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318 RICHARDSON'S BLUEBOOK OF PROJECTION
plate with drilled pin holes may be used, provided the experiment is made with a piano-face converging lens and the plate lies snugly against the lens.
(97) Projectionists may lay out the effective beam as per Fig. 140 in which line AB is on the optical axis and just long enough to reach from aperture B to plane of converger at A, broken lines representing the outline of the converger light beam as a whole. CC is the cooling plate.
All light outside of lines DD is wasted light and of no value when considering shutter action. The light between lines DD is the only light that is effective, hence it constitutes the "effective beam."
The Gear Train
(98) The gear train not only drives the entire motion picture projector mechanism, but also, as previously stated, locks the intermittent movement cam with the rotating shutter so that they rotate in exact synchronism— the shutter making one complete revolution to each complete cycle of action of the intermittent sprocket. At 90 feet per minute projection speed the intermittent sprocket acts 24 times per second. Through the gear train the master blade of the shutter is made to cut off all light from the screen and to turn it on again at precisely the right instant of time.
(99) There is lost motion even in a new gear train — about 3/16 of an inch free movement of the intermittent flywheel while the shutter is held stationary. Projectionists should watch this carefully. New or repaired mechanisms that have a greater ratio of lost motion should be rejected.
(100) As parts of the mechanism wear, lost motion will increase. It should never be permitted to be more than 5/16 of an inch. If the ratio is higher it is evidence that the mechanism needs a general overhauling.
The gear train may not be altogether responsible for lost motion. Often it is due to worn bearings and other factors in the intermittent movement.