航空術(shù)語詞典Dictionary of Aeronautical Terms 下

時(shí)間：2011-03-11 23:18來源：藍(lán)天飛行翻譯 作者：航空 點(diǎn)擊：次

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
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.

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