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THE LIGHT SOURCE 363
tivity is much higher than that of the carbon shell. The purpose of the core is to steady the arc and hold it in a fixed central position; also in the core certain materials, "rare earth" metals, are intermixed, and they serve to whiten or otherwise alter the color or tone of the light, and serve other useful ends.
(20) Carbon rods are made in all lengths and diameters which projection practice demands. They are relatively inexpensive.
(21) The carbon, except for its central core, is composed entirely of carbon powder very similar to the product known to us as "lampblack." This powder having first been subjected to the action of powerful magnets to remove metallic impurities, and put through other purifying processes, is thoroughly mixed with a binder of pure pitch. The resultant "dough" is then forced, under very heavy pressure, through a round role in the center of a steel plate or "die," the diameter of which corresponds to the diameter of the carbon it is desired to make. If a cored carbon is to be made, a steel needle is fixed in the center of the opening in the die, so that a hole is left in the carbon rod. The carbon comes from the die in the form of a continuous rod, which is afterward cut into proper lengths and subjected to a baking process in ovens or kilns. Baking reduces the pitch binder to carbon. After cooling the rods are removed and the core, if it is to be a cored carbon, is forced in under very high pressure.
The rods are then baked again to reduce the core to the necessary condition; they are cleaned, tested for straightness 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.