International projectionist (Jan 1961-Dec 1962)

INTERNATIONAL PROJECTIONIST Including a special Audio-Visual section relating to the operation and maintenance of A-Y equipment in the educational and industrial fields. Volume 36 Feb. 1961 EAST COAST OFFICE 545 Fifth Avenue New York 17, N. Y. Murray Hill 7-7746 RAY GALLO General Manager MIDWEST OFFICE 1645 Hennepin Avenue Minneapolis 3, Minnesota No. 2 FRANK W. COOLEY, JR. Editor and Publisher RAY GALLO General Manager R. ENTRACHT Associate Publisher R. A. MITCHELL Technical Editor In This Issue "Low-Powered" Projection 4 By ROBERT A. MITCHELL Technical Editor New Hillendale Theatre 8 Letters to The Editor 12 News Notes — Technical Hints — Miscellaneous Notes INTERNATIONAL PROJECTIONIST, published monthly by the International Projectionist Publishing Co. division of The Northern Publishing Co., Post Office Box 6174, Minneapolis 24, Minnesota. Editorial offices, 1645 Hennepin Avenue, Minneapolis 3, Minn. Subscription Representatives: AUSTRALIA— McGills, 183 Elizabeth St., Melbourne; NEW ZEALAND— Te Aro Book Depot, Ltd., 64 Courtnay Place, Wellington; ENGLAND and ELSEWHERE-Wm. Dawson & Sons, Ltd., Macklin St., London, W. C. 2. Subscription Rates: United States, Canada, and U.S. Possessions, $3.00 per year (12 issues) and $5.00 for two years (24 issues). Foreign countries: $4.00 per year and $7.00 for two years. Changes of address should be submitted four weeks in advance of publication date to insure receipt of current issue. Second-class postage paid at Minneapolis, Minn. INTERNATIONAL PROJECTIONIST assumes no responsibility for personal opinions appearing in signed articles, or for unsolicited articles. Entire contents copyrighted 1961 by INTERNATIONAL PROJECTIONIST PUBLISHING CO. MONTHLY CHAT Why Dim Screen Light With Good Arc Lamps? Check Your Carbon Trim and Current! IT IS A STRANGE and unfortunate fact that dim, discolored screen light seems to gravitate into the heart of metropolitan threatre districts. Projectionist pride in good equipment is high in the downtown areas, and the most modern arc lamps, projectors, and lenses are in use. Are urban screens inferior and dirty? Investigation disproves this theory. Why, then, the disconcerting prevalence of low light levels in many metropolitan houses, both large and small? A visit to suburbia provides a clue to the mystery. Smaller carbons are burned in most of the out-of-town theatres in relation to arc current and screen area, and the light is generally brighter and distincdy whiter. The conclusion is ineluctable: the better suburban theatres, wellmanaged and prosperous, employ higher current densities (not necessarily higher currents) in their arc lamps. Current density is measured in amperes per square millimeter of the cross-sectional area of the positive carbon. The cross-sectional areas of the standard sizes of positive carbons are: DIAMETER IN AREA IN MILLIMETERS SQUARE MM. 7 38»/2 8 50'/4 9 63 y2 10 78% 11 95 13.6 145V4 While the maximum current permissible with Suprex carbons is limited by the thickness of the copper coating (remember those "Victory" carbons?), tests wi.h the various brands reveal that maximum current should never exceed a current density of 1.4 amps, per sq. mm. in the case of a Suprex positive. More important, the minimum current density for white light and steady burning with Suprex positives should never fall below 1 amp /mm2. The specified current ranges for normally coated Suprex positives, therefore, are as follows (rounded off to the nearest 5 amps.): SIZE OF POSITIVE 7-mm Suprex 8-mm Suprex 9-mm Suprex MINIMUM MAXIMUM AMPS. AMPS. 40 50 65 55 70 90 «£» The maximum current density permissible for regular black 9-, 10-, and 11-mm carbons for rotating-positive mirror lamps is limited by the tendency to spindle, while the minimum current density is again determined by the necessity of holding a steady arc without excessive yellowing of the light. The maximum and minimum densities for all these positives are 1.4 and 1.1 amps/mm2. The 13.6-mm regular carbon, however, gives evidence of somewhat different burning properties. For this size of positive, the maximum and minimum appear to be 1.1 and 0.83 amp/mm2. (Continued on Page 18) International Projectionist February 1961