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368 PRINCIPLES OF CINEMATOGRAPHY
taken at a rate of 16 pictures per second are afterwards projected on a machine which will only run at a rate of 24 pictures per second the action will appear on the screen some 50 % faster than actually occurred at the time the record was made.
The film is then formed in a free loop between the supply and take-up sprockets so that, over a section of this loop, it may be passed through the film 'gate' and held under sufficient tension to keep it exceptionally flat and in the focal plane of the objective lens. The take-up sprocket is arranged to feed film to the take-up spool, at a rate equal to that at which the supply sprocket operates, so that a constant length of film always exists between these two sprockets. However, since the film is moved intermittently through the gate, the length of the loop between the supply sprocket and the top of the film gate will be at a maximum when the loop between the bottom of the gate and the take-up sprocket is at a minimum.
Since the circumference of the roll of film on the take-up spool will be continually increasing as more and more film is wound up, this spool must be driven through a clutch mechanism from the main driving train so that, although the drive to the clutch will remain at a constant rate, the frictional contact between the clutch and the spool will permit the spool to rotate at an ever decreasing speed until the whole film has been projected. The normal 400-ft (121-9-metres) spool has an internal diameter of approximately 1 • 49-inches (37 • 85-mm) and an external diameter of 7 • 00 inches (17-78-cms), that is, a ratio between these diameters of approximately 4-6:1. Because of this the spool will be required to rotate at a speed of 60 revolutions per minute at the commencement of projection and only 13 revolutions per minute as the film is about to come to an end. These figures presuppose that the rate of projection is at 16-pictures per second. If film is projected at a rate of 24-pictures per second these speeds are increased to 90 revolutions per minute at the commencement and 20 revolutions per minute at the end of projection.
When 1,600-ft (480-metres) spools are used the core diameter is increased to approximately 4-6-inches (11-7-cms) and the external maximum diameter is 14-0-inches (35-56-cms), that is, a ratio of approximately 3-0:1 exists between the two extreme positions. It is therefore seen that, although the larger spool is rather cumbersome to handle, it actually places less strain on the friction take-up clutch than does the smaller spool. At sound film projection speed this spool will rotate at a rate of approximately 30-revolutions per minute when projection commences and at 10