International projectionist (Oct 1931-Sept 1933)

October 1932 INTERNATIONAL PROJECTIONIST 23 an alkaline metal, known as a "getter," is vaporized within the tube to create a chemical reaction with any remaining oxygen or water vapor within the tube and bury other possible gases under the mirror-like coating which forms on the tulb. The tube is then sealed off from the manifold and the pressure at that time in the tube is of the order of one-billionth of an atmosphere, or equal to the 1 pressure of ^ millimeter of 1,300,000 mercury. Measuring Degree of Vacuum The importance of obtaining a high ■degree of vacuum has been described; how then is this degree of vacuum ascertained? By means of a rather simple affair, not unlike a vacuum tube, and called an ionization manometer, gas pressures may be measured to one-trillionth of an atmosphere. This manometer is sealed onto the pump station or to the tubes under test in the manner shown in Figure 5. (The manometer is the smaller of the two tubes.) The essential parts of an ionization manometer are a thermionic filament or cathode, an anode for collecting electrons and a collector for the positive ions. The filament is heated to a temperature at which electrons are emitted, the electron anode is raised to a potential of 100 volts or more, positive to the filament, in order to attract these electrons, and the ion collector is maintained at a potential of from 6 to 9 volts negative to the filament. When gas is present in the system, collisions will occur between the emitted electrons and the gas particles, thus forming positive ions which will be drawn to the ion collector. The positive ion current is indicated on an ion current meter and will be proportioned to the gas pressure in the tube. As the gas pressure becomes less, the meter reading becomes less. By knowing the characteristics of the manometer Fig. 4. Life test rack for testing Y^-ampere filament tubes and the value of the electron current, a true measurement of the degree of vacuum reached may be obtained. After an ageing process the tube is given a preliminary inspection to insure that it will meet all its requirements, then the base is put on and the tube is ready for its final test. Testing Process The tube is finally given a complete electrical and mechanical inspection. The filament resistance, the degree of vacuum, the impedance, the amplification factor and the amplification of the tube in a circuit — all are measured. The amplification in this circuit can be measured to within one-tenth of a decibel. The maintenance of satisfactory characteristics over a long life is one of the principal requirements of Western Electric vacuum tubes. Life testing, therefore, is one of the important functions carried out, and special equipment and circuits have been devised for this purpose. Because of the extreme length of life of the tubes and the large number that have to be tested, new circuits have recently been developed which combine economy of space and maximum ease in making these tests. Twelve circuits, each arranged for testing six tubes, are mounted on a sihgle bay of a relay rack. Additional bays are added when expansion of the testing facilities is required. (Figure 3.) When making the life tests the applied plate voltage is the same for all tubes under test, but the grid potential is separately adjustable for each circuit, and the filament current is separately adjustable for each tube. The cathode or filament temperature is the most important operating parameter of a tube, and in these tests it is controlled by regulating the filament current. Filament power is supplied by a small transformer in each circuit, which is fed from a 110-volt, 60-cycle source through a voltage regulator of the saturated iron type. The regulator is so designed that it supplies the transformer primary with a constant voltage regardless of load conditions or line voltage. Because of this constant voltage the filament current remains fixed at the correct value once the rheostat in the filament circuit has been adjusted. The grid-biasing potential is obtained from a battery of dry cells which is provided with end cell taps. These are run to each of the twelve grid control switches on each bay. Since the plate voltage remains fixed, the plate current is controlled only by the grid potential. Other features of the life-testing circuit are: protection against short circuits between the filament and grid, should the filament of one of the tubes burn out and fall against the grid; fus Fig. 5. New ionization manometer ing of both plate and filament circuits, and total absence of test meters from the test racks. To add meters to the racks would greatly complicate the wiring and reduce the effective test positions as well as increase the cost of the test equipment. A special mobile test wagon was therefore designed which can be wheeled into position in front of the test racks. On this wagon are all necessary test instruments, which may be connected to the tube test positions by means of a plug. (See Figure 4.) {To be Continued) SONOLUX EXCITER LAMPS Exciter lamps for all makes and types of sound picture reproducing systems are now being manufactured and distributed by the Sonolux Company, 3 Central Avenue, East Newark, New Jersey. Several of the larger theatre chains have standardized on Sonolux and many dealers throughout the country have ample stocks. Sonolux lamps are guaranteed by the makers to render perfect satisfaction over remarkably long periods of time, and because of their low cost to effect definite operating economies. NEW WEBER DISTRIBUTOR Motion Picture Sales & Service Co., with headquarters at 1501 Broadway, has been incorporated for exclusive distribution in the metropolitan New York area of products produced by the Weber Machine Corp. of Rochester, N. Y., manufacturers of Synchofilm sound equipment. Development of a new portable sound and visual 35 mm. sound-film projector complete with amplifier and speaker has also been announced by the Weber Machine Corp.