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thumbscrew. A type of machine screw with a flat head in line with the length of the screw. A thumbscrew
can be turned by hand by gripping the flat head between the thumb and first finger. Thumbscrews are used
to lock one part to another when the parts must be locked and released without the use of tools.
thunder (meteorology). The loud rumbling sound caused by lightning. When lightning (a large spark)
jumps from one cloud to another or from a cloud to ground, a great amount of heat is produced. This heat
causes the air to expand violently, and this expansion causes shock waves that travel outward at the speed
of sound. These shock waves cause the noise we know as thunder.
thundercloud (meteorology). The commonly used name for a cumulonimbus cloud. Violent up and down
air currents inside the cloud produce static electricity that discharges in the form of lightning. Lightning
causes the thunder heard inside thunderclouds.
thunderstorm (meteorology). A local storm invariably produced by a cumulonimbus cloud and always
accompanied by lightning and thunder.
thyratron (electronic component). A hot-cathode, gas-filled electron tube in which one or more control
electrodes start a flow of electrons between the cathode and anode. A pulse of high voltage on the control
grid ionizes the gas inside the tube and allows electrons to flow.
The grid loses its control as soon as the electrons begin to flow, and the only way to stop the flow
is to reduce the anode voltage to a value that no longer attracts the electrons.
thyrector (electronic component). A special type of semiconductor device used to protect a piece of
electronic equipment from being damaged by spikes of voltage. A thyrector contains two zener diodes,
connected together back to back. When the circuit voltage is below that for which the diodes are rated, no
current flows through them. But if a spike of voltage occurs in the circuit, one of the diodes breaks down
and conducts the spike to ground. Since the zener diodes are connected back to back, the thyrector protects
against spikes of voltage of either polarity.
Printed from Summit Aviation's Computerized Aviation Reference Library, 2/7/2007
Page 585
thyristor (electronic component). A semiconductor device used as an electrically controlled switch. Two
of the most widely used thyristors are silicon controlled rectifiers (SCRs) and triacs. An SCR is a
semiconductor device that does not normally allow electrons to flow through it in either direction. However,
if a pulse of voltage of the correct polarity is applied to a third electrode, the gate, the SCR conducts in its
forward direction.
The SCR continues to conduct until the voltage across it drops to zero. Once electrons stop
flowing through it, the SCR remains shut off until another pulse of voltage is applied to its gate. The triac
begins to conduct when a pulse of voltage of either polarity is applied to its gate. Once conducting, it can
be turned off only by reducing the voltage across it to a value low enough to stop the current flowing
through it.
tickler coil (electronic component). A small coil in series with the collector of a transistor in an oscillator
circuit. The tickler coil is part of a feedback circuit. Current flowing through the tickler coil induces a
voltage in an inductor in the base circuit.
tie rod (structural member). A high-strength steel rod used to tie parts of a structure together. Tie rods
carry only tensile loads.
tight-drive fit. An interference fit between mechanical parts in which the parts can be assembled only by
driving them together with a sharp blow from a 12- to 14-ounce hammer.
TIG (tungsten inert gas) welding. A form of electric arc welding in which the electrode is a
nonconsumable tungsten wire. TIG welding is now called GTA (gas tungsten arc) welding.
time constant (capacitive circuit value). The amount of time, measured in seconds, needed for the voltage
across a capacitor to rise to 63.2% of the voltage applied to the circuit. Time constant is determined by the
amount of both resistance and capacitance in the circuit.
time constant (inductive circuit value). The amount of time, measured in seconds, needed for the current
flowing through an inductive circuit to reach 63.2% of its final value. Time constant is determined by the
amount of both resistance and inductance in the circuit.
time constant of an electric motor. The amount of time, measured in seconds, needed for an electric
motor to accelerate from standstill to its final no-load speed. Time constant is measured when the motor is
connected to a power source having the correct voltage, phase, and frequency.
time-delay relay (electrical circuit component). A slow-acting electrical relay in which there is a
measurable delay between the time current is applied to the relay coil and the time the relay contacts close.
　
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