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CONSTANT POTENTIAL GENERATORS FOR MOTION PICTURE PROJECTION
By A. M. Candy
SYNOPSIS The following -paper discusses the development through which the constant potential generator has been carried for producing satisfactory results in projecting pictures in modern motion picture theatres. To eliminate any flicker or decrease of illumination on the screen when a second lamp is connected to the generator while the other lamp is being used for projecting pictures, has required very careful study of motion picture projection requirements by the design engineer. As a result, these generators, with their control equipment, have been perfected so that several lamps can be operated from the same generator, using different current values in the various lamps, without producing any visible flicker or decrease of illumination of pictures on the screen. The ability of the machine to carry several lamps at different current capacities will fulfill the requirements of modern theatres where it is frequently desirable to operate stereopticon lamps, spot lamps, flood lamps and a projector at the same time or any combination of these machines.
A FEW years ago, the public was satisfied to attend motion picture theatres which were equipped with only one projection machine necessitating a delay of from four to eight minutes at the end of each reel while the operator was removing the reel just projected, inserting, and rethreading a new reel. The electrical equipment required for supplying current for such a projection scheme was quite simple to design and operate.
To some unknown person, however, we are indebted for the idea that two projection machines can be used alternately and thereby materially reducing the time lost at the end of each reel, the same electrical generating equipment being used. Some other equally progressive person conceived the idea that if the lamp in the second projector could be started before the end of a reel, then the new reel could be started at the proper time so as to make a continuous projection of pictures on the screen. This necessitated the addition of electrical control equipment or ballast resistance for the second projector as well as for the first, see Figure No. 1. Obviously, when the second lamp was started, an overload was imposed upon the source of electrical power, usually a motor generator, see Figure No. 2, and as a result, the voltage of the generator was decreased which caused each lamp to receive less than its normal current, thereby decreasing the brightness of the picture on the screen. This is illustrated by Figure No. 3, which shows the characteristic curve of a generator designed for delivering 55 amperes to one arc only, with the arc operating at approximately 60 volts. When a constant voltage generator is used for this purpose, it is necessary to provide a resistance in series with the arc to make it stable. Since the generator is designed for 80 volts and the arc operates at approximately 60 volts, the
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