Transactions of the Society of Motion Picture Engineers (1916)

core material fills the entire length of the carbon. The composition of the coringmaterial is of considerable importance as it determines largely the burning quality and color of the arc. After coring, the carbons are dried, finished, pointed, inspected and placed in the shipping stock. The Carbon Arcs In the direct current arc, the crater of the positive carbon forms the principal light source. The positive crater is of relatively large area, while the negative spot is small and is not usually considered as a light source. While 95% of the light emitted by the arc comes from the positive crater, the characteristics of the negative carbon are of vital importance in securing steadiness of operation. In operation, the positive crater is set so as to face the axis of the optical system. In setting the carbons in this position, care must be taken to reduce to a minimum the shading of the crater by the negative carbon. In this respect, the direct current arc is superior to the alternating current arc. A direct current arc is longer and therefore gives less shading of the crater. The greatest advantage of the direct current arc is the fact that the current travels only in one direction and therefore the positive crater receives electrical energy continuously and consequently maintains a higher temperature. As was stated above the characteristics of the lower carbon on direct current are of greatest importance in securing steadiness of operation. The size of the upper carbon is determined by the power imputed to the arc. If the positive is too small the current wil overlap the end of the carbon and the arc will be noisy and unsteady. If too large, the crater covers the end of the carbon and the arc again will be unsteady, because the average temperature at the tip is lower. With the negative carbon, the carrying capacity is the important factor since the size of the negative carbon required by the negative spot is small. A small carbon keeps the arc steady and also eliminates the shadow due to the shading of the crater by the negative carbon itself. This problem has been solved by plating the solid negative over its entire length with a series of metallic coats forming a shell of metal of low electrical resistance around the carbon. This metallic coating volatilizes in the heat of the arc and thus prevents the spattering of the rear condenser lens with the heavy metal beads formed with the old style metal coat. The coating carries the major part of the current and makes possible the use of a small negative with the high currents required by long throws and dense films. The direct current arc is inherently stable and the range of arc voltage can be made whatever the projectionists desire, but there is one fact to be borne in mind that, for each given current value there is a definite arc voltage at which the arc operates at maximum efficiency. With a constant current value, gradually shortening the arc length, will finally produce an unstable arc; just previous to that point is the limiting voltage for the current chosen. Or, otherwise, a given current requires a certain arc length of voltage. To increase the current and not change the arc length, is equivalent to shortening the arc in the first case and the arc becomes noisy. For this reason increasing voltages are required for increasing currents. 25