Projection engineering (Sept 1929-Nov 1930)

Page 8 with the same lamp using a slit to produce a sound track of 250 mm. width — the size desired for the new "Grandeur" film. These recordings have shown ample light intensity for highly satisfactory recording on this new size of film. In this lamp a special glass is used which permits the passage of the violet and ultra-violet rays with a minimum amount of loss. At the point where the light is required to pass, the bulb is blown very thin, thus further assisting in the transmission of this highly important range of frequencies. In the early investigations it was found that glow lamps available for sound recording had an average life of approximately 500 to 600 feet of effective sound track. Beyond this footage the continued use of the lamps then available resulted in a blackening of the bulb and a consequent shutting off of the effective light. To avoid this premature blackening, it was found vital to select electrodes having the minimum tendency to vaporize under operating conditions. It was also found vital to select a mixture of gases and to determine a gas pressure, the combination of which should reduce this tendency toward vaporization to the lowest possible point. Furthermore, this choice of the gases and their pressure is also controlled by the characteristics of the desired light intensity. Naturally, the life of any lamp will depend upon the conditions under which it is operated, but the new Zetka glow lamp is capable, under ordinary conditions, of recording upwards of 25,000 feet of sound track. There have been some recordings made which ran as high as 60,000 feet. Every effort has been exerted to make this lamp as rugged as possible in order to withstand safely the shocks and abuse incident to express and parcel post shipments as well as the vibrations imposed by recordings under "field" conditions. The latest design Is illustrated in Fig. 1. Its physical dimensions are a maximum outside bulb diameter of 1 inch, with an overall length from the tip of the bulb to the end of the contact prongs of approxi Projection Engineering, July, 193& INPUT 500 OHM OR OTHER CONVENIENT LINE FROM AMPLIFIER O O STD. UX SOCKET \ RECORDING LAMP IMPEDANCE ADJUSTING TRANSFORMER BETWEEN LINE AND LAMP. (20.000 OHMS) 600 "B"8ATT'S OR "M-G"SET Fig. 2. Voltage c u rrent c h a racteristic new lamp. fll'IIMIIIUIIIIIIIIIIIIIIIIIIIIIIUII Fig. 3. Operating circuit of the glow lamp. mately 5% inches. It is mounted in a standard UX199 base, only two of the prongs of which are used for electrical connection. Characteristics The voltage-current characteristic curve of this lamp is shown in Fig. 2. The extraordinary linearity . of this characteristic curve is very unusual with gas-filled devices, and is very important from an electrical standpoint because it gives a constant impedance on the lamp during the signal swings. The impedance of the lamp of which Fig. 2 is the characteristic curve, is 20,000 ohms. Duplication of these lamps has been found perfectly feasible provided proper care is used at every stage of their manufacture. From Fig. 2, it is evident that, if the lamp is operated at 550 volts, 8 milliamperes, the heat dissipated within the bulb is only 4.4 watts which is not sufficient to cause any undue concern. When operated under these conditions the permissible voltage swing is about 100 volts peak, thus requiring a r.m.s. power from the amplifier of only threetenths of a watt. Under this particular condition it is easily possible to expose positive film when the lamp is used close up against the film with the usual one-thousandth of an inch — or even smaller mechanically, or quartz — slit. When used with an optical system, the light intensity is plentiful for "VD" film with a calculated slit, at the film, of about .0008 inch. Practical use of the Zetka lamp indicates that this new recording lamp represents a distinct advance in this particular field and overcomes the difficulties which have been inherent in lamps of this type in the past. The lamp can be used with any standard amplifier having suitable means for controlling modulation. Operating Circuit Fig. 3 shows the glow lamp circuit. In accordance with the usual practice with such delicate devices it is always essential to have a series resistance of at least 10,000 ohms in circuit with the lamp at all times. A variable resistance ranging from 10,000 to 50,000 ohms will be found satisfactory. Present demands in sound pictures require clear-cut recording with sharp definition and maximum freedom from "background" noise. Many years of intensive research and development in vacuum and gas-filled appliances, the study of various developments in glass and pyrex as well as means for their successful working, and painstaking study and experiment with the action of many metallic substances under varying conditions of vacua as well as their behavior at varying voltages in contact with some of th» rarer gases — all these have been required to produce in the lamp herein described a rigid, uniform, and substantial film recording tube giving absolutely dependable performance of exceptionally long life with entire freedom from any trouble or sputtering. CHICAGO SECTION SMPE ELECTS OFFICERS THE first meeting of the Chicago section of the Society of Motion Pictures Engineers was held recently, at which time officers were elected. J. A. Dubray was elected chairman and J. Elliott Jenkins, secretary. J. A. Dubray, J. E. Jenkins, O. F. Spahr and O. B. Depue were made governors, and J. A. Dubray, E. S. Pearsall, Jr. and Fred Kranz were made members of the papers and program committee with Mr. Dubray acting as chairman. B. W. Depue was appointed to handle publicity. The boundaries of the Section were set as a line running north and south through a point 50 miles west of Cleveland and a line running north and south through a point 50 miles east of Denver, the north and south boundaries to be those of the United States. It was also decided that a dinner meeting shall be held once every month.