. Motion picture electricity. tive crater, it is conceded, furnishesmore than 75% of the total illumination, and this is veryimportant for many purposes, because it centralizes theilluminating point in the crater of the positive carbon,permitting the crater to be used for projection. The illu-mination from the negative crater need not be consideredat all, and the illumination from the arc proper to a very 112 MOTION PICTURE ELECTRICITY small value. As a matter of fact, the arc in a direct cur-rent projector lamp is really only necessary to maintainthe high temperature in the positive crater. I
. Motion picture electricity. tive crater, it is conceded, furnishesmore than 75% of the total illumination, and this is veryimportant for many purposes, because it centralizes theilluminating point in the crater of the positive carbon,permitting the crater to be used for projection. The illu-mination from the negative crater need not be consideredat all, and the illumination from the arc proper to a very 112 MOTION PICTURE ELECTRICITY small value. As a matter of fact, the arc in a direct cur-rent projector lamp is really only necessary to maintainthe high temperature in the positive crater. In view ofthis fact, it becomes .necessary to centralize all possibleenergy at the positive crater and everything possibleshould be done to maintain the carbon separation onlygreat enough so that the negative carbon point will notinterfere with the emission of light from the positivecrater, and we should also reduce the arc mist to a mini-mum by using a solid negative carbon. In Fig. 59 the small bubbles or globules, above the. Fig. 60 Fig. 61 positive crater and below the negative crater, are intendedto show the accumulation of impurities in the generally drop off without being consumed, andfall in the bottom of the globe or lamp house, if in a pro-jection lamp. MOTION PICTURE ELECTRICITY 113 Fig. 60 illustrates what takes place when two coredcarbons are used. The arc separation becomes longer forthe same voltage drop, and the arc mist is almost 50%greater, thereby making this form of arc less efficient, andmore likely to be unsteady on account of draft inter-ference. Fig. 61 illustrates how a 45-volt open arc looks, main-tained between two solid commercial or ordinary carbonpoints. You will note that the points burn flatter, and acap formation is present, on the lower carbon point inparticular, and the lower carbon point covers almost en-tirely the positive crater, which makes this form of arcuseless for projection purposes. Of course, if a higherarc voltage is ap
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