ELECTRON TUBE

also called vacuum tube, or valve, device used in electronic circuitry to control a flow of electrons. Such devices include vacuum tubes, phototubes, gas-filled tubes, cathode-ray tubes, and photoelectric tubes. An electron tube typically consists of two or more electrodes enclosed in a glass or metal-ceramic envelope that is wholly or partially evacuated. Its operation depends on the generation and transfer of electrons through the vacuum from one electrode to another. The source of electrons is the cathode, usually a metallic electrode that releases a stream of electrons by one of several mechanisms (e.g., thermionic, secondary, or field emission; see below Electron emission). Once the electrons have been emitted, their movement is controlled by either an electric field or a magnetic field. An electric field is established by the application of a voltage between the electrodes in the tube, while a magnetic field may be produced outside the tube by an electromagnet or a permanent magnet whose field penetrates the vacuum envelope and influences the emitted electrons. In its simplest form, an electron, being negatively charged, is attracted and accelerated by an electric field from the positive electrode and is repelled and slowed by the field from the negative electrode. The electric fields therefore can be used to change the path of the electron stream, alter the number of electrons flowing (and thereby change the current), and modify their speed. The magnetic field serves primarily to control the movement of the electrons from one electrode to another. Electron tubes have certain unique properties that cannot be surpassed by solid-state devices for particular applications. Their thermal ruggedness, operating efficiency, and high-power capabilities are features well beyond those provided by the solid-state devices. As components of electronic systems, electron tubes are used as amplifiers, rectifiers, signal generators, and switches. Their applications are extensive, covering a wide range of power levels and frequencies. Electron tubes remain the dominant devices for applications requiring microwave frequencies (above 1,000 megahertz) and moderate- to high-power output. also called Vacuum Tube, or Valve, device usually consisting of a sealed glass or metal enclosure that is used in electronic circuitry to control a flow of electrons. The control exerted by such a device may take the form of rectification of an alternating current, amplification of a weak current, or the generation of oscillations, of X rays, or of images on a luminescent screen. Thomas Edison was one of the first to observe a flow of current between two separated charged elements (for Edison, a heated filament and a positively charged plate) in an evacuated glass bulb. The Edison effect remained a curiosity until it was shown by Sir J.J. Thomson in 1897 that the discharge observed in the tube was in fact a flow of electrons. From that clue J.A. Fleming designed a two-element tube, called a diode, that, when incorporated into a circuit, acted as a valve by permitting current to flow in one direction only. It could thus rectify an alternating current and, more importantly, serve as a detector of radio waves. In 1907 the U.S. engineer Lee De Forest built the first triode, or three-element tube, by inserting a grid between the emitting element (filament, later a cathode) and the collecting element (plate, or anode). An electrical potential imposed on the grid could increase or decrease the flow of electrons between the other two elements and thus enable the tube to act as an amplifier. If the current supplied to the grid were a weak signal, the triode tube would produce as its output a strong signal current. The development of practical radio communications derived directly from De Forest's invention. The typical electron tube consists of a glass or metal envelope from which air has been evacuated, a set of pins at the base by which connections are made to other circuit components, and the internal elements. These last consist of the cathode, usually of tungsten, which supplies electrons by the process of thermionic emission; a heater, or filament, which serves to heat the cathode so that it will emit electrons; the plate, or anode, which collects electrons; and one or more grids, including the basic control grid, a screen grid that increases the amplifying capacity of the tube, and a suppressor grid that controls secondary emission of electrons. A great variety of tubes have been developed for special purposes. The photoelectric tube contains a cathode made of a material, such as cesium, that emits electrons when struck by light. Electron-gun tubes use an arrangement of internal or external devices to control a beam of electrons from the cathode to a luminescent screen that takes the place of the plate. Cathode-ray tubes and television tubes are among the many types of electron-gun tubes. Other varieties of tubes include the ignitron, used to rectify very large currents in industrial applications, the gas tube, in which ionized mercury vapour carries the current instead of thermionic electrons, the klystron, used to generate extremely high-frequency microwaves, and X-ray tubes, in which the electron beam strikes a target of heavy metal to produce penetrating X-rays. For many applications the electron tube has been supplanted by the semiconductor device (q.v.). Additional reading Curtis L. Hemenway, Richard W. Henry, and Martin Caulton, Physical Electronics, 2nd ed. (1967), treats the fundamental physics of electron tubes. James T. Coleman, Microwave Devices (1982), provides a general treatment of vacuum devices, including fast-wave tubes. A.S. Gilmour, Jr., Microwave Tubes (1986), is a comprehensive treatment of modern electron tubes. Samuel Y. Liao, Microwave Electron-Tube Devices (1988), gives theoretical and experimental coverage of the basic and newer types of electron tubes. Estanislao Navarro Sosa