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RICHARDSON'S BLUEBOOK OF PROJECTION
jurious to light quality. Burning above rated capacity wastes both current and carbons.
Speed of Carbon Consumption
(76) The following approximate speeds of high intensity carbon consumption will enable projectionists within reasonable limits to estimate whether or not a positive carbon stub is long enough to run an additional reel, if the footage of the film on the reel is known. Rated amperage is assumed in each case.
9 mm positive, 4 minutes per inch — 15 inches per hour. 13.6 mm positive, 4 minutes per inch — 15 inches per hour.
(77) Often stubs of carbon are discarded which might serve for another reel, or at least a short reel, if the projectionist knows just how much time is required to consume each inch of carbon. The above figure may be accepted as correct for rated amperage. If the amperage is changed, the time, of course, will be altered. For example, while a 9 mm carbon will only burn 12.8 inches per hour at 75 amperes, it will consume 22.4 inches per hour at 85 amperes.
(78) We recommend that projectionists record the exact burning time of the positive and negative carbons for one full day. To do so, note the exact time the arc is struck and cut off at each run. When you have finished with the carbons divide the exact number of inches of carbon consumed by the number of minutes of running time — this will tell you what fraction of an inch of carbon is used per minute. Record this information for future use. Remember, if you change either the amperage or the carbon diameter you will alter the amount of carbon burned per minute but it is no big job to compile data covering all amperages used in a given theatre.
(79) The experienced projectionist can estimate current flow fairly well by observing the light source and rheostat settings. But for accuracy use a reliable, properly calibrated ammeter placed where the projectionist may observe it easily. "Guessing" by rheostat