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temperature. The contacts are normally mounted on a bimetallic strip that warps when its temperature
changes. When the strip warps, the contacts either snap closed or snap open, depending on whether the
switch is actuated by a descending or a rising temperature.
thermal relief valve (hydraulic system component). A pressure relief valve in a hydraulic system that
relieves the pressure which builds up when heat expands the fluid. Thermal relief valves are installed in
sections of the system in which fluid can be trapped, usually in the lines between the selector valves and the
actuators. When fluid in these lines absorbs heat and expands, the thermal relief valve momentarily unseats
and releases enough fluid back into the system return manifold to relieve the high pressure and prevent
damage to the system.
thermal runaway (nickel-cadmium battery malfunction). A condition that can occur in a
nickel-cadmium battery in which the battery overheats because of excess current. A nickel-cadmium
battery has a very low internal resistance, but if an excessive amount of current flows from it, even its low
resistance will cause it to overheat. The center cells get hotter than the outer cells, which transfer some of
their heat to the outside air, and when the center cells overheat, their voltage and resistance both drop.
When the generator puts current back into the battery, the center cells take the most current and
get even hotter. As they get hotter, their voltage and resistance continue to drop, and they take more current.
This current increase continues until the battery is destroyed.
thermal shock (engine operating condition). The rapid change in engine operating temperature that
occurs when engine power is suddenly reduced at the same time the airspeed, thus the cooling, is increased.
Thermal shock occurs when an aircraft is required to rapidly descend to a lower altitude.
thermionic current. The flow of electrons in a vacuum tube from a heated cathode to a positively charged
anode, or plate. When the cathode is heated, electrons gain so much energy that their orbits increase in size
enough for them to be influenced by the positive voltage on the plate. They leave the cathode and flow to
the plate.
thermistor. A semiconductor device whose electrical resistance varies with its temperature. The resistance
of a thermistor material decreases as its temperature increases. Thermistor materials are used in fire
detection systems. When the thermistor is cold, its resistance is high, but when it is heated in a fire, its
Printed from Summit Aviation's Computerized Aviation Reference Library, 2/7/2007
Page 579
resistance drops enough to operate the fire warning system.
thermistor material. A material with a negative temperature coefficient that causes its resistance to
decrease as its temperature increases.
thermoammeter. An ammeter used to measure the radio-frequency (RF) alternating current flowing in a
circuit. The RF current flows through a short piece of resistance wire and heats it. The amount the wire is
heated is proportional to the square of the current flowing through it.
A thermocouple is welded to the center of the resistance wire, and a DC ammeter measures the
current produced by the thermocouple. This current is proportional to the temperature of the resistance wire.
The dial of the thermocouple instrument is calibrated in amps of RF alternating current.
thermocouple. A device used to generate a small electrical current. A thermocouple is made up of two
dissimilar metal wires, such as iron and constantan, copper and constantan, or chromel and alumel. The
ends of these wires are joined to form a loop. Electrical current flows through the wires when there is a
temperature difference between the junctions, the points at which the wires join. The amount of current
flowing in the loop is determined by three things:
1. The types of metals used for the wires
2. The temperature difference between the two junctions
3. The resistance of the thermocouple wires
Iron and constantan or copper and constantan thermocouples are used to measure the cylinder
head temperature of reciprocating engines, and chromel and alumel thermocouples are used to measure the
exhaust gas temperature in both reciprocating and gas turbine engines.
thermocouple fire detection system. A fire detection system that works on the principle of the
rate-of-temperature-rise. Thermocouples are connected in series and installed in various locations in the
area to be protected. One thermocouple is surrounded by insulation that prevents its temperature changing
rapidly.
In the event of a fire, the temperature of all the thermocouples except the protected one will rise
immediately and a fire warning will be initiated. In the case of a general overheat condition, the
　
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