Transactions of the Society of Motion Picture Engineers (1916)

amount of current without glowing and oxidizing away sharply, so we are limited on A. C. to the physical characteristics of the carbon. Sketch (Fig. 5) shows the physical limitations for a ^-inch A. C. carbon. Using the old style alternating current carbon, a short air gap gives a hissing and sputtering arc which is very unstable. By using cored carbons, the cores of which are impregnated with carefully prepared chemicals, an absolutely silent and steady alternating current arc can be obtained. By using the proper chemicals a light source of high intensity is obtained which is far above that of the old cored carbons. This change in the construction of carbons for use with alternating current projection is one that has come to the front in the last year and has met with marvelous success. It has brought the alternating current arc in close competition with the direct current arc and it has allowed many houses who had seriously considered adopting other sources of illumination to continue with the alternating current arc without necessitating a single change in or about the lamp house or in the wiring. The mere substitution of these new carbons for the old style alternating current carbons makes the alternating current arc a very desirable and economical light source for projection. In addition to fulfilling the general requirements, the carbon arc has other characteristics which make it adaptable for motion picture work. These characteristics are: i — Color of light; 2 — Reliability; 3 — Flexibility; 4— Steadiness. Color oj Light: Until recently, the color of the light used for the projection of the high-class film was a source of much annoyance. It is obvious that where the picture is taken in the open and in bright daylight, the eflPect upon the screen would be inferior unless the projection light source approached that of daylight in color value. The light of the direct current arc is the nearest approach in color value to daylight of any of the known illuminants that could be used for motion picture projection. The light is a pure white of high intensity. The light of the alternating current arc using the modern high-grade projector carbon approaches that of the direct current arc both in color value and intensity. A pure white light is beyond doubt the proper kind of light to use for projection since it brings out the high lights and shadows and will project upon the screen a picture that will please the most critical audience. Reliability: The arc in the hands of an efficient projectionist, is a very reliable light source. It is not easily affected by fluctuations in line voltage and therefore will give an even screen illumination where other illuminants will fail. Carbons have a definite length of life and therefore the projectionist can guard against the failure of light in the middle of a reel of film. Flexibility: The carbon arc gives a steady, flexible light, variable at the will of the operator according to the density of the film. No two films are alike and no two parts of the same film are of the same density and consequently to give a true artistic presentation of any picture you must have a flexible light source. Steadiness: Both the direct and alternating current arcs are giving absolutely steady illumination. The traveling of the arc and negative 27