Page 341 - Electrical Engineering Dictionary

concentration of combustible dusts. A class datagram this is referred to as a trailer. Ad-
III hazardous location contains an ignitable dressing information and checksum are ex-
concentration of ignitable fibers. amples of the control information contained
in a header.
hazardous outage an outage which has
(2) a section of an image file, usually of
been assessed to have potential life threat-
fixed size and occurring at the start of the file,
ening consequences. The criteria for a haz-
that contains information about the image,
ardous outage varies considerably from one
such as the number of rows and columns and
utility to another. In some cases arcing lines
the size of each pixel.
is sufficient for a hazardous outage. Other
utilities would consider at least fire or explo-
sion to qualify an outage as hazardous. heap data storage structure that accepts
items of various sizes and is not ordered.
haze tails when self-converging deflec- Contrast with stack.
tion yokes are used on a CRT, overfocusing
occurs on the vertical center line of the beam
heat sink aluminum mountings of vari-
spot. Around the periphery of the screen, this
ous shapes and sizes used for cooling power
produces a hazy area above and below the
semiconductor devices. Heat sinks can be
spot, referred to as haze tails. This degrades
cooled by either natural convection or a fan,
picture contrast.
and heat dissipation can be improved by a
coating of black oxide or if the heat sink is
HBMT See hybrid bipolar MOS thyristor. made with fins.
heater See overload heater.
HBT Seeheterojunctionbipolartransistor.
Heaviside characteristic an activation
HDA See head disk assembly. function according to which the neuron out-
puttakesonthevalueunitywhenitsweighted
HDSL See high-speed digital subscriber input exceeds (or equals) the neural threshold
line. value and zero otherwise.

HDTV See high definition television.
Heaviside, Oliver (1850–1925) Born:
London, England
head an electromagnet that produces
switchable magnetic fields to read and record Heaviside is best known for his theoret-
bit streams on a platter’s track. ical work in electrical engineering. Much
of his work is contained in his three-volume
head disk assembly (HDA) collection of work called Electromagnetic Theory. The fi-
platters, heads, arms, and acutators, plus the nalvolumewaspublishedin1912. Heaviside
air-tight enclosing of a magnetic disk system. extended and improved the works of Hamil-
ton and Maxwell and deduced the concepts
head-medium gap the distance between of capacitance, impedance, and inductance.
the read–write head and the disk in magnetic Heaviside was self-taught and irascible. At
or optical disk memory devices. first, much of his work was dismissed as un-
orthodox or too theoretical to be of practical
header (1)adatastructurecontainingcon- value. Heaviside is best known in physics for
trol information that is placed at the head his correct prediction of the existence of the
of a datagram; when placed at the end of a ionosphere.

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2000 by CRC Press LLC