18WuD2x

684 three-phase rectifier • thumbwheel switch
for each phase. Also see POLYPHASE RECTIFIER,
THREE-PHASE BRIDGE RECTIFIER, THREE-
PHASE CIRCUIT, and THREE-PHASE HALF-
WAVE RECTIFIER.
three-phase system An alternating-current electrical
system in which three currents or voltages
exist simultaneously. They are of equal amplitude,
but are 120° out of phase with each other.
three-phase, three-wire system An electrical system
having three conductors, with an alternating-current
phase difference of 120° between
conductor pairs.
three-phase-to-single-phase transformer An alternating-current
transformer with three-phase
input and single-phase output (one terminal is
grounded).
three-phase-to-two-phase transformer An alternating-current
transformer with three-phase input
and two-phase output, the output currents
are 180° out of phase with each other.
three-phase voltage Voltage in a three-phase alternating-current
circuit. The voltages across the
three legs differ by 120°.
three-quarter bridge A bridge rectifier having
diodes in three arms and a resistor in the fourth.
three-space A mathematical continuum in which
each point is uniquely defined by three variables
in an ordered triple, such as (x,y,z), and each ordered
triple corresponds to exactly one point in
the space. A common coordinate system is the
Cartesian system consisting of three axes, x, y,
and z—each mutually perpendicular and intersecting
at the origin (0,0,0).
three-space coordinates Any set of coordinates
used for locating points or plotting graphs in
three dimensions.
three-state logic See TRI-STATE LOGIC.
three-way speaker A set of three individual speakers
contained in a single cabinet: a woofer (lowfrequency
speaker), a midrange speaker, and a
tweeter (high-frequency speaker). It is common in
high-fidelity music reproduction systems. Compare
TRIAXIAL SPEAKER.
three-wire system 1. An electric-power feed system
using three wires, the center one (neutral)
being at a potential midway between the potential
across the other (outer) two. 2. See THREE-
PHASE and THREE-WIRE SYSTEM. 3. See TWO-
PHASE and THREE-WIRE CIRCUIT.
threshold 1. The initial (observable) point of an effect
(e.g., threshold of hearing). 2. A predetermined
point, such as of minimum current or voltage, for
the start of operation of a circuit or device.
threshold component A value of current, voltage,
sound intensity, etc., selected as the minimum
level at which a circuit or device is to operate in
some prescribed manner, or beyond which a
certain condition will prevail. Also see THRESH-
OLD, 1, 2.
threshold current 1. The minimum value of current
at which a certain effect takes place. 2. The
smallest amount of forward current that flows
through a diode. 3. In a gas, the smallest level of
current for which a discharge will maintain itself
under variable conditions.
threshold detector A device that prevents a signal
from passing until its peak amplitude reaches a
certain value.
Input
On
Off
threshold detector
Output
threshold frequency 1. The cutoff frequency or frequencies
in a band-pass, band-rejection, low-pass,
or high-pass filter. 2. The lowest frequency for a
metal or semiconductor junction at which incident
radiation gives rise to the photoelectric effect.
threshold of hearing The minimum intensity level
at which sounds are audible in an environment
containing essentially no background acoustic
noise.
threshold of pain The intensity level at which
hearing a sound causes physical discomfort. This
is approximately 120 dB above the THRESHOLD
OF HEARING for most people.
threshold signal The weakest signal that can be
detected in a receiving system.
throat See HORN THROAT.
throat microphone A small microphone operated
in contact with the user’s throat.
throttle The feedback control device in a regenerative
detector or amplifier.
throughput In computer operations, an overall
quantitative indicator of processing power, expressed
in terms of the amount of data processed
in a given period of time.
throw-out spiral On a phonograph disc, a lead-out
groove.
thulium Symbol, Tm. A metallic element of the
rare-earth group. Atomic number, 69. Atomic
weight, 168.93.
thumbwheel potentiometer A potentiometer operated
by means of a knurled knob (usually protruding
perpendicularly through a panel) that is
turned with the thumb or with a finger.
thumbwheel switch A switch operated by means
of a knurled knob (usually protruding perpendicularly
through a panel) that is turned with the
thumb or with a finger.