Thermal
Expansion
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Thermal
expansion has been harnessed in a variety of
useful ways. It is most commonly used in
temperature measuring devices. Each has a
different method of utilizing the materials and
their expansion rates.
Thermometer
 A thermometer is made of a small
glass tube with mercury or alcohol. These fluids
then expand and contract as the temperature
changes. Since the coefficient of linear expansion
is known, the temperature can be calibrated then
the device be used to measure temperature. Mercury
and alcohol are used because they have a low
freezing point.
Thermostats
Thermostats also
use thermal expansion. In older non-digital
thermostats, heating and cooling systems were
controlled by a device with a bimetallic strip.
This bimetallic strip is made of two different
metal strips that are attached together and coiled
as shown in the picture at the right. One side has
a lower coefficient of expansion  than the other so when heated, the
different metals in the coil will expand at
different rates, causing the coil to expand. This
moving of the coil causes the glass tube with
mercury to tip. The conductive mercury connects
the wires that are at the bottom of the glass tube
which allows for a heating system to turn on at a
given temperature.
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