Richardson's handbook of projection (1927)

388 HANDBOOK OF PROJECTION FOR Fig. 114 and do no harm. They are caused by slight error in the con* sistency of the paste from which they are formed. Chip cracks running around the circumference, however, condemn the carbons, since there would be a tendency to break off at this point, though hair cracks are often found running around the circumference of good carbons. They are due to the same cause as the longitudinal cracks, and are of no consequence. EXAMINE CORES.— The ends of cored carbons should be carefully examined, and if too many of them show indication of imperfection in the core they should be rejected. It is absolutely essential to high class work on the screen that all cores be thoroughly continuous, also that the core adhere to the wall of the carbon sufficiently well to prevent short sections of it dropping out as the carbon is consumed. Carbon with a section of its core lacking is one of the most annoying things the projectionist is called upon to deal with. Absence of the core not only produces unsteadiness in the light while that section of the carbon is burned away, but also alters the tone of the light, and if the center of the crater is in focus at or near the film plane, the absence of the core will produce a dark spot at the center of the screen. We are assured by the manufacturer that all carbons produced by the National Carbon Company are very carefully inspected for these various faults and all carbons containing any of them are rejected and do not leave the factory. HARD SPOTS in the carbon due to faults in manufacture were of very common occurrence in the early days of projection, and were a source of great annoyance to the projectionist. Such spots are believed to be caused by the lack of thorough mixing of the carbon dough in the early stage of manufacture. They are, however, now so seldom encountered that they may be said to have been to all intents and purposes eliminated.