University of San Francisco - USF Don Yearbook (San Francisco, CA)

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THREE ELEMENT VACUUM TUBE 21 to escape from a surface whose con I our is irregular than from a smooth fiat or cylindrical surface. For this reason, since for the proper function of the Tube a copious flow is sometimes requisite, different shapes and materials have been tried for filament construction. The best example of unevenly surfaced filament is that which is composed of a platinum wire coated with an oxide of the alkaline earths. This filament, on account of its construction, has a weak electronic affinity and. therefore, a strong thermionic value which is to say that, when heated, electrons are copiously emitted and with such a speed as to cause the passage of a reasonably high current. For this same reason there is not a clear defined adjustment for the saturation current as there is in tubes employing a plain tungsten filament. Such a condition for general requirements is more of an advantage than otherwise. Conditions exist where light plays no unimportant part with reference to this electronic transmission. The well known scientist Hertz in 1887 noted that the distance which a spark would leap between two electrodes was increased when the gap was illuminated with ultra-violet light. Furthermore he found that an insulated metal plate, when electrically charged in the dark, became discharged when brought into the light. These phenomena take place in conjunction with the Vacuum Tube’s function and were subsequently explained by the electron theory. The force of the light wave striking upon the charged surface of the substance so stimulates the electrons that they escape with a velocity which varies as the strength or frequency of the light. The speed of the electrons is independent of the intensity of the light but has a direct relation to the frequency of its wave. There are some lights of so low a wave length that no electrons are stimulated to escape. Lights of this type come under the “photo-electric long wave-length limit.” The reason that this method of electronic dislodgment is scarcely ever used in practice is because photo-electric currents are always small and their action is somewhat erratic. Methods employing heat for this same

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20 THE I Gy AT I AN secondary one in the space intervening between the filament and the plate. The movement of the electrons through this space is called the “space current.” The process of the production of dislodged electrons, which are the ones which constitute the space current in the Vacuum Tube, is known as ionization. This can he accomplished by three different methods. (1) by collision of electrons and a substance. (2) by electro-magnetic radiation and (3) by heat. This last is the only method employed in the ordinary Vacuum Tube. Often in the operation of a tube a blue glow will appear at certain points of adjustment. This is due to the collision of electrons from the filament with the particles of residual air and is an instance of ionization by collision. The effect that this may have on the operation of the tube will he discussed later. It is well to repeat these outstanding facts upon which is based the function of the Tube—(1) Operation is secured by the passage of electrons. (2) these electrons emanate by disruption of the atom, under the influence of heat, from a solid conductor (the filament). Electrons are retained in substances by a force existing on the surface of the material. At ordinary temperatures few, if any. of these electrons possess the power to overcome this force. Heat increases the rapidity of their movement and transmits to them enough vigor to break through the restraining forces so that they escape as fast as they are being used. First a current is applied to the filament which raises it to a working condition. The electrons are then set free. If now there is a potential difference between the filament and the plate the free electrons will l)o attracted to the plate. A potential difference is a disparity in the pressure which causes an electrical current to flow through a circuit. When the current passing between the filament and the plate is of such a volume that an increase of plate voltage causes no corresponding increase of flow then this is known as the saturation current. Experiment has proved that it is easier for an electron

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22 T1IE 10 X ATI AX function arc more practical in every sense. The insertion of a grid or third electrode between the filament and the plate of the thermionic valve made it possible to adapt this device to operations that would otherwise have been impossible. The nature of the effect produced by the grid is as follows: Assuming that a space current is flowing between the anode and the cathode of a thermionic valve now if a grid is interposed and a negative charge impressed upon it then it is clear that the flow of ions will be retarded or completely turned back depending on the strength of the charge of the grid. This action is based on a fundamental principle in electricity which holds that like charges repel and unlike attract. For this reason a positive charge on the grid has the effect of drawing the electrons through to the anode. The filament, heated by the current from a battery, can be compared to a gun; continually bombarding the plate with electrons which must pass through the grid to reach the plate. The grid acts as a spasmodic shield; when a current in its positive phase is impressed on the grid, making it positive, the negative electrons are allowed free course to pass through the grid to the plate. When the negative phase of the current possesses the grid and makes it negative, it is as if an impenetrable shield were lowered between the filament and the plate completely stopping the electron barrage. Every time these electrons are allowed to pass from the filament to the plate they produce an impulse in the plate circuit. Through this impulse the wireless wave is detected, relays actuated etc., according to the function required by the specific instance. With respect to the impressed charges on the grid it is well to examine a few demonstrative conditions which are an index to its action under various circumstances. Suppose that the plate remains at a constant positive potential as regards the filament and that the grid is successively (a) positive (b) negative and (c) zero, (a) If there is a copious emission of electrons from the filament and the field between