electron-volt, abbr. eV, unit of energy used in atomic and nuclear physics; 1 electron-volt is the energy transferred in moving a unit charge, positive or negative and equal to that charge on the electron, through a potential difference of 1 volt. The maximum energy of a particle accelerator is usually expressed in multiples of the electron-volt, such as million electron-volts (MeV) or billion electron-volts (GeV). Because mass is a form of energy (see relativity), the masses of elementary particles are sometimes expressed in electron-volts; e.g., the mass of the electron, the lightest particle with measurable rest mass, is 0.51 MeV/c², where c is the speed of light.

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 oscilloscopes and video 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 widely used in high-power transmitters, some television cameras, specialty audio equipment, and as oscilloscope and video displays. 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.

electron paramagnetic resonance: see magnetic resonance.

electron microscope: see microscope.

electron, elementary particle carrying a unit charge of negative electricity. Ordinary electric current is the flow of electrons through a wire conductor (see electricity). The electron is one of the basic constituents of matter. An atom consists of a small, dense, positively charged nucleus surrounded by electrons that whirl about it in orbits, forming a cloud of charge. Ordinarily there are just enough negative electrons to balance the positive charge of the nucleus, and the atom is neutral. If electrons are added or removed, a net charge results, and the atom is said to be ionized (see ion). Atomic electrons are responsible for the chemical properties of matter (see valence). The name electron was first used for a unit of negative electricity by the English physicist G. J. Stoney in the late 19th cent. The actual discovery of the particle, however, was made in 1897 by J. J. Thomson, who showed that cathode rays are composed of electrons and who measured the ratio of charge to mass for the electron. In 1909, R. A. Millikan measured the charge of the electron. Combining these two results gives the mass of the electron (about 1/1,840 of the mass of the proton). Ernest Rutherford, in 1903, showed that beta rays (see radioactivity) are high-energy electrons. In 1927, Davisson and Germer, working with high-speed electron beams, discovered that electrons sometimes exhibit the wave property of diffraction. This confirmed L. V. de Broglie's hypothesis that electrons, which had previously been thought of as particles, also possess certain wave properties (see quantum theory). The wavelike properties of electrons are utilized in the electron microscope and other devices. The electron is the lightest particle having a non-zero rest mass. It belongs to the lepton class of particles and, together with its antiparticle, the positron, and its associated neutrino and antineutrino, constitutes a subfamily of the leptons. In any particle reaction involving any of the four members of the electron family, the total electron family number (+1 for ordinary particles, -1 for antiparticles) must be conserved (see conservation laws, in physics). As a consequence, an electron and a positron (total electron family number equals zero) can annihilate each other to yield two or more photons or a neutrino-antineutrino pair, but not two neutrinos (total electron family number equals two).

or electron paramagnetic resonance (EPR)

Technique of spectroscopic analysis (see spectroscopy) used to identify paramagnetic substances (see paramagnetism) and investigate the nature of the bonding within molecules by identifying unpaired electrons and their interaction with their immediate surroundings. Unpaired electrons, because of their spin, behave like tiny magnets and can be lined up in an applied magnetic field; energy applied by alternating microwave radiation is absorbed when its frequency coincides with that of precession of the electron magnets in the sample. The graph or spectrum of radiation absorbed as the field changes gives information valuable in chemistry, biology, and medicine.

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Lightest electrically charged subatomic particle known. It carries a negative charge (see electric charge), the basic charge of electricity. An electron has a small mass, less than 0.1percnt the mass of an atom. Under normal circumstances, electrons move about the nucleus of an atom in orbitals that form an electron cloud bound in varying strengths to the positively charged nucleus. Electrons closer to the nucleus are held more tightly. The first subatomic particle discovered, the electron was identified in 1897 by J. J. Thomson.

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