The motion picture projectionist (Nov 1929-Oct 1930)

22 Motion Picture Projectionist May, 1930 from a light source and yet he was quite familiar with the W. E. sound reproduction system. The photoelectric cell is very sensitive. Instead of being sensitive to sound, it is sensitive to light, and light variations falling upon a photoelectric cell cause it to affect the current in exact proportion to the intensity of the light. A weak light will cause but a small change in current and a stronger light a greater change. We may regard the photoelectric cell as a perfect insulator in the dark and a partial conductor when exposed to light. The cell, however, does not conduct a great amount of current even when it is subjected to powerful beams of light. At best, only a current of a few microamperes will pass through them. Types of Cells There are two types of photoelectric cells; one the gaseous type and the other the high-vacuum type. The W. E. cells are the gaseous type. In the gaseous type, there is admitted to the cell during its construction and after a high vacuum has been created, a very small amount of one of the rare gases, such as argon, neon or helium. Such gases, when subjected to the bombardment of electrons that are released when the cell is struck with light, become conductive to a degree depending upon the intensity of the light. Potassium hydride is one of the extremely photo-sensitive combinations, which is deposited upon the inside of the photoelectric cell, which is connected to the negative terminal of the cell. The other electrode of the cell takes the form of a grid anode which may be made up of either a single loop of fine wire, or a thin metal gauze suitably suspended and supported, which is connected to the positive terminal of the cell. When electrons are emitted from the cell surface, these electrons are drawn toward the anode conductor due to the positive potential, which is maintained by a battery. Selenium Cells While discussing the photoelectric cell, I might mention the Selenium cell. We must not make the mistake of confusing selenium cells with photoelectric cells, inasmuch as we are dealing with two different effects, although it may be said, broadly speaking, that both types of cells operate on a photoelectric principle if we take "photoelectric" to mean the control of current by light. In the photoelectric cell we are dealing with pure electronic emission surfaces. In the selenium cell, on the other hand, we are dealing with changes in ohmic resistance brought about by light. The cost of selenium cells as compared with photoelectric cells is much less. Selenium cells vary considerably in their quality. In every type of selenium cell there is always the vari able factor, which makes it very difficult to use them in film reproduction. However, they are being used at the present time. Every selenium cell has what is known as lag or inertia. The lag is the time that expires between the instant that the light falls upon the cell and the instant that the resistance of the cell drops in response to the light. This lag is controllable to a certain degree by the design of the cell itself and the method used in preparing the selenium. As a general rule, we might say that the higher the resistance of a cell, the less its inertia. It may further be claimed that the higher its resistance, the greater the ratio of sensitivity. This inertia not only opposes the drop in resistance when the cell is illuminated, but it also opposes to a much greater degree the return to normal resistance. This inertia effect can be reduced still further by enclosing the cell in exhausted glass tubes. This not only reduces the lag or inertia, but also adds considerable to the life of the cell. There are, at least, twenty-five different types of selenium cells, which are a result of the research work of many different experimenters. Selenium Supply Selenium is found among the rare minerals, and is related to both sulphur and tellurium in the periodic table. The source of the metal in this country is in the anode muds of our electrolytic copper refineries. Only a few tons of it are produced each year. In its raw, untreated state it shows a resistance as great as that of copper. When properly annealed by keeping it at a temperature just below its melting point for a long period of time and permitting it to cool slightly, it assumes a crystalline condition and in this condition its electrical resistance is considerably reduced and it at the same time becomes susceptible to changes induced by light. P. E. C. Amplifier As explained before, the photoelectric cell circuit is inherently one of high impedance. In spite of precautions, many projectionists continue to vary the photoelectric cell amplifier filament rheostat while the Movietone attachment is in operation, which results in popping and crackling noises which are often heard from the horns along with other reproduction. The current value of the p. e. c. amplifier tube filaments should always be set before starting the projector. Many projectionists, while running Movietone productions, will pull the switch controlling the p. e. c. amplifier and exciting lamp after making a changeover. This is not a good practice. Both photoelectric cell amplifier and exciting lamp currents come from the same source of battery supply. When the values are set, and the Movietone projector started, any variation in this circuit will be perceptible in the reproduction — therefore, it is advisable to always leave the circuit completed on the projector not in operation. However, the current values may be reduced. Many theatres, running continuously, will object to this method of operation, because of the drain on the "F" batteries. Every theatre, where W. E. equipment is installed, has two sets of "F" batteries, and therefore one set should always be fully charged while the other one is in use. It is imperative that Fader contacts be cleaned quite frequently. I have found many faders with contacts badly corroded and very dirty. When Fader contacts become dirty, noises will be perceptible on changeovers, and when increasing or decreasing the volume. Fader contacts should be cleaned with a soft rag dampened with carbona and after cleaning they should be polished with embossed paper and a very small amount of vaseline applied to the contacts, which will help ease the operation and besides, will help to keep the contacts from becoming corroded. Never use rags for cleaning rheostats, switches or contacts or any sound equipment, unless the rag is absolutely free from lint. A small piece of lint catching between contacts will often prevent the circuit from being completed. I. A. Convention Plans Representatives of all affiliated local unions of the International Alliance will convene in Los Angeles, California, on Monday, June 2. Eastern delegates to the convention will leave for the West Coast not later than May 28th, and many parties will leave even earlier. It is not unlikely that a special train will be made up out of New York City to accomodate New York State, New Jersey, and New England delegates. Los Angeles, convention city, is the home of five local unions chartered by the International Alliance, namely, L. U. 150, projectionists; No. 33, stagehands; No. 659, cameramen; No. 683, laboratory workers, and No. 37, studio stage mechanics. Nos. 33 and 150 meet in Los Angeles, and the other three in Hollywood. These five local unions have a total membership of more than 3,700. West Coast Local Unions, particularly the above-named five, are highly pleased at the choice of Los Angeles as the convention city, and they are making elaborate preparations to entertain visiting delegates. Ontario Safety Campaign A campaign has been launched in Ontario for the inspection of all theaters with a view to removing risks of fire and panic. Hon. Dr. J. D. Monteith, Provincial Treasurer, is behind the drive and expects to have all of the 428 houses in Ontario gone over within the next 12 months.