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218 RICHARDSON'S BLUEBOOK OF PROJECTION
tion, but do not flush out the well with kerosene or other liquid. If you do, some portion of it will remain to impair the lubricating properties of the fresh oil. Filling up the oil wells periodically will not do. As oil is used its lubricating qualities gradually wear away and if it is mixed with fresh oil, the fresh oil loses its full strength. Draining at regular intervals is therefore essential.
(42) Dates for draining wells may be determined by dividing 100 by the number of hours the theatre operates per day. The answer is the number of days between drainings and refillings.
Intermittent Sprocket
The intermittent sprocket is part of the intermittent movement. (43) The interval during which the intermittent sprocket is at rest plus the time it is in movement is termed the "speed of intermittent movement." In present day "90 degree movements" the intermittent sprocket is at rest, with the film motionless over the aperture, three times as long as it is in motion.
A quarter segment of a circle is 90 degrees. (A circle is divided into 360 degrees.) Pin D, in Fig 62, is an engagement with the star slot and therefore moves the intermittent sprocket through exactly 90 degrees, but is disengaged for the rest of the circle or (360°-90°) 270 degrees.
Because the intermittent sprocket is in motion onefourth of the complete intermittent cycle of action, the movement is a "three to one." The film lies over the aperture three times as long as it is in motion, though since the rotating shutter has two blades of equal width, the time of actual projection is reduced, approximately to half that time. In actual practice the period of projection is really a little more than that.
(44) The film is fed down to the aperture by upper sprocket A (Fig. 64) at a regular, unvarying rate of speed, namely, 90 feet per minute, or 18 inches per second. It is taken away by lower sprocket G, at the same rate of speed. Between these two sprockets it is moved intermittently by intermittent sprocket E — its