The motion picture projectionist (Nov 1931-Jan 1933)

November. 1931 Motion Picture Projectionist 15 Photoelectric Cell Theory and Construction The photoelectric cell is at once one of the most interesting and one of the most important factors in the reproduction of sound from film recordings. Without it, the sound film as we know it today would be a dream of the future instead of a reality of the present. In choosing the device as the subject of his article in this month's issue, Mr. Stier has selected a topic which is of concern to every worker with projection equipment.— The Editor. EVERY material is sensitive to the action of light. It does not matter in what state the material be — solid, liquid, or gas. This statement, from the viewpoint of the practical man, is true only from a relative point of view, since most liquids and solids, especially the metals, respond only to the extremely short X-rays or ultra-violet rays. Only a few of the metals respond to rays of light of the length of the infra-red or the visible light portion of the spectrum. These exceptions are the alkali metals of which sodium, potassium and caesium are excellent well known examples. These alkali metals are exceedingly sensitive to those light rays visible to the eye. Their sensitivity may extend into the infra-red region of the light spectrum. A type of photocell widely used in sound projector equipment today consists of a glass envelope containing within itself a curved cylindrical sheet or plate, termed the cathode, having in front of the cathode a vertical wire, called the anode. On the cathode is placed, during the processes of manufacture, a thin film or coating of caesium which has the property of emitting electrons when exposed to light. The number of electrons emitted by the cathode, under the influence of the incident light, varies directly as the intensity of the incident light. In other words the number of electrons released from the cathode is proportional to the quantity of light falling upon the cathode. This relationship is true only when the wavelength of the light remains constant, that is, when its color remains unchanged. The Caesium Cell Of the photocells commercially available, the caesium cell shows a greater sensitivity in ordinary usage than does the potassium or sodium cell. This is true because caesium itself is sensitive to the red and the infra-red portions of the spectrum (an exciter lamp is a very rich source of these rays) while the potassium and the sodium cells are most sensitive to the violet and the ultra-violet portions of the spectrum (in which the ordi tEngineering Department, RCA Photophone Tnc. By R. J. STiERf nary exciter lamp is comparatively deficient). When the light from the exciter lamp strikes such a cell electrons are emitted from the cathode very much as electrons are emitted, by thermal action, from the hot filament of the vacuum tube. In the case of the photocell, as in the case of the vacuum tube, an EMF is connected in series with the anode and the cathode so "poled" that the anode is positive with respect to the cathode. When light strikes a photocell so connected the electrons released will be attracted from the cathode (from which the light has released them) to the anode. They will then pass through the battery and return to the cathode thus completing the circuit. If a resistance be inserted in this battery line a potential difference will exist across the resistor terminals due to the voltage drop developed within the resistor. The Coupling Device As is common in sound projection practice a transformer's primary winding is substituted for the resistor mentioned above and the variation in current flowing through the transformer winding, connected in series with the photocell, is "repeated" across the transformer, hence a similar voltage appears at the secondary terminals of the transformer. Since the amount of current passed by the photocell varies in direct proportion to the amount of light falling upon the photocell cathode, it follows infallibly that the current passing through the transformer winding, connected in series with the photocell, varies directly as the amount of light reaching the photocell cathode. From this it is but a simple step to the conclusion that the voltage appearing at the transformer secondary terminals is a direct function of the varying amount of light incident to the photocell cathode. Introduction of Gas Up to this point we have discussed only a simple cell; one containing an anode and a lieht sensitive cathode presumably operating in a vacuum. The number of electrons emitted under such circumstances is very small, however. So small, in fact, that any means of increasing the electronic flow is justified. This may be accomplished by means of a phenomenon known as "ionization by collision" or "gas amplification." Gas amplification is accomplished by filling the cell with an inert gas, such as argon, at very low pressures. The cell then becomes a gas-filled cell. All matter, including the gas with which the cells are filled, is composed of molecules. Molecules are the smallest indivisible quantities of any material. This definition includes, of course, the inert gas with which the cell is filled. The gas molecules in turn are composed of equal numbers of electrons and protons. Electrons and protons attract each other with tremendous force, when one considers their infinitesimal dimensions, and repel each other with like force. Movement of Electrons The electrons, in moving through a definite space or region constitute the phenomenon we know as an electric flow, or more familiarly, an electric current. In the photocell, the battery exerts a force on these electrons in the direction of the anode. The light releases the electrons from the cathode and, under the accelerating effect of the force exerted by the anode potential, the speed of the electrons is increased as they travel toward the anode. Relatively few of the collected electrons reach the anode without having struck several molecules of the inert gas introduced into the cell. If the electrons strike the gas molecules with sufficient force they may disrupt the gas molecules into their constituent electrons and protons. However, since protons are approximately 1,800 times as heavy as electrons the latter are usually forced out from the gas molecule by force of impact. From one to five or more electrons may be removed from the gas molecule, the actual quantity depending upon the velocity with which the traveling electron strikes the molecule. Factors in Gas Amplification If the gas be introduced at ordinary atmospheric pressures, the molecules of gas are spaced relatively closely and the average distance between molecules is not great enough to permit sufficient electron acceleration to knock out additional electrons by such bombardment. If, however, the cell be filled with gas at a relatively low pressure (as is done) the electrons have an excellent opportunity to accelerate sufficiently before colliding with the gas molecules to release, by force of impact, several additional electrons from the gas molecule so struck. We then have many more electrons than were originally released by the incident light, which are collected by the positive anode, and a number of ions or remains of molecules charged due to their excess proton content. "Gas amplification" is the product of many variable factors the more important of which are these: 1. The pressure of the gas. Lower