18WuD2x

704 ultraviolet lamp • unblanking interval
ultraviolet lamp A lamp that delivers ultraviolet
rays. Common types are arc lamps and mercuryvapor
lamps.
ultraviolet light See ULTRAVIOLET RAYS.
ultraviolet power The power (in watts) of ultraviolet
radiation.
ultraviolet rays Radiation at frequencies in the ultraviolet
region (i.e., between the highest visiblelight
frequencies and the lowest X-ray frequencies).
ultraviolet therapy The use of ultraviolet rays by
physicians and other health personnel in the
treatment of certain disorders.
ultraviolet wavelength The wavelength range of
ultraviolet radiation [e.g., 390 nanometers (nm)
to 4 nm].
ultraviolet waves See ULTRAVIOLET RAYS.
umbilical cord 1. A cord through which missiles
and rockets are controlled and powered until they
are launched. 2. A cord connecting an astronaut
to his space vehicle during extravehicular activity
(e.g., during a space “walk”).
umbilical tower The vertical tower supporting the
umbilical cords extending to a rocket in the
launch position.
umbra 1. The region of total (conical) shadow behind
an object situated in the path of a radiation.
2. The comparatively dark central region in a
sunspot.
umbrella antenna An antenna consisting of a
number of wires, extending from the top of a vertical
mast to points on a circle below, at the center
of which the mast is mounted. The wires are
usually fed in parallel from the top.
Umklapp process A process responsible for thermal
resistance in nonconducting materials. It results
from collision between phonons, or
phonons and electrons.
UMW Abbreviation of ULTRAMICROWAVE.
unabsorbed field strength The field strength with
no absorption between transmitter and detector.
The unabsorbed field strength is an ideal quantity;
the actual field strength over a given distance
is always less. It is expressed in volts per meter.
unamplified back bias A negative-feedback voltage
developed across a fast-time-constant circuit
within a single amplifier stage.
unamplified feedback A positive or negative current
or voltage taken from the output of a system
and presented to the input without being boosted
through auxiliary amplification.
unamplified ALC Automatic level control (ALC) in
which the control-signal voltage is taken from a
point in the circuit and fed to the controlled point
without being boosted by an auxiliary amplifier.
Compare AMPLIFIED ALC.
unattended operation Operation, as of an electronically
programmed machine, with minimal (or
no) human supervision.
unattended time The period, excluding down time
for repair or checkout, during which a computer
is unpowered.
unattenuated Not reduced in intensity of amplitude.
Thus, an unattenuated signal is one that
has retained its original strength during its transmission
through a system.
unavailable energy The difference between the
quantity of heat energy supplied to a system and
the available energy of the system.
unbalance 1. Lack of balance or symmetry in a circuit,
line, or system. 2. The condition in which a
bridge (or the equivalent) is not nulled.
unbalanced circuit See SINGLE-ENDED CIRCUIT.
unbalanced delta system A three-phase circuit in
which the elements are connected in a triangular
(delta) configuration, but are of unequal
impedance. In an unbalanced delta system, no
definite relationship exists between line and
phase currents. Compare BALANCED DELTA
SYSTEM.
unbalanced input See SINGLE-ENDED INPUT.
unbalanced line 1. A transmission line in which
one side (conductor) is grounded (e.g., a coaxial
line). 2. A normally balanced transmission line in
which the currents in the two halves are out of
balance (i.e., they are not equal in amplitude and
opposite in phase).
unbalanced multivibrator A multivibrator in
which the time constant of one base or gate circuit
differs from that of the other base or gate circuit.
The asymmetry results in short-duration
output pulses separated by long time periods.
unbalanced output See SINGLE-ENDED OUTPUT.
unbalanced two-phase system A two-phase circuit
in which the load elements are of unequal
impedance.
unbalanced wire circuit A wire line whose (two)
sides are dissimilar.
unbalanced-wye system A three-phase circuit in
which the elements are connected in the familiar
wye, or star, configuration, but are of unequal
impedance.
unbalanced-Y system See UNBALANCED-WYE
SYSTEM.
unbalance-to-balance transformer A transformer
for matching an unbalanced transmission line to
a balanced line (e.g., a coaxial line to a parallelwire
line).
unbiased limiting Peak limiting that results from
overdriving an unbiased active device.
unbiased unit A device or circuit operated without
bias [e.g., a transistor without base bias (common
emitter and emitter follower) or emitter bias (common
base)].
unblanking Removal of the blanking pulse in a
cathode-ray-tube circuit (i.e., turning on the
beam).
unblanking generator A (usually pulse-type) signal
source for turning on the beam of a blanked
cathode-ray tube.
unblanking interval The period during which the
beam of a cathode-ray tube is turned on. Compare
BLANKING INTERVAL.