in thermal equilibrium with a third
body, then all three bodies are in
thermal equilibrium with each other.
See also enthalpy; free energy.
A
- A tutorial from the Division of Chemical
Education, Purdue University, Indiana - Thermodynamic properties from the
committee on Data for Science and
Technology
thermodynamic temperature
See temperature.
thermoluminescence*Lumines-
cence produced in a solid when its
temperature is raised. It arises when
free electrons and holes, trapped in a
solid as a result of exposure to ioniz-
ing radiation, unite and emit photons
of light. The process is made use of
in thermoluminescent dating, which
assumes that the number of elec-
trons and holes trapped in a sample
of pottery is related to the length of
time that has elapsed since the pot-
tery wasÜred. By comparing the lu-
minescence produced by heating a
piece of pottery of unknown age
with the luminescence produced by
heating similar materials of known
age, a fairly accurate estimate of the
age of an object can be made.
thermoluminescent datingSee
thermoluminescence.
thermolysis(pyrolysis) The chemi-
cal decomposition of a substance by
heat. It is an important process in
chemical manufacture, such as the
thermal *cracking of hydrocarbons
in the petroleum industry.
thermometerAn instrument used
for measuring the *temperature of a
substance. A number of techniques
and forms are used in thermometers
depending on such factors as the de-
gree of accuracy required and the
range of temperatures to be meas-
ured, but they all measure tempera-
ture by making use of some property
of a substance that varies with tem-
perature. For example, liquid-in-glass
thermometersdepend on the expan-
sion of a liquid, usually mercury or
alcohol coloured with dye. These con-
sist of a liquid-Ülled glass bulb at-
tached to a partiallyÜlled capillary
tube. In the bimetallic thermometer
the unequal expansion of two dissim-
ilar metals that have been bonded to-
gether into a narrow strip and coiled
is used to move a pointer round a
dial. The gas thermometer, which is
more accurate than the liquid-in-
glass thermometer, measures the
variation in the pressure of a gas
kept at constant volume. The resis-
tance thermometer is based on the
change in resistance of conductors or
semiconductors with temperature
change. Platinum, nickel, and copper
are the metals most commonly used
in resistance thermometers.thermoplasticSee plastics.thermosettingSee plastics.
thermostatA device that controls
the heating or cooling of a substance
in order to maintain it at a constant
temperature. It consists of a tempera-
ture-sensing instrument connected to
a switching device. When the tem-
perature reaches a predetermined
level the sensor switches the heating
or cooling source on or off according
to a predetermined program. The
sensing thermometer is often a
bimetallic strip that triggers a simple
electrical switch. Thermostats are
used for space-heating controls, in
water heaters and refrigerators, and
to maintain the environment of a sci-
entiÜc experiment at a constant tem-
perature.
THFSee tetrahydrofuran.thiamin(e)See vitamin b complex.thiazoleA heterocyclic compound525 thiazole
t