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National Projector Carbon
mended as positive and negative carbons respectively in old type, low intensity, D.C. lamps. DeI ailed discussion of their use is given in Chapter IV.
In adapting the carbon arc to alternating current, a different problem is encountered. The reversal in direction of current flow carries the current through a zero point on each reversal with a resulting tendency for interruption of the arc. The introduction of certain rare earth materials into the core of the carbon counteracts this tendency, supporting the arc through the point of zero current and, at the same time, adding color and luminosity to the arc stream itself. Since each carbon is alternately positive and negative, the crater formation is not as distinct on either carbon of the A.C. arc as it is on the positive carbon of the D.C. arc and the temperature of the carbon tips is much lower. For this reason, the crater light from one of the A.C. carbons is less than half of that from the positive carbon of the D.C. arc operated at the same amperage. It has been considered impracticable to focus the craters of both carbons on the aperture plate of the projector. Only the crater of the upper carbon is so focused. However, the combined effect of the crater light from the carbon and the luminescence of the adjacent portion of the flame-like arc stream provides an excellent projection light of snow-white quality.
The brilliant luminescence of the arc stream between White Flame A. C. carbons is clearly seen in Figure 11. The luminescent effect of the core material is quite evident when this illustration is compared with that of the D.C. arc between neutral cored carbons shown in Figure 9. In order to show clearly the character of the A.C. arc stream, the photograph reproduced in Figure 11 was taken with the carbons well separated. In actual projection, however, the lamp is operated with a very short arc, the relative position of the upper and lower carbons being that illustrated in Figure 34, page 39.
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