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298 MOTION PICTURE HANDBOOK
with liability to produce a dark, or multicolored streak across the center of the screen. This is due to the fact that the spot is merely an image of the crater (see Page 130), and with two craters there will be two images, which are not superimposed upon each other.
For years an effort was made to use both craters by means of what was known as the "jackknife" set, illustrated at B, Fig. 124, and A, Fig. 124. Some also attempted to utilize both craters by setting the lamp straight up and down, but these schemes have, for the most part, been relegated to the scrap heap, where they rightly belong, and today the b st men, men securing the best results and holding the best positions, almost invariably use practically exactly the same set (illustrated at C, Fig. 123, and in Fig. 126), both for A, C. and D. C., or else use a very modified jackknife set by setting the lower carbon so that it angles out very moderately with relation to the rackbars, angling the top carbon to meet it, as in A, Fig. 124. Even this scheme has, however, been largely discarded in favor of the regular D. C. set. Years ago I advised, both in my books and in the Projection Department of the Moving Picture World, the use of the same set for A. C. and D. C. / still advise it. Theoretically, setting the lamp straight up and down is better; practically, however, it is not. By using the straight up and down lamp set, or the jackknife set, one is enabled to get considerably higher candle power through the lens for a given amperage. That is a conceded fact, but the fly in that particular box of ointment is that a steady light absolutely cannot be maintained with these sets, or, in other words, the curtain illumination cannot he held at uniform brilliancy. I cannot recommend either the setting of the lamp and carbons perpendicularly, the jackknife set, or any other set except that shown in Fig. 126, Page 300, known as the "regular D. C. set."
At E, Fig. 124, we see the result of carrying an alternating current arc too short — the carbons too close together. The A. C. arc is very short — much shorter than the D. C., and this fault must be carefully guarded against. The D. C. current arc of 40 amperes will be one-quarter inch to threeeighth inch in length; the A. C. arc of less than 60 amperes will not be much in excess of one-eighth inch. It is thus made plain that the operator has slight leeway in handling the A. C. arc. It must be watched very carefully, fed frequently, and not allowed to vary from normal length. The condition shown at C, Fig. 124, is as good as you can hope for when using 60 amperes or less. It can only be obtained