International projectionist (Oct 1931-Sept 1933)

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22 INTERNATIONAL PROJECTIONIST September 1933 TABLE II Power Required for Equal Screen Light Using Resistance and Transformer Control Control Resistance Transformer * Assuming transformer efficiency of 95 per cent. Line Voltage Arc Voltage 115 30 115 30 Arc Power Cpnsumption Current in Watts 80 9200 80 2500* mm. carbons at 65 amperes and the 8-mm. carbons at 80 amperes. Wholly Satisfactory Results The uniformity of screen illumination is as good as that obtained with direct-current arcs, as shown by Table III. The same optical system, consisting of a reflector, aperture plate, and objective lens, was used in these tests. There was no rotating shutter in the system. It should be noted that these data were obtained on a laboratory set-up, and while they are perfectly comparable among themselves, they are not indicative of what might be obtained with any other set-up of optical system and screen. Table III shows that there is no question that the uniformity of screen illumination with the new alternatingcurrent arc is equal to that of the lowintensity, direct-current arc, with the very distinct advantage that the bluewhite color of the light resembles very closely that obtained from the directcurrent, high-intensity arc. There are not yet available detailed comparisons of the arcs of higher amperages, but a sufficient number of measurements have been made to be certain that Table IV provides a general idea of what may be expected from these alternating-current arcs in terms of well-known direct-current projection systems. In arriving at the values for line watts an efficiency of 80 per cent has been assumed for the motor-generator sets in the case of the direct-current arcs and 95 per cent for the transformers for the alternating-current arcs. One practical test has been in progress since September, 1932, in a theater with a 12 x 16^ -foot screen and a throw of about 100 feet. There have been no complaints concerning the quality of the screen light or the quantity, and no unusual operating troubles have been encountered. In order to furnish a general idea of the probable fields of use for this arc there is given in Table V a general summary of carbons, currents, voltages, wattages, and screen light in arbitrary units on a screen of a given size. The optical systems were the conventional ones employed with the different kinds of carbons and lamps in actual use in the theaters. Member: Since the arc is supplied in the evening when the greater part of the power load is taken off the line. Such a condition may be found in an Industrial district where the loads are heavy, but we have thought the voltage changes in our laboratories to be largely confined to our own lines on the low-voltage side of the power transformers serving both factory and laboratory. If difficulty is encountered with variable voltage, the trouble may be very easily corrected by a small iheostat in the lamp circuit or by variable taps on the transformers. Member: How do the a-c. carbons compare in price with the present d-c, low-intensity carbons? Operating Costs Mr. Geib: The operating cost of the new a-c. projector trim will be a little over 50 per cent higher than it is with the present low-intensity, d-c. trim, but this cost is just about one-half that of Hi-Low operating cost. The improvement in intensity and color of light with the new a-c. arc far more than offsets the slight increase in operating expense. Mr. Richardson: It was extremely difficult for projectionists to maintain steady screen illumination with the old style a-c. arc; the craters were small and difficult to handle. Will this trouble (Continued on next page) Line Carbons Cur. Line Volt. Line Watts Arc Cur. Arc Volt. 7-mm. Pos. 9 d-c. 110 990 9 d-c. 50 5-mm. Neg. Neutral Core 32832-2 8 d-c. 110 680 34 a-c. 19 ** Weston photronic cell. TABLE IV TABLE III Comparative Screen Light from Alternating-Current and Direct-Current, NeutralCored Carbon Arcs with the Regular Mirror Arc Optical System Foot-Candles on Screen** Middle Left Right Top Bottom 21 20 19 20 21 27 25 25 26 27 S. M. p. E. CHICAGO CONVENTION The Fall meeting of the Society of Motion Picture Engineers will be held at the Edgewater Beach Hotel in Chicago, October 16, 17 and 18, according to an announcement of the Board of Governors of the Society. Chicago is an ideal selection this year, according to the board, since members may visit the Century of Progress Exposition while attending the convention, and because of the unusually low transportation rates being offered. A feature will be the announcement and inauguration of the newly elected officers of the Society. directly from the a-c. supply lines through a transformer, every voltage variation of the outside lines will be transmitted to the arc. In view of the fact that all power companies allow themselves a variation of at least 5 per cent above and below the normal voltage (a total of 10 per cent between minimum and maximum) , what would be the effect of such variation on screen illumination and flickering? Mr. Downes: We have not found a 5 per cent variation in voltage in our laboratories and factories except at two very definite times each day: in the morning when the factory starts and Performance of Alternating-Current Arcs with Large Currents Screen Light Compared Carbons Current Volts Average Line Watts* Optical System with SRA Carbons at 35 Amp;, 55 Volt. Line Watts 6-mm. a-c. 40-45 22-25 945 Regular Mirror Arc 60-70% 3500 7-mm. a-c. 60-65. 23-26 1580 tt 85-95% 3500 8-mm. a-c. 75-80 24-29 2130 " 115-150% 3500 * Transformer efficiency. 90 per cent. HIGH FIDELITY INSTALLATIONS Contracts for the installation of Photophone High Fidelity sound equipment in nine theaters, eight of them in New England, have been received by RCA Victor Co. The houses are, in Massachusetts: Kameo, Pittsfield; Arcadia, Portsmouth; State, Quincy; Auditorium, Lynn, and Strand, Lawrence. In Connecticut: Empire, New London, and Strand, Norwich. Also the Park, Nashua, N. H., and the Rialto, Poughkeepsie, N. Y.