Vacuum Tubes
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Vacuum Tubes, electronic devices, consisting of a glass or steel vacuum envelope and two or more electrodes between which electrons can move freely. The vacuum-tube diode was first developed by the English physicist Sir John Ambrose Fleming. It contains two electrodes: the cathode, a heated filament or a small, heated, metal tube that emits electrons through thermionic emission; and the anode, or plate, which is the electron-collecting element. In diodes, the electrons emitted by the cathode are attracted to the plate only when the latter is positive with respect to the cathode. When the plate is negatively charged, no current flows through the tube. If an alternating potential is applied to the plate, the tube passes current only during the positive halves of the cycle and thus acts as a rectifier. Diodes are used extensively in the rectification of alternating current (see Electronics; Rectification).
The introduction of a third electrode, called a grid, interposed between the cathode and the anode, forms the triode, which for many years was the basic tube used for amplifying current. (The triode was invented in 1906 by the American engineer Lee De Forest.) The function of the grid is to control the current flow. At a certain negative potential, the grid, because it repels electrons, can impede the flow of electrons between the cathode and the anode. At lower negative potentials, the electron flow depends on the grid potential. The grid usually consists of a network of fine wire surrounding the cathode. The capacity of the triode to amplify depends on the small changes in the voltage between the grid and the cathode causing large changes in the number of electrons reaching the anode.
Through the years more complex tubes with additional grids have been developed to provide greater amplification and to perform specialized functions. Tetrodes have an additional grid, closer to the anode, that forms an electrostatic shield between the anode and the grid to prevent feedback to the grid in high-frequency applications. The pentode has three grids between the cathode and the anode; the third grid, close to the anode, reflects electrons that are emitted by the anode as it is heated by electron impact when the electron current in the tube is high. Tubes with even more grids, called hexodes, heptodes, and octodes, find applications as frequency converters and mixers in radio receivers (see Radio).
Vacuum tubes have now been almost entirely replaced by transistors, which are cheaper, smaller, and more reliable (see Transistor). Tubes still play an important role in certain applications, however, such as in power stages in radio and television transmitters, and in military equipment that must resist the voltage pulse (which destroys transistors) induced by an atmospheric nuclear explosion.
The introduction of a third electrode, called a grid, interposed between the cathode and the anode, forms the triode, which for many years was the basic tube used for amplifying current. (The triode was invented in 1906 by the American engineer Lee De Forest.) The function of the grid is to control the current flow. At a certain negative potential, the grid, because it repels electrons, can impede the flow of electrons between the cathode and the anode. At lower negative potentials, the electron flow depends on the grid potential. The grid usually consists of a network of fine wire surrounding the cathode. The capacity of the triode to amplify depends on the small changes in the voltage between the grid and the cathode causing large changes in the number of electrons reaching the anode.
Through the years more complex tubes with additional grids have been developed to provide greater amplification and to perform specialized functions. Tetrodes have an additional grid, closer to the anode, that forms an electrostatic shield between the anode and the grid to prevent feedback to the grid in high-frequency applications. The pentode has three grids between the cathode and the anode; the third grid, close to the anode, reflects electrons that are emitted by the anode as it is heated by electron impact when the electron current in the tube is high. Tubes with even more grids, called hexodes, heptodes, and octodes, find applications as frequency converters and mixers in radio receivers (see Radio).
Vacuum tubes have now been almost entirely replaced by transistors, which are cheaper, smaller, and more reliable (see Transistor). Tubes still play an important role in certain applications, however, such as in power stages in radio and television transmitters, and in military equipment that must resist the voltage pulse (which destroys transistors) induced by an atmospheric nuclear explosion.
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