The motion picture projectionist (Nov 1929-Oct 1930)

lune, 1930 Motion Picture Projectionist 29 has a marked influence upon its efficiency. "Open Arc" Type The earlier type of arc lamp was known as the "open arc" because it was designed with the electrodes exposed to the atmosphere. While this lamp was very efficient in comparison with other illuminants of its day and was widely used for street lighting, no redirector was provided to utilize the light normally emitted upward. Consequently, it illuminated a relatively small area immediately adjacent to the lamp to a high intensity while at points a relatively short distance away the intensity was low. Moreover, there was usually a dark spot immediately below the lamp due to the shadow cast by the lower carbon and its support. An improved type of arc lamp was the enclosed arc wherein the arc was surrounded by a glass globe. Although not airtight, the enclosing globe prevented free access of air to the electrodes with the result that the consumption of the carbons was greatly retarded. While the enclosed arc did not give as much light for the same wattage as the earlier type of open arc, it was unnecessary to make as frequent renewals of the carbon electrodes. As a result it was extensively used for street lighting. In ' the original carbon arc lamp, the hot gases in the arc gave off about 10 per cent, of the total light and the remaining 90 per cent, was emitted by the heated carbon electrodes. Later, it was found that if the electrodes were impregnated with certain salts, the arc gases became highly luminous. Thus, for a given amount of electrical energy, the socalled flaming arc lamp which resulted from this impregnation furnished considerably more light. These lamps were handicapped by the necessity for frequent carbon renewals. Attempts were made to enclose the flaming arc by providing a condensing chamber to remove the solid products of the combustion of the Mercury vapor lamp electrodes, but in spite of this a coating formed on the globes and in some cases the globes became roughened or pitted. Because high initial efficiency of these lamps could not be maintained, they did not prove particularly economical and therefore gradually disappeared from use. More light from the arc was also obtained by using electrodes of materials other than carbon, a notable example being the magnetite arc, some of which are still in use. In the magnetite arc a bar of copper is used for one electrode. The other electrode is a thin steel tube containing a composition of magnetite. Magnetite arcs are of high efficiency. The electrodes are not consumed rapidly, hence they require infrequent trimming. The light emitted is relatively white in color. Mercury-Vapor Lamp Another lamp, widely used for photographic purposes and to some extent for industrial lighting, is the Mercury-Vapor lamp, which can be recognized by the characteristic bluish green color of the light. This lamp consists of a long glass tube from which the air has been exhausted and which contains a small amount of mercury in a bulb at one end. An electrode is sealed into the glass at either end of the tube. The arc is started by tilting the tube slightly until a thin stream of mercury completes the circuit between the terminals. Returning the tube to its normal position breaks the circuit as the mercury runs back into the bulb and draws an arc behind it. The heat of the arc soon vaporizes enough mercury to fill the tube and it becomes luminous throughout its entire length. The peculiar color of the light is due to the presence of a relatively high percentage of green and blue rays and to the absence of red rays. Since photographic emulsions are most sensitive to green and blue, the mercury vapor arc is an efficient illuminant for photographic purposes. The Neon Tube When an electric current is forced through certain gases, they become luminous. Neon, one of the rare elements of the atmosphere, is a gas of this type which glows with a rich orange-red light. The Neon tubes used in electrical advertising operate on this principle. Glass tubes are formed in the shape of letters or any other desired patterns and an electrode is sealed into each end. They are then carefully exhausted of air and a small quantity of neon admitted. Due to the high Electric arc between carbon terminals resistance of the gas, voltages of from 8,000 to 12,000 volts are required to operate the tubes. Colors other than the characteristic orange-red are obtained by introducing small quantities of other gases such as mercury vapor, argon, and carbon dioxide, and by making the tubes of colored glass. Neon tubes have a bright, vivid, day and night appearance. They consume a relatively small amount of current but have a rather low power factor (about .50). Hot Cathode Neon Lamps A recent development in gaseous conductor tubes is the so-called "hot cathode tube." In this device one of the terminals is heated to incandescence. This allows the lamp to operate at a considerably lower voltage and provide a better efficiency in terms of light output per watt of energy consumed. Tubes of this kind have been used for spectacular lighting and for the floodlighting of buildings. Electric Eye Detects Gas Not only is every motor car which passes through the Holland Tunnels counted electrically, but a new application of the same electric eye, or photoelectric cell, has now been installed to give warning of dangerous exhaust gases from automobiles. Warm exhaust gases cast faint shadows across a beam of light, and the minute change is recorded by the sensitive photoelectric cell. This, in turn, by the change of current on the circuit, registers on a dial in the superintendent's office and enables the man in charge to turn a switch to speed up suction fans to carry the gases away. Fire Extinguishers _ For positive results in the operation of fire extinguishers, those containing soda and acid, or foamite solution, must be refilled every 12 months ; this is also a requisite of the Board of Fire Underwriters and most cities or municipalities. Those fire extinguishers containing tetrachloride solution are not necessary to refill once each year, but should be shaken up at intervals of every six months. Should only a portion of the content of any fire extinguisher be used, discharge the remaining content and refill. In addition, have placed on each container a tag showing the last inspection or filling.