used in 35mm. projectors. Fig. 5, showing movement accomplished during quarter revolution. Figs. 6 and 7, 35mm. and 16mm. sprockets. Fig. 8, an eight point star. Fig. 8a showing revclu'ion of shift pin disc required to move star one point. Figs. 9 and 10, the operation of the claw in 16mm. machines.