International projectionist (July-Dec 1934)

October 1934 INTERNATIONAL PROJECTIONIST 13 continuously while the arc is burning. The negative carbon is set at an angle tc the positive, usually 40 to 60 deg. below the horizontal. The feeding mechanism is designed to advance both carbons as they are consumed and should be adjusted to feed the positive carbon forward at the same rate it is being burned, thus maintaining the crater at a fixed position in relation to the condenser and to the point of intersection with the axis of the negative carbon. The rate of feed of the negative carbon is also adjustable relative to that of the positive, and when properly adjusted maintains an arc of uniform length. The reflecting principle of light concentration has been applied also to the high-intensity arc. In all high-intensity reflecting arc lamps the positive carbon is held in a horizontal position. The negative carbon also may be held horizontally or inclined to the positive, as shown in Fig. 7. H. I. Light Source Light from the high-intensity arc emanates from two distinct sources, the crater and the tail flame. The tail flame produces about 30 per cent of the total light from this type of arc, but is of little value for most applications since its size, shape, and position prevent its being focussed accurately. Therefore, a consideration of the characteristics of the high-intensity arc is concerned chiefly with the crater light from the positive carbon. The whiteness and intrinsic brilliancy of this crater light exceeds that of incandescent carbon volatilized at atmospheric pressure. Evidently its source is something more than the solid tip of incandescent carbon, since its color and brilliancy indicate a temperature of about 5,500 deg. C. This intensified light can be considered as coming from a portion of the luminous gases of the flame material retained in the crater of the positive carbon by the force of the negative arc stream. This explanation may not accord with physical facts but it aids the imagination in visualizing the reason for the remarkably high intrinsic brilliancy of the high-intensity carbon arc as compared with other artificial sources of illumination. Maximum intrinsic brilliancy from the high-intensity arc is attained by adjusting the position of the negative carbon so that the negative arc stream compresses within the crater of the positive carbon a substantial portion of the brilliantly luminescent vapors emanating from that point. This produces a bluish-white light of very high candle power and much brighter than can possibly be obtained from incandescent carbon alone. A photograph of the D. C. high-intensity arc is reproduced in Fig. 8. The energy distribution curve for a typical high-intensity carbon arc is shown in Fig. 9. On this is superposed the energy distribution curve for normal sunlight. Radiation from the arc as here measured is the radiation from the positive crater only, since in most applications of this type of arc the crater light is all that can be utilized. These curves show the striking similarity between this light source and natural sunlight through the entire range of the spectrum The candle power of the crater light directly in front of the arc is shown in Fig. 10. As would be expected, the FIG. 8. Front and side views of H. I., D. C. arc candle power increases with the current. When the same current is used with two different sized carbons of the same composition the smaller, that is, the one with the higher current concentration, gives the greater candle power. The term '"super high-intensity arc" has been given to the arc produced by special 16-mm. high-intensity carbons capable of carrying 195 to 200 amps, and producing 40 to 60 per cent higher candle power than the ordinary 16-mm. high-intensity carbon operated at 150 amps. FIGURE 9 Spectral energy distribution from H. I., D. C. arc compared with that from normal sunlight 120 ,000 (100,000 1 80,000 ■ £ 60,000 ■ 2 40 ,000 < a 20,000 o[ 16 MM/ 13.6 MM 9 MM, 60 80 100 120 MO 160 6,000 8,000 10,000 12,000 WAVE LENGTH IN ANGSTROM UNITS |4,000 ARC CURRENT, AMPERES FIG. 10. Candlepower of crater light from H. I. arc for carbons of 3 different sizes Motion picture projection in the larger theatres is the principal field of application for the high-intensity arc. There are many other applications, however, including theatre spotlights, motion picture studio sun arcs and rotary spots, army and navy searchlights, searchlights used for spectacular illumination, airport landing lights, and the famous Lindbergh beacon at Chicago. THE smallest D. C. high-intensity arc with rotating positive carbon in general use is the so-called "Hi-Lo" lamp which uses a 9-mm. positive carbon operating at 60 to 65 amps, and 45 to 50 volts. A rather large interval exists between the screen illumination available from this lamp and that practicable with the low-intensity reflector arc used in the majority of small theatres. There is also a marked difference in the color of light from these two types of arcs. The low-intensity arc gives a yellowish-white light, whereas the high-intensity arc gives a snow-white light generally considered more desirable for the projection of motion pictures. To bridge this gap and provide for the small theatre screen illumination of high-intensity quality and brilliancy, a metal-coated carbon of small diameter has been developed to operate at an intermediate current. The carbons for this small highintensity arc have been so designed that the positive carbon does not need to be rotated and the negative can be coaxial with the positive. The carbons are protected from oxidation and their electrical resistance reduced by the metal coating, making it practicable to hold the carbon at any convenient distance from the arc. Arc current and voltage ranges for the standard size trims are shown in Table I. (see page following). A. C, H. I. Arc Operation of a D. C. arc of any type from an A. C. power circuit requires the use of a motor generator set or a transformer and rectifier. The development ot an A. C. high-intensity carbon arc permits operation through a suitable transformer directly from the A. C. power line, thus eliminating all rotating intermediate apparatus as well as the heavy power loss in ballast resistance. This provides a very efficient source of projection light. The A. C. high-intensity arc, together