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PROJEl'TORS
THE INTERMITTENT MOVEMENT
The Heart of the Projector
A Description of the Intermittent HAovement
By FRED C. MATTHEWS
The very fact that motion picture films may be projected successfully is due largely to the action of the intermittent movement. This is readily apparent when we review what takes place while the projector mechanism is in operation.
When all standard projectors are properly threaded, the film follows a path from the upper magazine past the upper feed sprocket, through the film guiding system, past the aperture, over the intermittent sprocket, then over the lower feed (takeup) sprocket, and finally into the sound reproducer.
As the film comes down from the upper magazine, it is pulled down at a constant speed of one and one-half feet per second (or 24 separate photographs, termed "frames", by the upper feed sprocket, the shaft of which is attached to the projector mechanism gear train. After the film enters into the film guiding system, it must stop at the aperture and remain absolutely motionless for a period of l/32nd of a second, in order that the picture on the film may be properly projected on the screen. After this rest period, the movement of the film is sharply accelerated so that the succeeding picture may be pulled into position in front of the aperture. These successive
and continuous actions of stopping and starting must take place so rapidly that 24 successive pictures can be projected each second.
The portion of the projector mechanism that creates this intermittent action is known as the intermittent movement — ■often referred to as "the heart of the projector."
Upper and Lower Loops
Anyone who has ever seen a projector threaded has noticed that the projectionist leaves a slack of film between the upper sprocket and the entrance of the film guiding system which is known as the upper loop. Between the intermittent sprocket and the lower feed (take-up) sprocket a similar slack of film is left — this is known as the lower loop. These loops are absolutely necessary to permit the intermittent action of the film created by the intermittent movement without film breakage, as they absorb the stoppage of the film between the steadily moving upper and lower feed sprockets. While the film is motionless at the aperture, the upper loop of film becomes longer and the lower loop becomes correspondingly shorter. When the intermittent sprocket pulls down the film, the