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

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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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THREE ELEMENT VACUUM TI RE If) very young and no one ean foretell just where the Vacuum Tube will next assume the star role. General Theory of the Three Element Vacuum Tube The basis of thermionic action is the Electron Theory as formulated by 0. W. Richardson. An electron is defined as “A natural unit of electricity,” or the amount of charge carried by an atom of any univalent substance. If an electron becomes detached from an atom, the electron, which possesses a negative charge, leaves the remaining part of the atom with a plus charge. This partial atom is known as a positive ion. In like manner if an electron is added to an uncharged atom it communicates to it a negative potential and the combination of the two is called a negative ion. In this branch of therm ionics the electron behaves as if it possessed mass and can be so treated; also we are to deal with electrons that possess motion, for a stationary electron would be of no concern in a device like the Vacuum Tube. “What causes the electron to loose itself from the matter from which it comes and what subsequently impels it to fly through space!” When an atom becomes heated the electrons which it contains begin to move with increasing rapidity in a manner that might roughly be compared to the currents started when water is being heated to the boiling point. If this heating is continued, as in water, to a certain stage the movement becomes so vigorous that a portion of the electrons break away from the atom like steam from the water. This results in the decomposition of the atom when it takes place in a solid but. of course, does not hold for the water analogy. Whenever an electron is in such motion it is accompanied by an electromagnetic field. In this an electron produces an effect identical with a current of electricity. Thus a charge moving with a certain velocity is equivalent to a current possessing a strength equal to the charge multiplied by the velocity. When a Vacuum Tube is employed in a circuit the latter consists of two parts namely the ordinary wire circuit and a

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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