Theatre Catalog (1949-50)

Optimum Performance of lligh-Brightness Results of Tests Using Carbon Ares Both Water-Cooled and Air-Cooled Methods are Analysed and Compared BRIEF: The effects of positive-crater cooling are described, and a suitable apparatus for this purpose is illustrated... The combination of specially made highbrightness carbons with water-cooled operation permits the use of higher currents without unsteadiness, and so gives a higher brightness than has been achieved in conventional air-cooled operation .. . This is attributed to the fact that effective cooling of the positive carbon removes energy which would otherwise be dissipated in turbulent volatilization, so that a higher current density-can be achieved in the light-producing gas ball before overload turbulence occurs . . . A considerable part of the more efficient crater cooling is attributable to the carbons themselves, since they will operate without water cooling at higher currents and brightnesses than other types of equal size. Within the limit of satisfactory aircooled operation with a given carbon, efficient water cooling always reduced the light produced at a given current; the ability to operate with higher brightness at higher currents was thus gained at the expense of a lower current efficiency ... Carbons designed for efficient air-cooled operation gave no better result with water cooling; the current efficiency was sacrificed with no gain in maximum brightness. The high-intensity carbon arc finds extensive use in the motion picture industry because of several important attributes. First, it has a very high brightness over an area of adequate size and shape. An effective light-collecting sys tem thus can be designed to concentrate _ the necessary lumens on a projector aperture within the limits of optical speed which can be utilized effectively by the projection lens. Second, the light is of excellent color quality for the faithful photography and projection of both black-and-white and colored motion pictures. Third, the carbon-are lamp has a high degree of mechanical reliability insuring a constant trouble-free delivery of light during the period required to project one reel, or to photograph a scene. This first attribute of a continuously maintained high brightness has been the subject of investigation by many scientists both here and abroad. In our own laboratories, we are continually searching for ways of making and operating carbon arcs which will raise the ceiling of brightness, although we, in common with other investigators, have at times Reprint of a paper presented October 26, 1948, at the Society of Keeton Picture Engineers Convention in Washington; Central Section, S.M.P.E., January 13, 1949; an Rares in the Journal of the S.M.P.E., April, 1949, 392 By M. T. Jones ann F, T. Bowpircn National Carbon Company Cleveland, Ohio held the opinion that certain facts of nature have determined limits beyond which we may never be able to go. The present paper is concerned with a method of operating carbon ares which has been found useful whenever the highest brightness and smoothest operation, particularly at high currents, is desired. This involves the use of watercooled jaws for both the positive and negative carbons. When these jaws are properly employed in a manner to be described, they permit the effective utilization of the high-current densities required for optimum high-brightness performance. Mention has been made in previous publications of the advantages inherent in water-cooled jaw operation.” ? The continued confirmation and extension of these earlier findings has made appropriate this present paper, devoted more particularly to a description of the operating methods involved. The major source of brightness in the high-intensity carbon arc is the so-called line radiation resulting from energy exchanges between rare-earth atoms and electrons in the gas ball within the positive crater. It is apparent that the higher the current on a given-sized carbon, the higher the electron density in the crater will be. Thus a greater number of energy exchanges is to be expected, with a corresponding increase in crater brightness. However, as the current is increased beyond a rather critical value, an overload phenomenon is encountered, which is usually characterized by noise and unsteadiness. In practical operation, therefore, the user must be content with the brightness obtainable at currents below this overload point. With the present 13.6-mm super high-intensity projector carbon, for instance, the maximum recommended current is 170 amperes. A theory of overload has been advanced in an earlier publication.’ Briefly, it is thought to be analogous to the violent boiling of a kettle of water which accompanies a high rate of energy input from a turned-up burner. If, however, a cooling coil be inserted in the kettle (analogous to providing improved cooling of the positive-carbon crater) enough of the input energy can be absorbed so that the boiling will subside, and an even higher rate of energy input tolerated without turbulence. So it is with the carbon arc. Effective cooling of the positive carbon dissipates peaceably energy which might otherwise produce turbulence, so that a given-sized carbon can be designed to carry more current. Finkelnburg, in an accompanying paper,® points out that water-cooled operation is accompanied by a lower anode drop, so that this also contributes importantly to the reduction in anode energy per ampere. By means of optimum cooling, through the use of properly constructed carbons in water-cooled jaws, the gas ball in the crater space can thus be provided with a denser population of electrons before the limit of FIG. 1—Side view of arc-lamp mechanism incorporating water-cooled positive and negative jaws. THEATRE CATALOG 1949-50