Theatre Catalog (1949-50)

BRIGHTNESSC/MM2 ° 0.4 O68 L2 16 POSITIVE PROTRUSION-INCHES FIG. 6 Effect of positive protrusion on brightness of 13,6mm, '290-ampere”’ and super high-intensity carbons, bon is its high thermal conductivity, which is essential to the efficient transfer of heat from the floor of the crater to the water-cooled jaws. This is an important link in the cooling system required to postpone overload turbulence to higher current densities, in accordance with the theories previously expressed. Carbon composition, as well as water cooling, are thus involved in the achievement of crater brightness in excess of 2000 candles per square millimeter. An interesting demonstration of this fact is given by a comparison of the two following figures. Fig. 4 shows the relationship between crater brightness and are current for a new higher-current 13.6-mm carbon when operated first in water-cooled jaws at 44-inch protrusion and then in conventional air-cooled jaws at 14-inch protrusion. The outstanding feature of the water cooling, combined with the shorter protrusion which this makes possible, is the ability to carry much higher currents than with air cooling, and to attain higher brightness as a result. Within the limits of satisfactory air-cooled operation, however, the carbon reaches a higher brightness at a given current than when water-cooled, so that the current efficiency of the carbon is reduced by water cooling. The ability to carry higher currents with water cooling is not characteristic of all carbons, however. To illustrate this the performance of. the 13.6-mm super high-intensity projector carbon, representative of the usual type of carbon, is shown in Fig. 5. Here water cooling in no case produces a higher brightness than can be obtained with air cooling, and the current efficiency is always less. Thus with this, as with most conventional types of carbons, water cooling has no such advantage in increasing brightness as is exhibited by the “highbrightness” carbon of Fig. 4. FIG. 7—Effect of positive protrusion on current capacity of 13.6-mm, “290-ampere’’ and super high-intensity projector carbons. MAXIMUM CURRENT-AMPS. 1949-50 THEATRE CATALOG Referring again to Figs. 4 and 3, it will be noticed that sharp breaks occur in three of the four curves in the two figures, at the points indicated by the vertical arrows. These are the currents at which the carbon “overloads,” with the accompanying hissing and sputtering which is familiarly encountered in such cases. At higher currents, the arc is noisy and generally unsteady, prohibiting operation under practical conditions. It is the practice, of course, to operate a carbon at a current somewhat below this ““maximum” value at which overload occurs. The 13.6-mm super high-intensity projector carbon, for example, overloads at about 176 amperes, whether wateror air-cooled, so that 170 amperes is the recommended maximum operating current for this carbon. The high-brightness carbon (Fig. 4) reaches a similar overload condition at 282 amperes when aircooled. However, in interesting contrast to the usual types of overload, this carbon does not behave in the manner just described when water-cooled, even at currents up to 500 amperes. It operates quietly up to about 325 amperes. At higher currents, the light remains steady, but a sort of droning noise gradually develops, which is altogether different in quality and much lower in intensity than with the conventional type of overload, and quite tolerable in many applications. We have found this clear-cut difference to exist to the extent described only with carbons having relatively thin shells (less than 2 mm thick with the 13.6-mm carbon). High-brightness carbons having thicker shells (of which the 13.6-mm, “290-ampere” carbon’ is an example) exhibit tendencies toward the hissing type of overload common to usual types of carbons, so that their “maximum” current is fairly well defined. Another manifestation of the unique properties of the high-brightness type of carbon is the relation of brightness and of are current to positive protrusion. Figs. 6 and 7 show these relationships for the 290-ampere. 13.6-mm carbon. As the protrusion is lessened to give improved crater cooling, the “maximum” current and the brightness increase. The usual type of carbon, exemplified again by the 13.6-mm super high-intensity positive carbon, exhibits little or no change in brightness and “maximum” current with change in protrusion. High brightnesses have been obtained with these special carbons at significantly higher current and carbon efficiencies than have been reported by other investigators. For instance, the maximum performance predicted by Hallett’ is exceeded by all of the 15 high-brightness carbons for which the data are plotted on Fig. 8. These carbons are from 9 to 13.6 mm in diameter and exceed the predicted performance at a given current density by as much as 10 per cent, although the general shape of Hallett’s master curve is followed quite well. Another interesting property of these carbons is their ability to produce a much higher brightness at a given consumption rate than was characteristic of the carbons which Finkelnburg* examined in Germany. Data on many of our high-brightness carbons ranging in size from 9 to 16 mm and burned in watercooled jaws are plotted on Fig. 9. The : 3 3 MAXIMUM BRIGHTNESS-GANOLES PER MM* o 1 2 3 CURRENT DENSITY-AMPERES PER Mid? 4 FIG.8—Brightness variation with current density for “‘high-brightness” carbons, brightness at a given consumption rate exceeds that reported by Finkelnburg by more than 50 per cent in all cases. REFERENCES (1) M. T. Jones R. J. Zavesky, and W. W. Lozier, ““A new carbon for increased light in studio and theatre projection,"” J. Soc. Mot. Pict. Eng., vol. 45, pp. 449-459; December, 1945. _ (2) F. T. Bowditch, “Light generation by the highintensity carbon an," J. Sve. Mot. Pict. Eng., vol. 49, pp. 209-218; September, 1947. _ G) W. Finkelnburg, “The influence of carbon cooling on the high-current carbon arc and its mechanism,” J. Soc. Mor. Pict. Eng., this issue, pp. 407-417. (4) W. Finkelobure. “The High-Current Carbon Are,”’ Field Information Agency, Technical, Office of Military Governmen: for Germany (US), Final Report 1052. (Office of Technical Services P.B. No. 81644.) Review published J. Soc. Mot. Pict. Eng., vol. 52, pp. 112-113; January, 1949. (5) C. G. Heys Hallett. ‘Recent developments in carbon arc lamps,"” J. Brit. Kinematograph Soc., vol. Il, p. 188; December, 1947. FIG. $—Brightness variation with ti rate for “high-brightness’ carbons. Sarasa Set : Fy o 3 r=} 3 é 5 & : = 395