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Invisible lines of magnetic flux leave the north pole of the magnet and travel by the easiest path to the south
pole. These lines leave and enter the magnet at right angles to the surface. A magnet attracts pieces of iron
or steel (ferrous metals), and if a wire is moved through the lines of magnetic flux between the poles,
electrons are forced to flow in it.
There are two basic types of magnets: permanent magnets and electromagnets. Permanent magnets
are pieces of metal with the magnetic domains in alignment. The metal used as a permanent magnet has a
high retentivity; therefore it keeps, or retains, its magnetism.
Electromagnets are made of a coil of wire wound around a core of soft iron. When direct current
flows through the coil, the domains in the soft iron line up, and the core becomes a strong magnet. Iron has
very low retentivity, however, and as soon as the current stops flowing in the coil, the domains in the iron
core lose their alignment, and the iron is no longer a magnet.
magnetic amplifier (magamp). A type of electrical control transformer in which a small amount of current
flowing in the control winding determines the amount of load current allowed to flow in the load winding.
Direct current flowing through the bias and control windings of the magamp varies the magnetic saturation
of the core, and the condition of saturation determines the permeability of the core.
Varying the permeability of the core varies the amount of inductive reactance opposing the AC
flowing in the load winding. Magnetic amplifiers are also called magamps and saturable reactors.
magnetic bearing (navigation). The direction to or from a radio transmitting station measured relative to
magnetic north.
magnetic brake (electric motor component). A type of friction brake that automatically releases when
current flows through the motor windings. As soon as current to the motor is shut off, a spring forces a
stationary friction disk against a disk that rotates with the motor armature. The friction prevents the
armature turning. An electromagnet pulls the stationary disk away from the rotating disk when current
flows in the motor.
magnetic bubble memory (digital computers). A method of storing information for use in a digital
computer. Tiny areas of magnetism can be formed in a thin film of magnetic garnet crystal. These tiny
areas, or bubbles, can be moved about within the film to change the logic conditions they represent.
Magnetic bubble memory allows an extremely large amount of digital information to be stored in an
exceedingly small area.
magnetic chuck. A special work surface used with certain types of metal-machining tools. An
electromagnetic force holds the material being machined tightly to the surface.
magnetic circuit. The complete path in an electrical machine that is followed by lines of magnetic flux.
Lines of flux leave the north pole of the magnet, flow through all the circuit components, and re-enter the
magnet at its south pole.
In a motor or generator, the magnetic circuit is made up of the pole shoes, the armature core, the
field frame, and the air gaps between the pole shoes and the armature
magnetic circuit breaker. An electrical component that opens a circuit any time excessive current flows.
A magnetic field is produced by current flowing through the circuit breaker, and the strength of the field is
proportional to the amount of current. When more current flows through the circuit breaker than it is rated
Printed from Summit Aviation's Computerized Aviation Reference Library, 2/7/2007
Page 367
to carry, the magnetic field becomes strong enough to snap open a set of contacts and break, or open, the
circuit.
magnetic course (navigation). A desired flight path referenced from magnetic north. A true course
measured on a navigational chart is referenced to the geographic north pole but a magnetic compass points
to the magnetic north pole. The angular difference between a true course and a magnetic course is called
variation.
magnetic deviation (navigation). A form of compass error caused by local magnetic fields in the aircraft
interacting with the magnetic field of the earth. Deviation error is minimized by counteracting the
interfering magnetic fields by adjusting small compensating magnets that are built into the compass
housing. This compensating procedure is called “swinging the compass.” Deviation error differs with each
heading of the aircraft.
magnetic-drag tachometer. An instrument used in an aircraft to show the rotational speed of the engine
crankshaft. A flexible shaft, or cable, turned at one-half crankshaft speed, spins a permanent magnet inside
the tachometer.
An aluminum drag cup rides over the spinning magnet, but does not touch it. A pointer shaft is
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航空术语词典Dictionary of Aeronautical Terms 中(119)
