Motography (Jan-Jun 1913)

April 19. 1913 MOTOGRAPHY 275 Motion Picture Making and Exhibiting By John B. Rathbun T CHAPTER I. (CONTINUED). THE FILM GATE. HE principal function of the film gate is to steady and flatten the film so that it will not move edgewise in the gate during the period of projection, or cause distortion of the image through the curling of the him. It also acts as a check to the momentum of the film so that it will not "follow" nor buckle when the film and reels are brought suddenly to rest. The opening or aperture in the gate is slightly smaller than the picture on the film, and prevents the light from escaping around the edges of the picture. In effect, the gate is a form of friction brake that acts directly on the film, the frictional drag acting against the pull of the feed sprocket so that the film is kept taut and straight at all times whether moving or stationary. To prevent sidewise motion, or to prevent the film from assuming a diagonal position in front of the aperture, the plate is provided with two side rails which act as guides, the distance between the rails being just enough to clear the width of the film and no more. Friction is produced by the action of springs that press a plate on the back of the film, forcing the front face against the main gate platen. The tension on the film may be varied to compensate for the wear on the surfaces by increasing or decreasing the spring tension. When the shutter and film shift mechanism are not operating in the proper relation to the gate, so that the picture is not exactly in the center when the shutter opens, the picture is said to be "out of frame." When this condition exists, the top of the picture no longer coincides with the top of the illuminated area on the screen, with the result that parts of two adjacent pictures are thrown on the screen simultaneously. To avoid Fig. 14. — A Reel of Film Ready for Projection. stopping the machine for readjustment, all commercial machines are provided with devices by which the proper relation may be re-established by the operator while turning the crank. This operation is known as "framing UP-" Framing the picture may be accomplished by four different methods, all of which depend upon changing the relative positions of the gate, lens and film sprocket. Moving the lens and gate up or down will bring the picture in frame, advancing or retarding the film shaft with the lens and gate stationary will give the same result. Increasing or decreasing the amount of slack in the film loop will change the relation of the film and film gate, and is therefore effective in framing the picture. No matter what system is used, the control lever that frames the picture is invariably capable of a film displacement equal to the height of one picture, or even more. When the operator looks at the screen and discovers that the picture is out of frame, a slight movement of the framing lever in one direction or the other will raise or lower the picture in the gate to the correct position. THE SAFETY SHUTTER. The light concentrating effect of the condenser lens produces a very high temperature in the film gate, much higher than the ignition temperature of the celluloid film, and for this reason the film must be kept moving rapidly in order to prevent the light rays from setting it on fire. Fig. IS.— A Belt-driven Take-up Reel. The Reel is Driven by the Belt (3) that Passes Over the Pulley (4). The Friction Adjustment is Made by the Thumb Screw (6). Should the film stop for even a few seconds in the gate, it will burst into flames. Any accident or neglect that will cause the film to stop or slow up, such as film breakage, or failure to turn the machine at the correct speed, might cause fire unless some device is provided that will automatically cut off the light from the condenser as soon as the trouble occurs. A device of this nature, known as a "safety shutter," is provided in some form on all projectors, although in the majority of cases it is effective only in cutting off the light when the machine slows down beyond a certain point. A trustworthy safety shutter acting automatically when the film stops, and not merely when the machine stops, seems not to have been developed up to the present time. The many conflicting conditions that must be met with such a device makes the design of a fool-proof safety shutter a difficult proposition, and from the failures recorded it would seem that the only solution of the problem is non-inflammable film. In practically all projectors, the safety shutter consists of a metal gate placed between the condenser and the film. A centrifugal type of governor driven by the motion of the operating crank is connected to the sliding gate in such a way that any decrease in the cranking speed slides the gate between the film and light. The governor consists of a vertical spindle on which are pivoted two small balls, or weights. When the spindle is revolved, the weights tend to assume a horizontal position, and in swinging up from the vertical plane, they