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338 RICHARDSON'S BLUEBOOK OF PROJECTION
ness and possible defects, cut to proper lengths, pointed, and, after inspection, packed for shipment.
We have outlined very roughly the process of manufacture. There is very much more to it, such as the inspection and testing of materials at various stages, etc. It is impractical to set forth the many elements that enter into carbon cores. They are, for the most part, either patented or well guarded trade secrets, promulgation of which would serve no useful purpose so far as concerns the projectionist.
The carbon rod surrounding the core (where there is a core) provides most of the crater floor area. The core serves several purposes as already indicated but in addition the various chemical elements with which it is impregnated make it either a high or low intensity carbon, or any one of several other sorts, including White Flame and a.c. carbons.
(12) What is known as the "crater" of the electric arc is the light source. It is formed by the heating of the positive carbon tip through current action, to the point where the carbon particles are volatilized (at about 3,500 degrees Centigrade or 6,300 Fahrenheit) into gases. In this manner a shallow, saucer shaped depression is formed and its floor or bottom called the "crater floor," is the light source, except in the case of high intensity, where gases contained in the crater form the light source..
(13) It is essential to good results that the entire area of the crater floor have, as nearly as possible, an even temperature, which gives a uniform luminosity. This can be accomplished only by the use of very pure carbon materials. Should impurity occur at any point, or carbon density vary at different points of the crater floor, there will be a variation in the resistance to the current; as a result current flow and luminosity will not remain constant until the fault, whatever it be, has burned away. This being self evident, we may readily understand the importance of pure materials and of good mixture to provide uniform density of the carbons at every point.