Vacuum Tubes
. 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 ca