Journal of the Society of Motion Picture Engineers (1930-1949)

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416 FRANK BENFORD [J. S. M. p. E. amperes the arc drop changes by a smaller amount and the filament current is not changed by as large a percentage as before. The electrodes radiate greatly increased quantities of energy and the net gain in incandescent light is now much greater. Perhaps a better picture of the relations between input and radiation is gained by examining the watt-lumen characteristics. In ouuu 7OOO 60OO 5OOO 4OOO 300O WOO • 7Z I 1 /A/ CftA/OES* 0^ **( 2-*_— /O 20 30 FIG. 14. Ampere-lumen characteristic of Type S-l. Fig. 15 the arc output of visible radiation is seen to approach a maximum between 300 and 350 watts, while the tungsten radiation increases in more nearly the customary manner of an incandescent lamp. The arc has its maximum efficiency at slightly over 300 watts while the tungsten efficiency increases steadily with the current. The presence of well-marked engineering optima will appeal to the engineer and they seem a guide to good engineering practice. In Table I a summary is made of some of the therapeutic and luminous properties of the sun and two therapeutics arcs. It will be