The motion picture projectionist (Nov 1931-Jan 1933)

24 Motion Picture Projectionist December, 1931 Let us consider what that means. First, the supply voltage was 100 in both cases. At "A," the rheostat drop, which is the difference in voltage between the line and the arc, was 100 minus 63 or 37 volts. In the case at "B" it was 100 minus 83 or 17 or less than fifty per cent of "A." It means that with the rheostat at "A ' set up for 115 amperes, the current would be approximately 121 amperes, while the arc at "B" would have only approximately 87 amperes using the total output of a 180 ampere rheostat. We cannot blame the projectionist at "B" for complaining because of the poor light on the screen but suppose rheostats were built on the basis of his operating conditions. Such a rheostat will be satisfactory for his application, but what happens if an identical rheostat is shipped to "A"? Plenty! "Why, man alive, the rheostat gets red hot! Surely that's no way for a rheostat to operate." Everybody looks worried and wonders why. The answer is that if a rheostat is designed for a 17 volt drop and it actually has to drop 37 volts it is only carrying an overload of 475 per cent. This is the reason why manufacturers wish an arc had more definite characteristics so that they could figure on definite and fixed values and that all rheostats operating on similar applications would operate with equal voltage drops. Standard Values Desirable The problem would be more serious except for the general cooperation of the projectionists who quickly sense the cause of the effect and either operate the arcs with voltage values in accordance with the present specifications, at least attempt to follow the recommendations of the lamp house manufacturer, or change the connections on the rheostat so that the voltage drop across the rheostat is complementary to that across the arc. However, it would clear the. situation if the lamp house and carbon manufacturers could agree to some standard values of arc voltage for various carbon combinations and current ranges. Another interesting problem that is always presenting itself in one form or another to rheostat manufacturers reads about as follows : "Please furnish ballast rheostat suitable for 180 ampere arc to operate from an 80 volt generator." Have you tried to operate a 180 ampere arc from an 80 volt generator? Yes, it can be done, but only by an expert and then only if the arc voltage is below normal. Usually a question of this kind is handed back with advice to consult with the generator manufacturer, and it usually returns with the generator voltage specification changed to 100 volts or higher. Probably somebody was figuring on' saving a few watts lost in the ballast. Well, those watts are a good investment as they pay for themselves by insuring a steady light on the screen. "Power House Practice" Have you ever walked into a rheostat room and noticed the number of switches on each rheostat and wondered why so many switches? Rheostat manufacturers wonder also but the only answer seems to be "it is standard practice." Standard practice be hanged! Why should a rheostat be rated 30 to 200 amperes and look like an old-fashioned power house with eight or more switches for current adjustment when it is going to be used on a 180 ampere arc and might just as well be rated at 140-200 amperes and supplied with four switches and furnished with a 90 ampere tap for warming up the carbons. Who expects to use a 200 ampere rheostat on a 30 or 60 ampei^e circuit? If the explanation is that 30 amperes is desired for warming up, it can be so furnished but less the extra switches, which, by the way, are not furnished gratis. Now considering the same 30-200 ampere rheostat, what arc voltage should be used as a basis for the design of the rheostat? If 60 or 65 volts is used, it will be satisfactory for a 125 ampere arc but unsatisfactory for a 180 volt arc with 80 to 85 volts across the carbons. If designed for the latter condition, it will be overloaded on the lower range. The above is perhaps an extreme case taken as an example but every manufacturer meets similar problems quite frequently. Report of the Screens Committee Because of lack of space it was impossible to complete the S.M.P.E. Screens Committee's report in our November issue. The information furnished is of great practical value and it has, therefore, been deemed advisable to conclude the report and the discussion concerning it in our present issue. — The Editor. Illumination THE study of screen illumination is one of the primary aims of the Screens Committee. We hope to determine average values of brightness encountered in theatres and to discuss these in relation to stray light, print density and physiological factors. Also, we plan to consider and standardize methods of measuring brightness, which, at the present time, because of their lack of uniformity, render the comparison of data difficult. Some information on screen brightness has been accumulated but not sufficient for presentation at this time. Rear projection is attracting wide attention at the present time in New York and promises to develop into a field of interest throughout the country. The manufacturers of this type of screen are not as yet willing to release engineering information so that we are postponing discussion of this for our later report. Discussion President Crabtree: How are the screens cleaned? If the brush method is used, how are they brushed? Is the screen taken down from its position or is it brushed in place? Also, how is the screen resurfaced? What is the cost of resurfacing in comparison with the cost of the screen? Is it worth while? Mr. Falge: The screen is cleaned in position with a very soft, longhandled brush. Cleaning is very simple, but is often neglected. Some one in every theatre should be given the responsibility of keeping the screen clean. The cost of taking down the screen, packing and shipping it to be resurfaced, and mounting again is so great that it is better to clean the screen in position. Screens may be resurfaced in a number of ways, the spray process being the most satisfactory. The cost of this treatment varies in different places, from 10 to 20 cents per foot. A new screen may cost from 2V2 to 4 times the cost of resurfacing, depending upon the amount of surface to be treated. Screens can be resurfaced satisfactorily, but in general the process is not satisfactory, as the material used for resurfacing becomes yellow and is not always put on uniformly. Spraying Screen President Crabtree: What is the effect of spraying a beaded screen? Is it cleaned by spraying, or were you referring to diffuse screens? Mr. Falge: I was referring to diffuse screens. No good is accomplished by spraying a beaded screen, as the spraying causes the beads to lose their directive qualities. In general, it is extremely difficult properly to clean screens on account of the wide expanse of the flat surfaces. Beaded screens can be cleaned satisfactorily, but the process is very complicated. The interpretation of measurements must be left to the discretion of one closely acquainted with the measuring conditions. A general attenuation in loudness, as judged from the measured screen transmission characteristic, greater than 1 db., is not considered tolerable. Although this limit may appear rather stringent, there are many screens available which meet this requirement. It seems advisable to maintain this high standard for sound transmission. President Crabtree: Could some solvent be used for cleaning the beaded screen?