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MANAGERS AND PROJECTIONISTS 399
portion to the increased amperage, because up to that point none of the condenser is working up to capacity, but beyond that point it begins to slow up, and as more and more of the condenser comes up to maximum working capacity, the slowing up is more rapid, until a point is reached at about 120 amperes where very little if any further perceptible screen brilliancy can be had.
VALUABLE DATAj — The foregoing conclusion is supported in its entirety by experiments conducted by Mr. W. R. Mott, of the National Carbon Company's laboratories, which were described in an article, "As to Increased Amperage and Screen Illumination," Page 1186, May 24, 1919, issue of Moving Picture World, in which the experiments were described as follows :
"The screen candle power was determined as the average of many readings with a Scharpe Miller Photometer Lamp standardized by the Bureau of Standards at Washington. The screen was a 7 by 9 foot with a 67 foot throw. The entire purpose was to obtain the percentage gain in candle power with increase in amperage, using proper size trims corresponding to the amperage. Columbia core uppers were used with Columbia Silver Tip negative lowers.
"Increase in current from 30 amperes to 50 amperes gave 160 per cent gain in screen candle power.
"Increase in current from 50 amperes to 70 amperes gave 46 per cent gain in screen candle power.
"Increase in current from 70 to 90 amperes gave 18 per cent gain in screen candle power.
"Increase in current from 90 to 110 amperes gave 15 per cent gain in screen candle power.
"Increase in current from 110 to 130 amperes gave 12.5 per cent gain in screen candle power.
"The above data were obtained with the usual lens system consisting of 6y2 and 7^2 inch focal length condensers, with a fourth size projection lens.
"Beyond 120 amperes D. C. the increased amperage gives no appreciable increase in screen brilliancy, nor is it possible with a projector aperture of the present size, to change this condition."
ARC VOLTAGE. — A projection arc at a given amperage operates most efficiently with the carbon tips a certain given distance apart. Any alteration in this distance will operate to decrease the brilliancy of the light. This distance varies with the amperage, and what we call the "voltage of the arc" is the E. M. F. necessary to force the current from one carbon tip to the other against the comparatively high resis