Source : WordNet®

electron tube
     n : electronic device consisting of a system of electrodes
         arranged in an evacuated glass or metal envelope [syn: {tube},
          {vacuum tube}, {thermionic vacuum tube}, {thermionic
         tube}, {thermionic valve}]

Source : Free On-Line Dictionary of Computing

electron tube
     
         (Or tube, vacuum tube, UK: valve, electron
        valve, thermionic valve, firebottle, glassfet) An electronic
        component consisting of a space exhausted of gas to such an
        extent that {electrons} may move about freely, and two or more
        electrodes with external connections.  Nearly all tubes are of
        the thermionic type where one electrode, called the cathode,
        is heated, and electrons are emitted from its surface with a
        small energy (typically a Volt or less).  A second electrode,
        called the anode (plate) will attract the electrons when it is
        positive with respect to the cathode, allowing current in one
        direction but not the other.
     
        In types which are used for amplification of signals,
        additional electrodes, called grids, beam-forming electrodes,
        focussing electrodes and so on according to their purpose, are
        introduced between cathode and plate and modify the flow of
        electrons by electrostatic attraction or (usually) repulsion.
        A voltage change on a grid can control a substantially greater
        change in that between cathode and anode.
     
        Unlike {semiconductors}, except perhaps for {FET}s, the
        movement of electrons is simply a function of electrostatic
        field within the active region of the tube, and as a
        consequence of the very low mass of the electron, the currents
        can be changed quickly.  Moreover, there is no limit to the
        current density in the space, and the electrodes which do
        dissapate power are usually metal and can be cooled with
        forced air, water, or other refrigerants.  Today these
        features cause tubes to be the active device of choice when
        the signals to be amplified are a power levels of more than
        about 500 watts.
     
        The first electronic digital computers used hundreds of vacuum
        tubes as their active components which, given the reliability
        of these devices, meant the computers needed frequent repairs
        to keep them operating.  The chief causes of unreliability are
        the heater used to heat the cathode and the connector into
        which the tube was plugged.
     
        Vacuum tube manufacturers in the US are nearly a thing of the
        past, with the exception of the special purpose types used in
        broadcast and image sensing and displays.  Eimac, GE, RCA, and
        the like would probably refer to specific types such as "Beam
        Power Tetrode" and the like, and rarely use the generic terms.
     
        The {cathode ray tube} is a special purpose type based on
        these principles which is used for the visual display in
        television and computers.  X-ray tubes are diodes (two element
        tubes) used at high voltage; a tungsten anode emits the
        energetic photons when the energetic electrons hit it.
        Magnetrons use magnetic fields to constrain the electrons;
        they provide very simple, high power, ultra-high frequency
        signals for radar, microwave ovens, and the like.  Klystrons
        amplify signals at high power and microwave frequencies.
     
        (1996-02-05)