International projectionist (July-Dec 1934)

Fig. 2. 8-mm. a-c. high intensity carbons, underloaded: 60 amperes, 24 volts; showing different positions of the arc as it "flops" about on the ends of the carbons Fig. 1. 8-mm. a-c. high-intensity carbons, overloaded: 90 amperes, 35 volts OPERATING CHARACTERISTICS OF THE NEW A. C. ARC D. B. Joy and E. R. Geib THE new high-intensity a-c. arc was first discussed before the S.M.P.E. at the Spring, 1933, Meeting1 at New York, and additional data2 were given in connection with another subject at the Fall Meeting of the same year at Chicago. Subsequently it was the subject of much consideration by the Projection Practice Committee. The writers contributed to the discussion and were later asked to put their comments in written form as it was felt that this information would be of value to both the users of the high-intensity a-c. arc and the equipment manufacturers. The aforementioned papers1-2 gave a general description of the arc, the current and voltage ratings of the carbons, and approximate consumption rates. This is summarized in Table I. A number of lamps and auxiliary equipment for using the a-c. high-intensity arc have recently appeared on the market and have been installed in a number of theatres. For that reason, a more detailed description of the arc and its operating characteristics will be helpful to those responsible for the operation of such equipment. The high-intensity a-c. projection arc is essentially a high current-density, lowvoltage arc. At the rated currents the current density (970-1090 amps, per sq. in. of carbon cross-section) is very much higher than that of the mirror arc carbons (140-188 amps, per sq. in.), and 1 Joy, D. B., and Downes, A. C. : "A New Alternating-Current Projection Arc." /. Soc. Mot. Pict. Eng., XXI (Aug., 1933), No. 2. p. 116. 2 Joy D. B., and Downes, A. C. : "DirectCurrent High-Intensity Arcs with Non-Rotating Positive Carbons," /. Soc. Mot. Pict. Eng., XXII (Jan., 1934), No. 1, p. 42. somewhat higher than even that of the high-intensity d-c. positive carbons (450900 amps, per sq. in.). Another difference is that in the conventional d-c. high-intensity lamp the positive carbon is gripped at a point close to the arc. whereas in the a-c. high-intensity lamps both carbons are clamped near the holder end. It is therefore necessary to increase the conductance of the electrode by coating it with metal, which in this case is copper. The copper does not enter the arc stream, its only function being to furnish a low-resistance path for the current from the carbon holder to a point near the arc. By carefully observing the high-intensity a-c. arc in operation, it will be seen, as shown in the accompanying illustrations, that the copper coat ends an appreciable distance from the arc. As the carbons are consumed the copper coat continually melts away, so that it never is sufficiently close to the tip of the carbon to enter the arc stream itself. This copper coat is designed to take care of the current rating of the carbon. If the current is too great, the copper will melt to a considerable distance from the arc, as shown in Fig. 1. The arc then becomes unsteady and is apt to blow out, and the arc voltage and consumption of the carbons are increased to such an extent that they may be outside the range of control of the arc-feeding mechanism. If, on the other hand, the current is too low, the copper will not melt away as far from the arc, the light will be very much reduced, and the current and voltage will not be constant. This condition results in an unsteady arc, which "flops" from the top to the bottom of the carbon, as illustrated in Figs. 2 (A) and 2 (B). If the current and voltage limits recommended in Table I are observed neither of these undesirable conditions will be encountered. The illustrations of the arcs shown in this paper are all traced from actual arc images, and show the true relationship between the different parts of the arc. Arc Length Important It is essential for good operation of the arc and good light projection that the high-intensity a-c. arc be maintained within certain definite arc lengths and that it have a characteristic shape which is easily identified. Fig. 3 shows the high-intensity a-c. arc burning under the correct conditions at 80 amperes and 25% volts between two 8-mm. carbons. The copper coats end 0.35 inch (8.9 mm.) from the ends of the carbons. The arc length is 0.27 inch (6.9 mm.) long. The end of the electrode is 0.225 inch Characteristics of Copper-Coated A-C. High-Intensity Carbons TABLE I 8-mm. Current (Amperes) 75-80 Approximate Arc Voltage , 24-29 Consumption (Inches per Hour) 4.0-5.5 Current Density (Amps, per Sq. In.) .. 970-1040 7-mm. 60-65 23-26 4.0-5.5 1000-1090 [8]