International projectionist (Jan-Dec 1947)

LIGHT GENERATION BY H. I. CARBON ARCS (Continued from page 18) may be employed, with a minimum protrusion of the positive carbon beyond these jaws.5 The more efficient 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. DISCUSSION Mr. Doffer: The speaker mentioned that carbon is a very good electrical conductor, therefore you had a low loss. Why do some carbons have a copper coating on them? Mr. F. T. Bowditch: Carbon is not so good an electrical conductor as copper. I didn't mean to imply that. But as compared with other materials which remain solid at a comparable temperature, carbon is an excellent conductor. A copper coating is applied to carbons in services where the current must be carried throughout the length of the carbon from fixed clamps at the end farthest from the arc. Bare carbons are ordinarily employed with current jaws which grip the carbons close to the arc, and through which the carbon is pushed forward by a feeding mechanism at the rear. Mr. Read: Does the alternation of the electron path from points A-B to points A-C take place about once a second or does it occur at a rate which produces an audible note? Mr. Bowditch: I did not mean to imply there was any alternation whatever. Mr. Read: I don't mean alternating current. Mr. Bowdtch: The picture we have, and the picture I intended to portray, is that at reasonable currents, well below the overload limit, practically all the electrons take the longer path to the base of the crater. This is so because it only requires an anode drop of perhaps five volts to transfer electrons from B Jones, M. T., Zavesky, R. J., and Lozier, W. w\: *'A New Carbon for Increased Light in Studio and Theatre Projection," /. Soc. Mot. Pict. Eng., Dec, 1945, p. 449. the arc stream into the core at this point, as compared with 35 volts directly to carbon at the crater lip. Now as the crater becomes deeper, more and more electrons consistently take the shorter path to the crater lip (not first this way and then that way), until a sufficient electron concentration is developed to generate a very intense heat at that point. Every electron entering the carbon releases 35 electron-volts of energy as it lands there, and so is responsible for a very high concentration of energy. Mr. 0. E. Miller: Do you have positiveion current in appreciable amount? Mr. Bowditch: Our measurements indicate that the positive ions carry only a small percentage of the total current. There is such an effect. However the energy relationships indicated by the crater radiation spectrum show that only a very small percentage of the rare-earth atoms are ionized. Positive-ion current, while present, is therefore considered to be a minor factor in the light-producing relationships involved. Kodak's 15,000 Tons of Ice Daily To control production and processing conditions and to air-condition the many buildings where Kodak films and papers are made, a refrigerating unit with an average daily production equivalent to 15,000 tons of ice is operated at Kodak Park. The Inside Story on the New fOREST Electronic Projection Lamp One look at the inside of the new Forest Electronic Projection Lamp will demonstrate the reason for its dependable operation, economical maintenance and trouble-free performance. Simplicity is the keynote of Forest construction. Note the lack of complicated gearing mechanisms, cams and other fast moving parts. Through the use of Electronic timing and solenoid actuation of carbon feeds, Forest has successfully eliminated more than 300 components. The result is a lamp that maintains constant, maximum brilliance without continual attention from the operator, that reduces current and carbon consumption to a minimum, and that simplifies maintenance and overhaul. / Forest Electronic Advantages • Electronic Control • Easy Maintenance • No Fast Moving Parts • Low-Cost Operation • Electronic Timers • Unit Construction • Solenoid-Actuated Carbon Feed ToSIsT Electronic Company, Inc. 744 Broad St., Newark, N. J. Now Available For Quick Delivery! See Advertisement Page 31 for Data on Rectifiers for Use with Forest Electronic Projection Lamp INTERNATIONAL PROJECTIONIST November 1947 2T