The Audion Tube, an article on early radio communication from the December 31, 1916 Los Angeles Times  

The New York Times, December 31, 1916, p.4:



    Yesterday we discussed the principles of the two-element vacuum tube, which, although proved quite useful in itself, was to be the forerunner of that wonderful device known as the audion. This little instrument has been called by many names, such as the triode, pilotron, three element thermionic vacuum tube, three-element tube, vacuum tube [and, in Britain, "valve"], etc. However, the inventor, Lee De Forest, proposed the name "audion," which has lately come into quite common usage.

    The invention of the audion marks the greatest advance ever made in radio communication. Audions are now used universally in radio communication. They form the very heart of all receiving sets [not including crystal radios]. They serve both to detect or to rectify the incoming waves, and to amplify the resulting currents thousands of times in order to actuate the various types of recording instruments.

    Audions are also the principal parts of the radio telephone transmitters, being used to generate the high-frequency currents, to vary or modulate these currents, and to amplify the small variations of current caused by the action of the human voice on the microphone so that many kilowatts of energy may be controlled in accordance with the words spoken. Audions are also used in line telephony as amplifiers, thus making transcontinental line communication possible.

    The operation and construction of the audion is quite similar to the two-element tube described yesterday. The tube essentially consists of an evacuated glass vessel or bulb containing
    (1) a filament for a cathode, which may be heated by an external battery;
    (2) a sheet of metal or plate [anode] surrounding the filament;
    (3) a "grid" or mesh of wire placed between the filament and plate.

    There are two types of filament now in use: the tungsten filament and the coated filament. The former is constructed of a fine tungsten wire. Coated filaments are made by coating a thin ribbon of platinum with oxides of calcium and barium. It has been found that these coated filaments emit electrons at much lower temperatures than the pure metal filaments and hence have much longer operating lives.
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    In shape filaments are usually made either straight, or in the form of an inverted V.

    The grid may be a spiral of fine wire surrounding the filament at a small distance from it. In some tubes the grid is merely two rectangular wire grids placed on opposite sides of the filament.

    The plate [anode] is usually a thin cylinder of metal, and incloses both the grid and the filament. Another common form is to make it of two rectangular plates, placed facing each other outside of the grid and filament.

    The battery used to heat the filament is known as the "A" battery. The plate or anode potential is maintained by the "B" battery.


    The simple operation of the audion is as follows:
    As the filament is heated by the "A" battery current, it emits electrons. If the plate is then connected to the positive terminal of the "B" battery, and the filament to the negative terminal, the escaping electrons will flow to the plate, thus giving rise to the plate current exactly similar to the operation of the two-element tube.
    If now the grid be given a negative charge, it will repel the escaping electrons and consequently prevent them from reaching the plate. When the grid becomes positive, it draws electrons toward it. Moving in high velocities, these electrons pass through the grid spaces and fall upon the plate, thus increasing the number which would normally reach it. This results in a great increase in the plate current.

    The grid therefore serves as a valve, through which the plate current can be varied or controlled by a very small amount of energy.

    Tomorrow we shall describe some of the practical applications of audions.
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