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

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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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18 THE 1GXATIAS experimenter tli ‘ tremendous possibilities tlmt were eon-neeted with it. In 190( Dr. Lee deForest was allowed a patent for a detector of electro-magnetic waves employing two electrodes in an open flame. One year later diligent endeavor on the part of this same gentleman had brought forth the real three electrode Vacuum Tube. Jt is well to understand at the outset that the vacuum bulb in this form was intended as a wireless detector but was subsequently found to function in a multitude of different ways. Some of the most important variations of actions will form the main body of this treatise. The major service performed by the tube is in wireless. It functions as a detector, an amplifier and an oscillation generator. The first two terms are self explanatory and by an “oscillation generator” is meant the tube when so acting that it generates radio waves of continuous amplitude and frequency. It is this last application that has made the radio phone so perfect and has caused the displacement of the old fashioned spark set in transmission of telegraphy and the substution of the new bulb Continuous Wave transmitter which is less costly, more silent, and possesses an efficiency that the old spark transmitters could in no way equal. We can attribute the rapid development of this device to nothing else but the fact that its applications are so numerous and so diverse and to its efficient action under varying conditions. It is truly an exemplification of tin fabled “Genic in a bottle.” The war added impetus to investigations that were already under way. There was a crying need for such a device and scientists of all nations delved deep into the subject of Thermionics. Important discoveries were made both in America and in Kuropc but on account of the absence of intercommunication no scientific data could be exchanged or no mutual hell) secured. However the result has becu so gratifying as to satisfy even the wildest hopes. The study is still

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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