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National Projector Carbonc
carbon equal to or near the diameter of the positive,
it is impossible to hold the arc steadily on one spot. This results in a changing intensity of light on the
en. To overcome this condition, some projectionists shortened the arc to such an extent that the negative carbon obstructed some of the light from the positive crater. Obviously, another means of correcting this condition was necessary. It was found that the application of a metal coating to the negative carbon (increasing the current carrying capacity so that a /'..-inch diameter negative, for example, would carry the same current previously requiring the use of iVinch diameter plain carbon imparted the desired steadiness to the arc.
The Silvertip Negative Carbon, introduced to the industry a number of years ago by National Carbon Company, Inc., wras the first metal-coated negative carbon placed on the market. This was later superseded by the Orotip Negative Carbon. The Orotip differs in every respect from negative carbons previously manufactured; new shell, new core, new coating. The composition of the shell is such that it burns longer than any other metal-coated negative carbon on the market. The structure of the core is such that, even under the most severe operating conditions, it will not recess or burn back but maintains a quiet and steady arc. The introduction of metalcoated negative carbons brought about a decided change in motion picture projection. In a short time they were adopted by the majority of theatres operating on direct current.
It is evident from the foregoing discussion that best results can be obtained only when carbons are used in proper combination, that is, with the respective diameters of positive and negative carbons in proper relation to each other, and to the current at which the lamp is to be operated. It is clearly shown in Chapter III that imperfect projection is certain to result when carbons are operated
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