The motion picture projectionist (Nov 1931-Jan 1933)

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16 Motion Picture Projectionist November, 1931 gas pressure results in a greater relative spacing between molecules and the electron may then attain a greater velocity in this spacing which results in a greater impact of collision between the electron and the molecule. On the other hand the gas pressure may not be too low for under these circumstances the relative scarcity of molecules results in a greater percentage of electrons striking the anode without colliding with the gas molecules. Average suitable gas pressures are about 0.01 mm. 2. The nature of the inert gas. Some gas molecules ionize at lower potentials than others; the magnitude of impact required to disrupt the molecular structure of the gas molecule varies accordingly. 3. The voltage of the photocell polarizing potential. The potential gradient existing within the cell determines the acceleration of the emitted electron. If the gas is under greater pressure than that ordinarily considered optimum a greater polarizing potential is required to accelerate the electrons sufficiently, in the smaller distance, in order that they may strike the molecule with the same impact as before. The electrons released by collision are attracted to the anode, as are the electrons emitted from the cathode. Similiarly they accelerate and strike other molecules with sufficient force to disrupt additional electrons which in turn travel towards the anode only to strike still other molecules en route. Thus the effect of the electronic bombardment of the gas molecules is seen to be cumulative. A stable operating point is reached when the polarizing potential is properly adjusted, the light source is not too intense and the gas pressure is not too great. Under proper conditions of polarizing potential, light source and gas pressure, the ionized increment added to the electronic stream is directly proportioned to the electronic emission of the cathode and hence to the light source. If any of these values is excessive, however, the ionization may proceed to the point of saturation producing the familiar glow caused by an excessively ionized condition within the cell. This glow is harmful to the cell and should be stopped as soon as possible after its establishment. Principle of Photo Conduction No discussion of photocells is complete without reference to cells operating by virtue of a principle known as photo conduction. Photo conduction is the property any material may possess to change its electrical resistance. It is caused by the action of incident light. Selenium is a notable example of an element possessing photo conductive properties. Under any conditions selenium is a very poor conductor of electricity but under the influence of light the resistance of the selenium decreases and permits an increased current to flow. Selenium has a sluggish response characteristic — that is, its response to changes in the amount of light falling upon it is slow as compared to the practically instantaneous response of the true photo electric cell — hence its use, at present, in sound projection work is not most desirable. • Selenium, in its ordinary forms, lacks the ability to "follow" actual changes in light intensities occurring in a modulation of the original incident light (such as those caused by the passage of a sound track in the sound gate) and its inability to follow these changes is more apparent with increase in frequency. While the cell may be used for sound projector work its use is not desirable because of its high frequency cut-off. Later work has developed structures of selenium cells which have less lag than the earlier types. Thallium Oxysulphide Cell There has been developed one additional type of photocell conductive, which uses a compound called thallium-oxysulphide in place of the more familiar selenium. Thalliumoxysulphide is quite sensitive to the red and infra-red rays, in which the ordinary exciter lamp is quite rich, and has a favorable lag characteristic. It is used in one system of talking picture equipment. The cell requires the use of a special light filter to reduce the rate of speed at which the sensitive sulphide changes to the insensitive sulphate. Cells operating upon the photo voltaic principle are available on the commercial market. Photo voltaic cells operate upon the principle of creation of an electromotive force within an electrolytic cell when an electrode or the electrolyte is exposed to illumination. Most photo voltaic cells use a construction employing a sensitive electrode rather than a sensitive electrolyte. Silver halides and cuprous oxide are most commonly used for electrodes. If a cell is made of two electrodes of copper similarly coated with cuprous oxide and immersed in an electrolyte the potentials set up are equal and opposite, hence no current would flow in an external circuit were one connected. If, however, one of the electrodes is illuminated an additional electromotive force is produced, which may be measured, and current will flow in the external circuit, if one be connected. The general dynamic response of the ordinary photo voltaic cell is inferior to the selenium cell and is far inferior to the gas filled photo electric cell. A New Departure in Spotlights (Continued from page 14) The entire arc mechanism may be raised or lowered for centering the crater with respect to the lens, and also may be tilted to obtain the correct burning angle, which depends on the horizontal tilt of the lamp. This angle once set for a lamp in a particular location, always remains the same. A metal shield, positioned in advance of the arc, eliminates the projection of the arc flame. Marked scales and dials are provided for the assistance of the operator. One indicating the horizontal position of the hood, another the vertical position of the hood, and the third the position of the arc mechanism— thus allowing the operator to set his lamp for any degree spot and at exactly the desired position, without first shinning the light on the stage. Five eolorframes are provided, enclosed in a colorbox permanently mounted on the front of the spotlight and operated through an ingenous arrangement of levers which extend along the side of the hood. These are situated in the most practical location and the new operating mechanism eliminates the use of strings. Each colorframe has its individual lever, marked to designate the color it controls. The position of the lever indicates the position of the colorframe, and a quarter turn throws the screen in or out of position. Stand ard 8" eolorframes are used, which are slipped into their holders, thus permitting quick and easy changing of colors. Slide grooves are also provided on the front of the color box for an extra color frame, color wheels, special effects, and spotlight attachments. An iris shutter is part of the permanent equipment operated by a handle at the back of the spotlight and by an auxiliary handle on the side near the front of the spotlight. Markings on the spotlight and the position of the handle indicate the relative opening of the shutter. A curtain shutter is made part of the permanent equipment, and it too is operated by a handle at the rear of the spotlight. A quarter turn of the handle closes the shutter from full opening to black-out, or vice versa. A fixed resistance for connection in series with the arc is furnished as a separate unit so that it may be mounted back of the switchboard or in some other out-of-the-way location. The carbons recommended for use are %"xl2" cored projector carbons in the top holder, and %" x 6" silvertip carbons in the bottom holder. The spotlight is furnished complete with color frames, asbestos leads, and an enclosed knife switch mounting on the base. An automatic blackout shutter, operated from the stage switchboard can be quickly installed on each lamp, making an instantaneous blackout for all lamps when required.