pole, magnetic: see magnetic pole.

permeability, magnetic: see magnetism; flux, magnetic.

nuclear magnetic resonance: see magnetic resonance.

magnetic tape: see computer; tape recorder.

magnetic resonance imaging (MRI), noninvasive diagnostic technique that uses nuclear magnetic resonance to produce cross-sectional images of organs and other internal body structures. The patient lies inside a large, hollow cylinder containing a strong electromagnet, which causes the nuclei of certain atoms in the body (especially those of hydrogen) to align magnetically. The patient is then subjected to radio waves, which cause the aligned nuclei to "flip"; when the radio waves are withdrawn the nuclei return to their original positions, emitting radio waves that are then detected by a receiver and translated into a two-dimensional picture by computer. Unhampered by bone and capable of producing images in a variety of planes, MRI is used in the diagnosis of brain tumors and disorders, spinal disorders, multiple sclerosis, and cardiovascular disease. The procedure is considered to be without risk, but the scanner may interfere with pacemakers, hearing aids, or other mechanical devices. Although the images are similar in many ways to those of CAT scans, they are obtained without X rays or other radiation, and generally provide more contrast between normal and abnormal tissue.

magnetic resonance, in physics and chemistry, phenomenon produced by simultaneously applying a steady magnetic field and electromagnetic radiation (usually radio waves) to a sample of atoms and then adjusting the frequency of the radiation and the strength of the magnetic field to produce absorption of the radiation. The resonance refers to the enhancement of the absorption that occurs when the correct combination of field and frequency is reached. The procedure is analogous to tuning a radio dial exactly to a desired station.

Several distinct kinds of magnetic resonance exist. In cyclotron resonance the magnetic field is adjusted so that the frequency of revolution of a charged particle around the field lines is exactly equal to the frequency of the radiation. This principle is used to produce beams of energetic particles in particle accelerators.

Other magnetic resonance phenomena depend on the fact that both the proton and electron exhibit intrinsic spin about their own axes and thus act like microscopic magnets. Electron paramagnetic resonance (EPR) arises from unpaired electron spins in liquids or solid crystals. Because of their own magnetism, the spins line up with the external magnetic field. For a given magnetic field the spins can be made to "flip" to the opposite direction when they absorb radiation at a corresponding "resonant" frequency. From the point of view of quantum mechanics, the spin flips can be considered as transitions between states that become separated in energy when the magnetic field is applied. The effect is related to the splitting of spectral lines when an atom is subjected to a magnetic field (see spectrum; Zeeman effect).

Nuclear magnetic resonance (NMR) is analogous to EPR; however NMR is produced by the much smaller magnetism associated with unpaired nuclear spins. The NMR resonant frequency (usually that of protons in complex molecules) is slightly shifted by interactions with nearby atoms in the sample, thus providing information about the chemical structure of organic molecules and other materials. NMR is now extensively employed in medicine, although the use of the word "nuclear" is avoided, the preferred name being magnetic resonance imaging (MRI). The technique provides high-quality cross-sectional images of internal organs and structures. Paul Lauterbur, an American physicist, and Peter Mansfield, a British physicist, shared the 2003 Nobel Prize in Physiology or Medicine for pioneering contributions that later led to the application of magnetic resonance in medical imaging.

Magnetic resonance can also occur without an external magnetic field from interactions of the electron and nuclear spins; such resonance produces the fine and hyperfine structure of atomic spectra.

magnetic pyrite: see pyrrhotite.

magnetic pole, the two nearly opposite ends of the planet where the earth's magnetic intensity is the greatest, as the north and south magnetic poles. For the magnetic north, it is the direction from any point on the earth's surface linking the horizontal component of the magnetic lines of force with the observer and north magnetic pole; it is similar for magnetic south. The north magnetic pole, first located (1831) by British explorer Sir James C. Ross, is now about 78°N and 104°W in the Queen Elizabeth Islands of northern Canada. The south magnetic pole, reached (1909) by Australian geologists Sir T. W. E. David and Sir Douglas Mawson, is now about 66°S and 139°E on the Adélie Coast of Antarctica. The magnetic poles are not fixed but follow circular paths with diameters of about 100 miles (160 km). Studies of paleomagnetism also indicate that the earth's magnetic field has reversed its polarity many times in the geologic past. The best hypothesis to date for the origin of terrestrial magnetism is the self-exciting dynamo theory, where the earth's magnetic field is generated by the interaction of motion and electrical currents in the earth's liquid outer core.

magnetic levitation or maglev, support and propulsion of objects or vehicles by the use of magnets. The magnets provide support without contact or friction, allowing for fast, quiet operation. In a typical system, the vehicle, which resembles a railroad car, travels above a guideway. Arrays of magnets of like polarity in both the vehicle and guideway repel each other, producing the lifting force. By continuously changing the polarity in alternate magnets, a series of magnetic attractions and repulsions is created that moves the vehicle along the track. The electrical energy required for such a system is great and the use of superconducting materials offers the only realistic potential for this means of transportation. Research into such systems has been conducted since the 1960s in the United States, Great Britain, Japan, and Germany. Maglev technology was applied in England in the construction of a fully automated, low-speed shuttle in Birmingham, but the line was closed because of maintenance problems. In 1996 funding was approved in Germany for a maglev train linking Berlin and Hamburg, but it was canceled in 2000. In 2004 a maglev line linking Shanghai's financial district with its airport began commercial operation; the train can reach speeds of 268 mph (432 km/h) along its 18.6 mi (30 km) route.

magnetic core: see computer.

flux, magnetic, in physics, term used to describe the total amount of magnetic field in a given region. The term flux was chosen because the power of a magnet seems to "flow" out of the magnet at one pole and return at the other pole in a circulating pattern, as suggested by the patterns formed by iron filings sprinkled on a paper placed over a magnet or a conductor carrying an electric current. These patterns are called lines of induction. Although there is no actual physical flow, the lines of induction suggest the correct mathematical description of magnetism in terms of a field of force. The lines of induction originate on the north pole of the magnet and end on the south pole; their direction at any point is the direction of the magnetic field, and their density (the number of lines passing through a unit area) gives the strength of the field. Near the poles where the lines converge, the field and the force it produces are large; away from the poles where the lines diverge, the field and force are progressively weaker.

electric and magnetic units, units used to express the magnitudes of various quantities in electricity and magnetism. Three systems of such units, all based on the metric system, are commonly used. One of these, the mksa-practical system, is defined in terms of the units of the mks system and has the ampere of electric current as its basic unit. The units of this system—the volt, ohm, watt, and farad—are those commonly used by scientists and engineers to make practical measurements. The two other systems are both based on the cgs system. Electrostatic units (cgs-esu) are defined in a way that simplifies the description of interactions between static electric charges; there are no corresponding magnetic units in this system. Electromagnetic units (cgs-emu), on the other hand, are defined especially for the description of phenomena associated with moving electric charges, i.e., electric currents and magnetic poles. The two cgs systems have been widely used in the past and are still found in many texts and papers. The official body for maintaining such units in the United States is the National Institute of Standards and Technology.

Selective absorption of very high-frequency radio waves by certain atomic nuclei subjected to a strong stationary magnetic field. Nuclei that have at least one unpaired proton or neutron act like tiny magnets. When a strong magnetic field acts on such nuclei, it sets them into precession. When the natural frequency of the precessing nuclear magnets corresponds to the frequency of a weak external radio wave striking the material, energy is absorbed by the nuclei at a frequency called the resonant frequency. NMR is used to study the molecular structure of various solids and liquids. Magnetic resonance imaging, or MRI, is a version of NMR used in medicine to view soft tissues of the human body in a hazard-free, noninvasive way.

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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, magnetic resonance imaging is used to produce images of human tissue.

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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 diamagnetism) have constant relative permeabilities of slightly less than 1. Paramagnetic materials (see paramagnetism) have constant relative permeabilities of slightly more than 1. The relative permeability of ferromagnetic 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.

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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 moving 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.

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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 continuously changing. Magnetic fields are commonly represented by continuous 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.

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Tiny magnet with subatomic dimensions, equivalent to the flow of electric charge around a loop. Examples include electrons circulating around atomic nuclei, rotating atomic nuclei, and single subatomic particles with spin. On a large scale, these effects may add together, as in iron atoms, to make magnetic compass needles and bar magnets, which are macroscopic magnetic dipoles. The strength of a magnetic dipole, its magnetic moment, is a measure of its ability to turn itself into alignment with a given external magnetic field. When free to rotate, dipoles align themselves so that their moments point predominantly in the direction of the magnetic field. The SI unit for dipole moment is the ampere-square metre.

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