Motography (Apr-Dec 1911)

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138 MOTOGRAPHY Vol. V, No. 6. We next come to the sprockets. In every machine with the geneva star movement and take-up we have three sprockets, namely : the upper feed sprocket which pulls the film out of the upper magazine, the intermittent which is attached to the star wheel shaft and feeds the film past the aperture, and the take-up sprocket which feeds the film unto the lower reel. The sprocket should be kept thoroughly clean. Emulsion from the film, especially when using first run, gathers on the sprockets, becomes mixed with particles of dust, and the soft film in passing over becomes scraped and full of scratches, causing the effect of "rain" on the picture. This accumulation also gives the film a tendency to jump the sprockets. The teeth of the sprockets wear rapidly. In pulling down the film, the under side of the teeth have little ridges worn in them. This is especially true of the upper feed sprocket. When the sprockets get in this condition, they should be renewed. The sprockets must be examined at close range in order to see this defect. The same is also true of the pins which take the place of the intermittent sprocket in the machines with the cam movement. These pins should be removed from the machine while being examined, and renewed at once when worn, as the film becomes caught in these ridges and will not pass down, causing the gate to jerk open. These grooves in the pins and sprocket teeth also cause a vibration of the picture on the screen. There are many things which must work perfectly in order to give a steady picture and this is one of them. Sprockets are provided with idlers or rollers attached to brackets with springs. The purpose of these rollers is to keep the film on the sprockets. The rollers should never rest directly on the sprocket, every roller being equipped with a screw so that it may be kept at the correct distance from the sprocket. They should be set so that they are a little more than the thickness of the fiim away from the sprocket. If allowed to rest heavily on the film any dirt on them will scratch the emulsion. The Powers No. 6 machine is equipped with two rollers on each sprocket, one revolving on a fixed spindle, and the other a holding roller mounted on a spring pressed bracket pushed out of the way when threading up the machine. The use of two rollers gives the film a contact of about six teeth on each side of the sprocket. This is especially desirable on the take-up sprocket as riding of the film on the sprocket is prevented. Covering the film in the passage through the machine is the film gate. At the top of the gate is the guide roller which guides the film over the track. The most important part of the gate is the film tension springs. These tension springs have two duties to perform. One is to flatten the film prefectly straight before the aperture, and the other is to hold the film absolutely still while being projected. The film does not become absolutely stationary when the intermittent sprocket ceases moving, but tends to slide a trifle further. The springs should exert just enough pressure on the film to keep it from moving. Some operators screw the tension spring very tight to keep the film from moving which causes the picture to shake on the screen, instead of adjusting the intermittent movement. This extra pressure on the springs causes a heavy drag and consequently wears out the driving elements, aperture plate, intermittent movements and film. Most of the machines are provided with screws for adjusting the pressure of the tension springs. In regard to keeping the film flat, it is understood that unless the entire film lies in one plane no matter how fine a lens you have you will not get a sharp, clean cut picture on the screen, but part of it will be blurred. If as above stated, the springs are very short or are set too tight they wear a corresponding depression in the film tracks. In its passage over the film track the film becomes cupped or enters this depression, and the best lens in the world will not project a sharp picture. Sometimes one spring presses more on one side than the other causing one side of the picture to be blurred. When the film or aperture plate becomes worn from the extra pressure of the springs it should be renewed at once. These plates can easily be removed on most machines. In threading each film the operator should run his finger around the edge of the aperture as dirt and particles of emulsion collect, especially on the bottom edge, and cause the picture to have ragged edges. This cleaning is but the work of a moment, but if dirt is left on it certainly betrays the qualifications of the operator. The tracks on the film gate should be kept clean by rubbing them with a flat piece of wood. Vaseline or wax applied to the tracks and tension springs will prevent new film from sticking on these parts. Apply sparingly and wipe off the excess leaving the slight hollow places filled. The purpose and value of the automatic fire shutter is familiar to every operator. They work either with a lever attached to the crank which raises the shutter when the speed of the crank reaches a certain degree; or by a centrifugal governor and a lifting lever controlled by the governor and connected to the shutter. Whichever style you have it must be kept scrupulously clean and well oiled. Any oil having a tendency to gum will cause the parts to stick and the shutter will not operate properly. Therefore in order to avail yourself of the protection afforded by this shutter it must be well taken care of. The hand cut-off or "dowser" if worked with a balance weight should be properly adjusted so that it will close with a touch of the hand and remain closed. We have two kinds of revolving shutters, namely, the inside and the outside. All revolving shutters serve two purposes. One is to cut off the light when the film moves down one picture, and the other purpose is the elimination of the flicker. Some inside shutters have a wide and narrow blade. The wide blade of the shutter should cut off the light while the film is in motion, and the narrow blade travels across the light while the film is at rest. This, of course, shortens the period of projection of the still film but the object is as follows. If the shutter had but one blade it would be more noticeable in its passage across the screen, but when two or three blades pass over the screen in the same interval of time as the one the period between these black streaks across the light are not so far apart, and are consequently not so noticeable. The same thing may be seen in the operation of an electric fan. If the fan had but one blade and even if turned at full speed you would be able to see it more or less plainly, but when it has four blades and is running full speed nothing but the whir of the blades can be seen. Also, the faster the crank is turned the faster the shutter will revolve, consequently the distance between the periods of darkness will be de