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magnetic poles. The locations on a magnet at which the lines of flux leave and enter. All magnets have
both a north pole and a south pole. The lines of flux leave the magnet at right angles to the face of the north
pole and enter at right angles to the face of the south pole.
magnetic saturation. The condition in which all the magnetic domains in a piece of magnetized material
are lined up in the same direction. When a piece of unmagnetized iron or other magnetizable material is
held in a magnetic field, increasing the magnetizing force increases the magnetism. But when all the
domains are aligned in the same direction, increasing the magnetizing force no longer increases the strength
of the magnet. The magnet is said to be saturated.
magnetic shunt (electrical measuring instrument component). A piece of soft iron placed across the air
gap in a magnet used in an electrical measuring instrument to vary the amount of magnetic flux across the
gap. The permeability of soft iron is much higher than that of air (flux can pass through iron much easier
than it can pass through air), and the flux passes through the iron, rather than crossing the air gap. The
position of the magnetic shunt is changed to calibrate the instrument.
magnetic variation (navigation). A compass error caused by the difference in the locations of the earth’s
geographic north pole and its magnetic north pole. All navigation charts and maps are drawn with reference
to the geographic north pole, and all magnetic compasses point to the magnetic north pole. These two
locations are not the same, and they are only in alignment along the agonic line.
At any particular location on the surface of the earth, the variation error is the same for all
headings. Variation is called declination in land navigation. See agonic line, isogonic lines.
magnetism. A property possessed by certain materials that causes them to attract or repel other materials
having this same property. Magnetism also causes electrical current to flow in a conductor when it is
moved across the lines of magnetic flux extending out from the magnet.
magnet keeper. A piece of soft iron placed across the poles of a horseshoe permanent magnet when the
magnet is not in use. The permeability of the soft iron is much greater than that of the air between the poles,
and by keeping the flux concentrated, the flux loss is minimized and the magnet is prevented from
becoming demagnetized.
magneto (reciprocating engine ignition system component). The source of high-voltage electrical energy,
used to produce the spark to ignite the fuel-air mixture inside the cylinders of a reciprocating engine. A
magneto contains a small alternating-current generator with a set of breaker points to interrupt the current
the instant the spark is needed.
Current produced by the AC generator flows through the primary winding of a step-up transformer,
the magneto coil, and this current causes lines of magnetic flux to expand out across the many turns of wire
in the secondary winding of the coil. When the breaker points open, the primary current immediately stops
flowing, and the magnetic field instantly collapses, cutting across the secondary winding and producing a
high voltage in it. A capacitor, installed across the breaker points, prevents arcing as the points open.
A magneto also incorporates a distributor, or high-voltage selector switch, to direct the high
voltage to the correct spark plug.
magnetometer. An instrument used to measure and plot the strength and direction of the earth’s magnetic
field. Magnetometers are often towed behind low-flying airplanes to find deposits of magnetic minerals by
noting the change in the direction of the local magnetic field. These aerial surveys are referred to as MAD
(magnetic anomaly detection) surveys.
magnetomotive force (mmf). A force that produces a magnetic field. The magnetomotive force caused by
current flowing in a coil of wire is determined by both the amount of current and the number of turns of
wire.
The gilbert is the unit of magnetomotive force, with one gilbert being the amount of mmf needed
to cause one maxwell (one line of magnetic flux) to flow through a magnetic circuit having a reluctance
(the opposition to the passage of magnetic flux) of one unit. One gilbert is also the amount of mmf
produced by 0.7957-ampere turn. The constant 0.7957 is 10 divided by four π.
magneto safety check (aircraft reciprocating engine operation check). A check made by the pilot of an
Printed from Summit Aviation's Computerized Aviation Reference Library, 2/7/2007
Page 370
aircraft to determine that the ignition switch is operating properly. The engine is idled, and the ignition
switch is momentarily placed in the OFF position. The engine should stop firing, but the switch is returned
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航空术语词典Dictionary of Aeronautical Terms 中(121)
