Magnetic \Mag*net"ic\, Magnetical \Mag*net"ic*al\, a. [L.
magneticus: cf. F. magn['e]tique.]
1. Pertaining to the magnet; possessing the properties of the
magnet, or corresponding properties; as, a magnetic bar of
iron; a magnetic needle.
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2. Of or pertaining to, or characterized by, the earth's
magnetism; as, the magnetic north; the magnetic meridian.
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3. Capable of becoming a magnet; susceptible to magnetism;
as, the magnetic metals.
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4. Endowed with extraordinary personal power to excite the
feelings and to win the affections; attractive; inducing
attachment.
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She that had all magnetic force alone. --Donne.
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5. Having, susceptible to, or induced by, animal magnetism,
so called; hypnotic; as, a magnetic sleep. See
Magnetism. [Archaic]
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Magnetic amplitude, attraction, dip, induction, etc.
See under Amplitude, Attraction, etc.
Magnetic battery, a combination of bar or horseshoe magnets
with the like poles adjacent, so as to act together with
great power.
Magnetic compensator, a contrivance connected with a ship's
compass for compensating or neutralizing the effect of the
iron of the ship upon the needle.
Magnetic curves, curves indicating lines of magnetic force,
as in the arrangement of iron filings between the poles of
a powerful magnet.
Magnetic elements.
(a) (Chem. Physics) Those elements, as iron, nickel,
cobalt, chromium, manganese, etc., which are capable
or becoming magnetic.
(b) (Physics) In respect to terrestrial magnetism, the
declination, inclination, and intensity.
(c) See under Element.
Magnetic fluid, the hypothetical fluid whose existence was
formerly assumed in the explanations of the phenomena of
magnetism; -- no longer considered a meaningful concept.
Magnetic iron, or Magnetic iron ore. (Min.) Same as
Magnetite.
Magnetic needle, a slender bar of steel, magnetized and
suspended at its center on a sharp-pointed pivot, or by a
delicate fiber, so that it may take freely the direction
of the magnetic meridian. It constitutes the essential
part of a compass, such as the mariner's and the
surveyor's.
Magnetic poles, the two points in the opposite polar
regions of the earth at which the direction of the dipping
needle is vertical.
Magnetic pyrites. See Pyrrhotite.
Magnetic storm (Terrestrial Physics), a disturbance of the
earth's magnetic force characterized by great and sudden
changes.
magnetic tape (Electronics), a ribbon of plastic material
to which is affixed a thin layer of powder of a material
which can be magnetized, such as ferrite. Such tapes are
used in various electronic devices to record fluctuating
voltages, which can be used to represent sounds, images,
or binary data. Devices such as audio casette recorders,
videocasette recorders, and computer data storage devices
use magnetic tape as an inexpensive medium to store data.
Different magnetically susceptible materials are used in
such tapes.
Magnetic telegraph, a telegraph acting by means of a
magnet. See Telegraph.
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magnetic tape
(Or "magtape", "tape" - paper tape is now
obsolete) A data storage medium consisting of a magnetisable
oxide coating on a thin plastic strip, commonly used for
backup and archiving.
Early industry-standard magnetic tape was half an inch wide
and wound on removable reels 10.5 inches in diameter.
Different lengths were available with 2400 feet and 4800 feet
being common. DECtape was a variation on this "round
tape".
In modern magnetic tape systems the reels are much smaller and
are fixed inside a cartridge to protect the tape and for
ease of handling ("square tape" - though it's really
rectangular). Cartridge formats include QIC, DAT, and
Exabyte.
Tape is read and written on a tape drive (or "deck") which
winds the tape from one reel to the other causing it to move
past a read/write head. Early tape had seven parallel tracks
of data along the length of the tape allowing six bit
characters plus parity written across the tape. A typical
recording density was 556 characters per inch. The tape had
reflective marks near its end which signaled beginning of tape
(BOT) and end of tape (EOT) to the hardware.
Data is written to tape in blocks with inter-block gaps
between them. Each block is typically written in a single
operation with the tape running continuously during the write.
The larger the block the larger the data buffer required in
order to supply or receive the data written to or read from
the tape. The smaller the block the more tape is wasted as
inter-block gaps. Several logical records may be combined
into one physical block to reduce wastage ("blocked
records"). Finding a certain block on the tape generally
involved reading sequentially from the beginning, in contrast
to magnetic disks. Tape is not suitable for random
access. The exception to this is that some systems allow
tape marks to be written which can be detected while winding
the tape forward or rewinding it at high speed. These are
typically used to separate logical files on a tape.
Most tape drives now include some kind of data compression.
There are several algorithms which provide similar results:
LZ (most), IDRC (Exabyte), ALDC (IBM, QIC) and
DLZ1 (DLT).
See also cut a tape, flap, Group Code Recording,
spool, macrotape, microtape, Non Return to Zero
Inverted, Phase Encoded.
(1997-04-05)
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