18WuD2x

680 texture sensing • thermal shock
roughness of a surface. One common scheme
uses reflected light or infrared. Scattered reflections
indicate a rough or matte surface.
tgt Abbreviation of TARGET.
TH Abbreviation of true heading.
Th Symbol for THEORIUM.
TH Symbol for heater temperature.
thallium Symbol, Tl. A metallic element. Atomic
number, 81. Atomic weight, 204.38.
thalofide cell An evacuated photoconductive cell
using thallium oxysulfide as the light-sensitive
material.
THD Abbreviation of TOTAL HARMONIC DISTOR-
TION.
theory A reasonable proposition put forth to account
for the behavior of, or the relationships between,
bodies and forces, or to explain concepts
and their relations. When a theory has stood up
under exhaustive tests, it might reveal a scientific
law.
therm A gas heating unit. 1 therm = 100,000
British thermal units (10 5 Btu).
thermal agitation Random movement of particles
(such as electrons) in a substance, because of
heat.
thermal-agitation noise See THERMAL NOISE.
thermal ammeter See HOT-WIRE AMMETER.
thermal anemometer See HOT-WIRE ANEMO-
METER.
thermal conductivity The heat-conducting ability
of a material. Compare electrical conductivity (see
CONDUCTIVITY).
thermal conductivity
Thermal conductivity
Element
(mW/m/°C)
Aluminum 22.0
Carbon 2.4
Chrominum 6.9
Cooper 39.0
Gold 30.0
Iron 7.9
Lead 3.5
Magnesium 16.0
Mercury 0.8
Nickel 8.9
Platinum 6.9
Silicon 8.4
Silver 41.0
Thorium 4.1
Tin 6.4
Tungsten 20.0
Zinc 11.0
thermal-conductivity device An instrument or
control unit using a heated filament whose temperature
and, accordingly, conductivity is varied
by some sensed phenomenon. See, for example,
GAS DETECTOR, HEATED-WIRE SENSOR, HOT-
WIRE ANEMOMETER, HOT-WIRE FLOW-ME-
TER, and HOT-WIRE MICROPHONE.
thermal-conductivity gasmeter See GAS DETEC-
TOR.
thermal detector 1. See BOLOMETER. 2. In a security
or fire-protection system, a device that
closes a circuit or actuates an alarm if the temperature
rises to a specific level.
thermal emf See SEEBECK EFFECT.
thermal gasmeter See GAS DETECTOR.
thermal imaging See THERMOGRAPHY.
thermal instrument See HOT-WIRE METER and
THERMOCOUPLE-TYPE METER.
thermally sensitive resistor See THERMISTOR.
thermal meter See HOT-WIRE METER and THER-
MOCOUPLE-TYPE METER.
thermal neutron A neutron that is essentially in
thermal equilibrium with the surrounding medium
or environment.
thermal noise Frequency-independent electrical
noise caused by the agitation of particles (e.g.,
atoms and electrons) in a material by heat. Thermal
noise is proportional to bandwidth, resistance,
and absolute temperature.
thermal radiation See HEAT.
thermal recorder A graphic recorder in which a
strip of paper coated with a thin layer of opaque
wax is drawn between a knife-edge platen and a
heated writing stylus that melts the wax beneath
its point, exposing the underlying black paper as
a fine line.
thermal resistance Symbol, RT. For a semiconductor
device, the rate of change of junction temperature,
with respect to power dissipation; RT =
dT/dP, where RT is in degrees Celsius per milliwatt,
T is the temperature in degrees Celsius, and
P is the power in milliwatts.
thermal resistor A resistor that is sufficiently
temperature-sensitive to be used as a heat sensor.
Examples: thermistor and germanium
diode.
thermal response time For a power-dissipating
component, the elapsed time between the initial
change in power dissipation and the moment at
which the temperature has changed by a specified
percentage (usually 90%) of the total value.
thermal runaway A destructive process resulting
from cumulative temperature effects. In bipolar
transistors, this can occur if the collector current
increases as the temperature rises. As the unit
gets hotter, the collector-base junction dissipation
increases, generating still more heat. The
ultimate result, if the process continues
unchecked, is destruction of the component. The
process can also occur in certain batteries when
they are charged too rapidly or at excessively high
temperatures.
thermal shock The effect of applying heat or cold
to a device so rapidly that abnormal reactions occur,
such as rapid (often catastrophic) expansions
and contractions.