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1948-49 Theatre Catalog, 7th Edition, Page 405 (392)

1948-49 Theatre Catalog, 7th Edition
1948-49 Theatre Catalog
1948-49 Theatre Catalog, 7th Edition, Page 405
Page 405

1948-49 Theatre Catalog, 7th Edition, Page 405

FIG. 4. Comparison of the core sizes and craters of typical low- and high-intensity carbon arcs. (The deep high-intensity crater shown in the upper cross section was formed on a 13.6-mm carbon at 150 amperes. the almost flat low-intensity crater on a 12-min carbon at 30 amperes.)

Carbon is an ideal material from which to construct the electrodes for such an arc because of three important properties: (a) it is a good electrical conductor, (b) it remains in solid form to a very high temperature (approximately 6500 F (3600 C)), and (c) it volatilizes directly without passing through a messy molten statef

The positive electrode of the high-intensity carbon arc differs from that em< ployed with the low-intensity arc in two important respects. The core is not only much larger, but it is also heavily loaded with the flame materials whose lightproducing function has just been described. In low-intensity positive carbons, the core hole is no more than one fourth the diameter of the shell: in the high-intensity positive carbon, the core is at least one half and is frequently a much greater proportion of the outside diameter of the shell. This is illustrated by Fig. 4. The current density is also much higher, a one-half inch lowintensity carbon operating at about 35

amperes as compared with well over 100 amperes for the same-sized high-intensity positive carbon.

Because of the lower voltage drop from the arc stream to the core as compared with the voltage drop to the carbon shell, most of the electrons forming the high current in the arc stream are encouraged to travel to the central core. Here the concentration of energy is so great that the core and the immediately surrrounding shell are vaporized faster than the shell at the outside. Thus, as shown by Fig. 4, a cup or crater is formed on the end of the positive carbon, which is filled with the rich light-producing mixture of rare-earth vapors and electrons. As the current is increased, the depth of this crater likewise increases to a limiting value determined by what is called the ftoverload" of the carbon. Overload is characterized by the fact that beyond a particular current value the are no longer burns smoothly and quietly, but becomes unsteady and noisy. Since, for all impor FIG. 5. Diagram illustrating the mechanism oi overload in a high-intensity carbon arc. ABZNormal electron path to core. AC=overload electron path to carbon shell.


i' *


tant uses, the arc must be both stable and quiet, operation is always confined to currents well below this overload value.

As shown in Fig. 5, the mechanism of overload is visualized as follows. Since the electrons encounter a much lower voltage drop in entering the positive carbon through the central core, the resulting crater becomes deeper and deeper with increasing current. At the same time, the tendency for electrons to fiow directly to the shell C at the mouth of the crater, instead of taking the longer path to the bottom B, becomes greater and greater. Finally, the difference between the voltage drop over the longer arc stream AB to the bottom of the crater, and the voltage drop along the shorter path AC to the crater lip, becomes sufficient to counteract the effect of the more favorable electron entrance into the core. When this happens,

' the electrons travel in increasing num bers to the shell. Here the rate of energy release increases to a point where carbon is volatilized violently and noisily. Thus, an upper limit of about 35 volts (the anode drop to pure carbon) seems to be imposed on the voltage component of the energy which can be released within the positive crater. The current component of this energy is less rigidly limited. For instance, watercooled positive jaws may be employed, with a minimum protrusion of the positive carbon beyond these jaws.3 The more efiicient heat dissipation thus obtained permits the use of substantially higher currents and the achievement of correspondingly higher brilliancies. The added complication of arc operation with water-cooled jaws has so far prohibited their use in many applications. However, in motion picture studios where background projection is frequently employed to provide the setting for a physically distant location, operation with water-cooled jaws to achieve the brightest possible background image is receiving active experimental consideration.

It is hoped that these theoretical considerations, assembled in the course of arc-carbon research and development, have proved of interest outside that limited field. Not only the designers and the operators of the many types of burning mechanisms which facilitate the generation and release of this benign form of atomic energy, but also the many artisans engaged in the control of this energy to create wanted effects on film and on the motion picture screen, are all dependent upon the radiant output of the carbon arc. It is to them that this paper has been directed.


'Kalb, W. (7.. "Progi'css in Projection Lighting." J. Soc. Mot. Pict. EDI-2.. 35, 1 (July 1940), ll. 17.

2Lindvrmun. I{. (L. l'Iundlcy, (J. W , and Rodgers. A.. "Illumination in Motion Picture Production." J. Soc. Mot. Pict., 40. (1 (June 1943) p. 333.

i'MacPhorson, H. G.. uA Suggested Clarification of Carbon Arc Tm'minology as Applivd to Motion Picture Industry," J. Soc. Mot. Pict. Eng, 37, 5 (Nov. lEMl). n. 480.

*Chzlncy. N. K., Hamish-r. V. (3., and Glass, S. W.. HProperties of (Jul-hon at iht.- Ai'c. Temporaturn." Trans. Amer. Electrochcm. $06., 67. (1935), p. 201.

5.10in, M. T., ZnVPSl-(y. R. J., and Lozicr. W. W., "A Now Carbon for Increased Light in Studio and Theatre Projection," J. Soc. Mot. Pict. Eng., 45, 6 (Dec. 1945), p. 449.

1948-49 Theatre Catalog, 7th Edition, Page 405