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and an accurate magnetic compass. The platform is leveled and pointed toward true north by applying the
correction for declination, or variation, to compensate for the difference between magnetic north and true
north.
An azimuth scale (a scale that measures the horizontal direction the telescope moves) measures
the bearing from the theodolite to any object visible through the telescope. A vertical scale measures the
angle the telescope is inclined upward from its level position.
In weather observation, the theodolite is used to track the movement of a weather balloon to find
the direction and velocity of the wind at various altitudes. A theodolite is also used with a ceiling light to
measure the height of the base of clouds at night.
therapeutic mask adapter (oxygen system component). A type of adapter used in a constant-flow
oxygen system to increase the flow of oxygen to a mask being used by a passenger known to have a heart
or respiratory problem.
thermal (meteorology). A column of rising air. Buildings and automobile traffic heat the air above cities
and this heated air rises in a column, called a thermal.
Sailplanes gain altitude by circling in a thermal and being carried upward by it. Thermals are
easily found, because clouds normally form at their top. Air containing water vapor is carried upward until
it reaches a level at which the vapor condenses and becomes visible droplets of liquid water. These droplets
form the cloud.
thermal anti-icing system (TAI). An aircraft anti-icing system that uses hot air to prevent ice forming on
the leading edges of the wings and tail surfaces. Hot air from the turbine-engine compressor or from a
shroud around the exhaust system of a reciprocating engine flows through ducts in the leading edges of the
wings and tail surfaces to heat them so ice cannot form on them.
thermal barrier (high-speed aerodynamics). A term used to describe one of the limits to the speed at
which an aircraft can fly. The thermal barrier is the speed limit caused by heat produced by friction of the
air passing over the aircraft surfaces. At this speed, the heat is severe enough to damage the aircraft
structure.
thermal circuit breaker. A type of circuit breaker that opens an electrical circuit when excessive current
flows through it. Movable contacts are mounted on a bimetallic strip which is bent in such a way that they
are forced tightly against fixed contacts in the circuit breaker housing. When more current than the circuit
breaker is designed to carry flows through the bimetallic strip, the strip heats up and warps. When it warps,
it snaps the contacts apart and opens the circuit.
thermal coefficient of resistance. The ratio of the change in resistance of a material to a change in its
temperature. The coefficient is positive if the resistance of the material increases as its temperature goes up.
The coefficient is negative if the resistance becomes less as the material gets hotter. The thermal coefficient
of resistance is usually expressed in such units as ohms per degrees Celsius (Ω/°C).
thermal conduction. A method of heat transfer in which heat energy is transferred from one body into
another when the bodies are in direct contact.
thermal conductor. A material that transfers heat energy by conduction with very little loss. Most metals
are good thermal conductors, and such materials as paper, wood, and water are poor thermal conductors, or
are thermal insulators.
thermal cutout (electric motor component). A special circuit breaker mounted inside an electric motor.
The thermal cutout opens the power lead to the motor if the motor overheats for any cause. These causes
could be: too great a load, too low an operating speed, or lack of cooling airflow. As soon as the motor
Printed from Summit Aviation's Computerized Aviation Reference Library, 2/7/2007
Page 578
cools down, the thermal cutout automatically resets itself, and the motor runs again. Thermal cutouts are
also called automatic-reset circuit breakers.
thermal dimpling. See hot dimpling.
thermal efficiency (heat engine specification). The ratio of the amount of useful work produced by a heat
engine to the amount of work that could be done by the heat energy in the fuel used.
thermal expansion. The change in the physical size of a material caused by the material absorbing heat.
Thermal expansion causes a material to expand both in its length and in its volume.
thermal expansion coefficient. A dimensionless number that relates to the change in the physical
dimensions of a material as the temperature of the material changes. The thermal expansion coefficient of
aluminum is approximately twice that of steel.
thermally actuated switch (electrical component). A switch whose contacts are actuated by changing
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