magnetic field Region around a magnet, electric current, or changing electric field in which magnetic forces are observable. The field around a permanent magnet or wire carrying a steady direct current is stationary, while that around an alternating current or changing direct current is con¬ tinuously changing. Magnetic fields are commonly represented by con¬ tinuous lines of force, or magnetic flux, that emerge from north-seeking magnetic poles and enter south-seeking poles. The density of the lines indicates the magnitude of the field, the lines being crowded together where the magnetic field is strong. The SI unit for magnetic flux is the weber.
magnetic force Attraction or repulsion that arises between electrically charged particles that are in motion. While only electric forces exist among stationary electric charges, both electric and magnetic forces exist among moving electric charges. The magnetic force between two mov¬ ing charges is the force exerted on one charge by a magnetic field created by the other. This force is zero if the second charge is traveling in the direction of the magnetic field due to the first and is greatest if it travels at right angles to the magnetic field. Magnetic force is responsible for the action of electric motors and the attraction between magnets and iron.
magnetic permeability Relative increase or decrease in the magnetic field inside a material compared with the magnetic field in which the material is located. In empty space the magnetic permeability is 1, because there is no matter to modify the field. Materials may be classified by the value of their magnetic permeability. Diamagnetic materials (see diamag¬ netism) have constant relative permeabilities of slightly less than 1. Para¬ magnetic materials (see paramagnetism) have constant relative permeabilities of slightly more than 1. The relative permeability of fer¬ romagnetic materials (see ferromagnetism) increases as the magnetizing field increases, reaches a maximum, and then decreases. Pure iron and some alloys have relative permeabilities of 100,000 or more.
magnetic resonance Absorption or emission of electromagnetic radiation by electrons or atomic nuclei in response to certain magnetic fields. The principles of magnetic resonance are used to study the atomic and nuclear properties of matter; two common laboratory techniques are NUCLEAR MAGNETIC RESONANCE and ELECTRON SPIN RESONANCE. In medicine, MAG¬ NETIC resonance imaging is used to produce images of human tissue.
magnetic resonance imaging (MRI) Computer production of images from magnetic resonance. The structural and biochemical infor¬ mation it provides is helpful in the diagnosis of abnormalities without the possibly harmful effects of X rays or gamma rays. It is invaluable in detect¬ ing and delineating tumours and in providing images of the brain, the heart, and other soft-tissue organs. MRI may produce anxiety because the patient must often lie quietly inside a narrow tube. Another disadvantage is that it requires a longer scanning time than other computer-assisted forms of scanning, which makes it more sensitive to motion and of less value in scanning the chest or abdomen. However, MRI images provide better contrast between normal and diseased tissue than those produced by other computer-assisted imagery.
magnetism Phenomenon associated with magnetic fields, the effects of such fields, and the motion of electric charges. Some types of magnetism are diamagnetism, paramagnetism, ferromagnetism, and ferrimagnetism. Mag¬ netic fields exert forces on moving electric charges. The effects of such forces are evident in the deflection of an electron beam in a cathode-ray tube and the motor force on a current-carrying conductor. Other applica¬ tions of magnetism range from the simple magnetic door catch to medi¬ cal imaging devices and electromagnets used in high-energy particle ACCELERATORS.
magnetite or lodestone or magnetic iron ore Iron oxide mineral (Fe 3 0 4 ), the chief member of one of the series of the spinel group. Min¬ erals in this series form black to brownish, metallic, moderately hard octahedrons and masses in igneous and metamorphic rocks and in gran¬ ite pegmatites, stony meteorites, and high-temperature sulfide veins. Mag¬ netite, as the name implies, is strongly attracted to a magnet. It is a common constituent of iron ores. Magnetite with an intrinsic magnetic field (a natural magnet) is known as lodestone.
magneto \mag-'ne-to\ Permanent-magnet alternating generator used mainly to produce electrical current for the ignition system in various types of internal-combustion engines, such as aircraft, marine, tractor, and motor¬ cycle engines. The main parts of a magneto are a permanent-magnet rotor, a primary winding of a small number of turns of coarse wire, a second¬ ary winding with a large number of turns of fine wire, a cam-type circuit breaker, and a capacitor. As the rotor turns, it produces a current in the primary winding, charging the capacitor. The cam breaks the circuit, and the magnetic field around the primary winding collapses. The capacitor releases its stored current into the primary winding, causing a reversed magnetic field. The collapse and reversal of the magnetic field produce a current in the secondary winding that is sent to the spark plugs.
magnetosphere Region around a planet (such as Earth) or a natural satellite that possesses a magnetic field (see geomagnetic field) where mag¬ netic phenomena and the high atmospheric conductivity caused by ion¬ ization strongly influence the behaviour of charged particles (see electric charge). A planet or moon’s magnetic field, like its gravitational field, becomes weaker with distance from the body. The solar wind sweeps a body’s magnetosphere out away from the Sun in a “tail” trailing well beyond it.
magnitude In astronomy, the measure of the brightness of a star or other celestial body. The brighter the object, the lower the number assigned as a magnitude. In ancient times six magnitude classes were used, the first containing the brightest stars (see Hipparchus). In the present
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magnolia ► Maharashtra I 1169
system a difference of one magnitude is defined as a ratio of brightness of 2.512 times. Thus, a difference of five magnitudes corresponds to a brightness ratio of 100 to 1. Apparent magnitude is an object’s brightness as seen from Earth (e.g., -26.7 for the Sun, about -11 for the Moon). Absolute magnitude is an object’s brightness as it would be seen at a dis¬ tance of 10 parsecs (32.6 light-years; e.g., 4.8 for the Sun). See also albedo; PHOTOMETRY.
magnolia Any of about 80 species of trees and shrubs in the genus Magnolia , native to North and Central America, the Himalayas, and East Asia. They are valued for their fragrant flowers and handsome leaves. Magnolia is one of 12 genera in the family Magnoliaceae, which contains 210 species. Magnolias are among the most primitive of flowering plants; their primitive features include long floral axes, spiral arrangement of flower parts, and simple water-conducting cells.
Magnus II Eriksson (b. 1316, Norway—d. Dec. 1, 1374, Sweden) King of Sweden (1319-63, as Magnus II) and Norway (1319-55, as Mag¬ nus VII). Grandson of Norway’s Haakon V and nephew of the Swedish king, Magnus became ruler of both countries. Since he spent almost all his time in Sweden, Norwegian nobles arranged for his son Haakon’s succession, and Magnus abdicated in 1355. He antagonized many Swed¬ ish nobles, raising taxes and curbing the economic power of the church and nobility. In 1356 he was forced to cede half his Swedish kingdom to his son Erik, and he began making concessions to the nobility. When he renewed his efforts to control the Swedish nobles, they deposed him.