International projectionist (Oct 1931-Sept 1933)

September 1933 INTERNATIONAL PROJECTIONIST 21 Showing Differences in Light Sources of Various Fig. 1. Direct-current, high-intensity arc C "U \ Now in Common Use Fig. 2. Direct-current, low intensity arc why this arc is easily adaptable to an optical system employing either a mirror or condenser lens. This type is used in light source Classes I and II. Fig. 2 is a direct-current, neutralcored carbon arc (used in Class III light sources), in which the light source is the brilliant crater on the positive carbon. This figure shows the much less brilliant negative tip and the very faint arc stream between the carbons. This shows, as in the high-intensity arc in Fig. 1, that by far the largest part of the light comes from the positive crater. Fig. 3 is an alternating-current, white-flame arc, burning carbons containing cerium group compounds. The light source is the brilliant flame between the electrodes ; but the electrodes themselves are relatively dim, and emit only a very small fraction of the light. Such an arc, while producing a great deal more total light than a neutral-cored carbon arc using the same power on direct current, is obviously unsuitable for projection, as it is too large to be readily focused through an optical system and its intrinsic brilliancy is too low. The light emitted by the white-flame arc at currents up to 30 or 35 amperes Fig. 4. Alternating-current arc increases directly as the voltage, but as the square of the current. The rate of increase in light, with increasing current, decreases above 30 or 35 amperes until at very large currents the rate of light increase becomes equal to the rate of current increase. Increasing the current, which, if no other adjustments are made, will also cause a small increase in arc voltage, will therefore very materially increase the light produced. If, instead of permitting the arc voltage to increase with increasing current, the arc voltage and also the arc length be decreased, the arc becomes steadier and the sources of light are concentrated into smaller volumes near the electrode tips, as shown in Fig. 4. This arc is quite different in character from any other arc of which we know today. Either of these small, brilliant light sources can be focused by means of a mirror, and therefore can be used for projection. Despite the fact that practically all the useful projection light comes from only one of these sources, early experiments showed that with this alternating-current arc at least 15 per cent more light could be projected to a Fig. 3. Alternating-current, white-flame arc screen with a given optical system and aperture plate than with a lowintensity, direct-current arc using the same line power. The color of the screen light from the alternatingcurrent arc was blue-white, resembling that produced by the direct-current high-intensity arc. Additional Advantages In addition to these advantages is the fact that ballast resistances, expensive switchboards, and motorgenerators are unnecessary. The motor-generator and ballast resistance are replaced by a relatively low-priced transformer, and the switchboard can be very simple and cheap. A number of years ago Mott found that if two alternating-current arcs, one with resistance and the other with reactance ballast, were operated with the same line power, the arc with reactance ballast produced 33 per cent more light. There is no reason why the new alternating-current arc can not be controlled with a ballast resistance ; but Table II shows that since the transformer will cost little or no more than a good resistance unit, there should be no incentive to use the resistance. The carbons for this alternatingcurrent arc service have been made in 6-mm., 7-mm., and 8-mm. sizes for use at 40-45, 60-65, and 75-80 amperes, respectively. A large number of laboratory tests have been made on these sizes. Since the current densities are about 800 to 1,000 amperes per square inch of cross-section, it is necessary to vise metal-coated carbons. The carbon consumption is approximately 4 to 4.5 inches per hour for the 6-mm. carbons at 45 amperes, and 4.5 to 5.5 inches per hour for the 7 i