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CARBONS 339
In the high intensity carbons the source of light is very largely the highly luminous gas or vapor contained in the positive carbon crater and held therein by the negative gas stream. This in no degree lessens the necessity for evenness of resistance at all points of the crater floor, excepting only the difference inherent between the core and surrounding carbon.
Effect of Moisture
(14) Best results cannot be had from carbons impregnated with moisture. Under the action of heat moisture will be converted into steam, which, because it cannot be instantly expelled, will set up high pressures at points within the body of the carbon and create a very harm
FiG. 82. — Effects of overloading a negative carbon. It gives a poor light because it is badly "spindled."
ful situation. Minute particles will blow off from the crater floor causing the light to flicker.
This is what is often called "sputtering of the arc." Tt is most in evidence when the moisture is contained in the positive carbon, though the effect may, in lesser degree, be produced from moisture in the negative.
(IS) Carbons should be kept in. a warm, dry place for several days before they are used. (16) Two or three bundles of carbons, of all types and sizes should be kept constantly in stock, stored in the warmest, driest place available. It is a good plan to lay in a three months' supply, replenishing the stock every thirty days thereafter, always using those longest in stock.
(17) We suggest that before purchasing any carbons newly marketed, the projectionist obtain a small number of the right diameters for a trial in his own theatre. Tf they seem to work well, it is safe to purchase a full bundle of positives and negatives to try them out thoroughly.
High Intensity Carbons
High intensity carbons differ but little if at all from