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electron tube

electron tube, device consisting of a sealed enclosure in which electrons flow between electrodes separated either by a vacuum (in a vacuum tube) or by an ionized gas at low pressure (in a gas tube). The two principal electrodes of an electron tube are the cathode and the anode or plate. The simplest vacuum tube, the diode, has only those two electrodes. When the cathode is heated, it emits a cloud of electrons, which are attracted by the positive electric polarity of the anode and constitute the current through the tube. If the cathode is charged positively with respect to the anode, the electrons are drawn back to the cathode. However, the anode is not capable of emitting electrons, so no current can exist; thus the diode acts as a rectifier, i.e., it allows current to flow in only one direction. In the vacuum triode a third electrode, the grid, usually made of a fine wire mesh or similar material, is placed between the cathode and anode. Small voltage fluctuations, or signals, applied to the grid can result in large fluctuations in the current between the cathode and the anode. Thus the triode can act as a signal amplifier, producing output signals some 20 times greater than input. For even greater amplification, additional grids can be added. Tetrodes, with 2 grids, produce output signals about 600 times greater than input, and pentodes, with 3 grids, 1,500 times. X-ray tubes maintain a high voltage between a cathode and an anode. This enables electrons from the cathode to strike the anode at velocities high enough to produce X rays. A cathode-ray tube can produce electron beams that strike a screen to produce pictures, as in some oscilloscopes and older television displays. Gas tubes behave similarly to vacuum tubes but are designed to handle larger currents or to produce luminous discharges. In some gas tubes the cathode is not designed as an electron emitter; conduction occurs when a voltage sufficient to ionize the gas exists between the anode and the cathode. In these cases the ions and electrons formed from the gas molecules constitute the current. Electron tubes have been replaced by solid-state devices, such as transistors, for most applications. However they are still used in high-power transmitters, specialty audio equipment, and some oscilloscopes. A klystron is a special kind of vacuum tube that is a powerful microwave amplifier; it is used to generate signals for radar and television stations.

See also magnetron; photoelectric cell.

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A framework of parallel, crisscrossed lines or bars forming a pattern of uniform size; sets of intersecting members on a square or triangular matrix, which make up a three-dimensional structural system.

power grid

A network of power transmission and distribution facilities used to provide electricity to users such as homes, businesses, and industry. Large power plants, wind-power-generating facilities, and small power producers such as photovoltaic farms feed electrical power into the grid for distribution to users.
Illustrated Dictionary of Architecture Copyright © 2012, 2002, 1998 by The McGraw-Hill Companies, Inc. All rights reserved


(computer science)
In optical character recognition, a system of two groups of parallel lines, perpendicular to each other, used to measure or specify character images.
(design engineering)
A network of equally spaced lines forming squares, used for determining permissible locations of holes on a printed circuit board or a chassis.
A metal plate with holes or ridges, used in a storage cell or battery as a conductor and a support for the active material.
Any systematic network, such as of telephone lines or power lines.
An electrode located between the cathode and anode of an electron tube, which has one or more openings through which electrons or ions can pass, and serves to control the flow of electrons from cathode to anode.
A system of uniformly spaced perpendicular lines and horizontal lines running north and south, and east and west on a map, chart, or aerial photograph; used in locating points.
(mining engineering)
Imaginary line used to divide the surface of an area when following a checkerboard placement of boreholes.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.


1. See gridiron.
2. See grillage.
3. In surveying, closely-spaced reference lines which are perpendicular to each other; elevations usually are taken at the intersections of these lines.
McGraw-Hill Dictionary of Architecture and Construction. Copyright © 2003 by McGraw-Hill Companies, Inc.


A GEOREFF grid. This is superimposed on map.
i. The system of two sets of parallel lines uniformly spaced and crossing at 90° to each other to form a pattern of squares. This grid is designed so that any point on the map can be designated by its latitude and longitude or by its grid coordinates, and a reference in one system can be converted into a reference in another system. Such grids are usually identified by the name of the particular projection for which they are designated. See universal transverse Mercator's grid.
ii. The electrodes in an electron tube between the cathode and the anode that permit and direct the passage of electrons or ions.
iii. Two sets of mutually perpendicular lines dividing a map or chart into squares or rectangles to permit the location of any point by a system of rectangular coordinates as in a military grid, world geographic reference system, or GEOREFF. See GEOREFF.
iv. Pertaining to or measured from a reference grid, such as a grid azimuth, grid latitude, or grid meridian.
An Illustrated Dictionary of Aviation Copyright © 2005 by The McGraw-Hill Companies, Inc. All rights reserved


1. a network of horizontal and vertical lines superimposed over a map, building plan, etc., for locating points
2. the grid the national network of transmission lines, pipes, etc., by which electricity, gas, or water is distributed
3. Electronics
a. an electrode situated between the cathode and anode of a valve usually consisting of a cylindrical mesh of wires, that controls the flow of electrons between cathode and anode
b. (as modifier): the grid bias
5. a plate in an accumulator that carries the active substance
Collins Discovery Encyclopedia, 1st edition © HarperCollins Publishers 2005


(1) Any interconnected set of nodes such as the electric power network or a communications network.

(2) "The Grid" is a nickname for Internet2. See Internet2.

(3) In a vacuum tube or gas-filled electron tube, the grid is a perforated electrode through which electrons may pass. The term typically refers to the control grid in a triode, tetrode or pentode vacuum tube. In these cases, the grid is used to control the amount of current flow between the cathode and plate (anode). As the voltage potential is varied on the control grid, the amount of current allowed to pass through also varies. Relatively small fluctuations in the grid's potential can control substantially larger amounts of current flow through the tube. This phenomenon is referred to as "gain." Tetrodes and pentodes use additional grids to regulate current flow and effect gain. See screen grid and suppressor grid.
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