International projectionist (July-Dec 1934)

TECHNICAL AND ECONOMIC ASPECTS OF THE SUPREX ARC A Report by the Projection Practice Committee, S. M. P. E. INADEQUATE screen illumination has long been a problem of major importance in the projection field generally. Satisfactory screen illumination has been confined mainly to the larger theatres, the smaller theatres being either unable or unwilling to make the expenditure necessary to improve the quality of screen light. Exhibitors have recognized the need for more and better light on the screen; and they have recognized, too, the fact that the problem was more economic than technical. The situation, already serious, promised to become acute with the increasing use of color in motion pictures. Happily, the solution of the problem appears to be at hand, in the form of a new type d-c. projection arc which not only materially improves screen illumination but also satisfies the economic urgencies of the situation. This arc, using the new copper-coated Suprex carbons, is the topic of this report, the importance of which to the exhibition field is emphasized by the Committee. Based on Extensive Tests This report is based upon an extensive series of tests of the new arc made under actual operating conditions. Through the courtesy of various manufacturers there was made available to the Committee a group of motor-generators, both single and three-phase rec' tifiers, and arc lamps of practically all the new types. The Committee desired to obtain the answers to the following questions: (1) What is the carbon consumption per hour for values of current from 40 to 50 amperes, using the 6 and 7-mm. combination ; and from 50 to 65 amperes, using the 6.5 and 8-mm. combination? (2) What is the ratio of burning of the positive and negative carbons at different current densities? (3) What effect does the arc gap exert upon the burning rate of either carbon ; and what arc gap affords the best results? (4) Is there a difference in arc voltage with different sources of supply, such as rectifiers and generators? (5) Is there any difference in the quality of the projected light when power is derived from either rectifiers or generators? This article needs no introductory or explanatory statement. It speaks for itself — clearly, concisely, thoroughly and impartially. It avoids theory and treats only with facts. It is far and away the best of a long line of fine jobs done by the Projection Practice Committee of the S. M. P. E., and as such is a splendid tribute to the Committee, composed mainly of practical projection men, and to the Society, as ivell as to the craft generally. Mere ivords would not suffice to properly evaluate the worth of this paper to the industry. It is proudly presented in these columns. — Editor. (6) What increase of light occurs with an increase of carbon current density? (7) How do the various lamps now available compare as to light intensity, for a given current? (8) What is the efficiency and powerfactor of the several power sources used with the new lamps ? (9) What can be done to protect the reflector against pitting? (10) What are the over-all advantages of this new type of light source? (1) Tests were made to determine first the rate of carbon consumption for various current densities, the arc gap being maintained constant — that is, at from 5/16 to 11/32 inch; using as sources of current a polyphase rectifier, a single-phase rectifier, a motor-generator, and the regular d-c. power line. Carbon Burning Rate The tests (Fig. 1) indicated that the consumption of the 6-mm. negative carbon was constant for currents between 40 and 50 amperes, being 3% inches per hour or % inch for each ten minutes. The burning rate of the positive carbon, however, varied with the current. For 40 amperes, the rate was 6% inches per hour, or 1% inches for each ten minutes; for 45 amperes, 10% inches per hour, or 1-11/16 inches for each ten minutes; and for 50 amperes, 13V2 inches per hour, or 2%. inches for each ten minutes. Similar tests were made using the 6.5 and 8-mm. carbon trim at currents from 50 to 65 amperes, with the following results: For 50 to 57 amperes, the 6.5-mm. negative carbon burned at the rate of 3% inches per hour, or % inch for each ten minutes — which is identical to the burning rate of the 6-mm. negative at 40 to 50 amperes. However, at 65 amperes, the 6.5-mm. negative carbon burned at the rate of 4% inches per hour, or % inch for each ten minutes. The consumption of the 8-mm. positive carbon (Fig. 2) at 50 amperes was 6 inches per hour, or 15/16 inch in ten minutes; at 55 amperes, 8^ inches per ? 5 rt.. Bv?/V//V6 ft/ir£ or Svp&cjt ?Jft3o/vs 1 /^oj/r/ri C*/rao* 7tf/i. / 1 5 $ 4/£GAT/l/£ C/i/raon 6»„ 45 : FIGURE 1 Burning Rate of Suprex carbons; 6-mm. negative and 7-mm. positive trim [8]