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CH. XI] MOVING PICTURE FILM AND MECHANISM 419
The gate D is then opened and the idlers, iii are pushed away from the sprocket wheels a, b and c. A sufficient length of film is unrolled from F to reach to the take-up reel G and the film is put under the sprocket wheel a, so that the teeth fit into the holes at the edges of the film. Care must be taken that the film goes over or under the sprocket wheels in such a way that as the crank is turned forward all of the sprocket wheels tend to move the film in the same direction, otherwise they will tear it apart. The arrangement may differ in different machines.
After putting the film on the sprocket wheel a, so that the teeth pass through the holes of the film, the idler i, is pushed over to hold the film in place. This can be done with one of the fingers while holding the film in place with the thumb and forefinger. The film is then engaged with the lower sprocket wheel b, leaving an extra length of film to form the two loops L and M. This can best be determined by experience, it must be enough so that the intermittent sprocket will not jerk the film in two and not long enough so that the loops will strike any shields there may be to cover them.
The film is held against the intermittent sprocket c, so the loops L and M, are about equal in size and held straight on the tracks of the aperture plate when the gate D, is closed.
The end of the film is now pushed through the fire-trap opening in the lower magazine and fastened to the take-up reel G. This is accomplished by slipping the end under the spring on the spindle of the reel, in such a direction that the film will not be folded as the reel is turned. The reel is turned to insure the end of the film being well fastened. Fig. 233 shows a mechanism with the film in position and ready to operate as soon as the magazine doors are closed.
If the picture is not directly in front of the aperture plate but is above or below (misframed) , it can be put in its proper position by a lever which lowers the mechanism and film without disturbing the position of the aperture plate and objective.
§ 585. Direction of motion. — The normal direction of motion to secure the proper sequence of events in the order in which they occurred is secured by moving the film downward, and results