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number is 90, and its atomic weight is 232. Thorium is slightly radioactive and is used as an alloy with
tungsten to make filaments for incandescent electric lights. It is also a potential source of nuclear energy.
thread (machine fastener component). Helical grooves cut onto the surface of a bolt or screw. Mating
threads are cut into the inside of a nut. The threads inside the nut match the threads on the outside of the
bolt, and the nut is turned down over the threads of the bolt to produce a clamping action that holds parts of
a machine together. A threaded bolt is a form of inclined plane.
thread (textiles). Long filaments made by twisting together short lengths of natural fibers, such as cotton
or linen, to form a continuous strand. Threads of synthetic materials are made of a single continuous
filament. These are called monofilament threads.
thread pitch (threaded fastener specification). The linear distance from the apex (peak) of one thread to
the similar position on the next thread on a threaded fastener.
three-axis automatic pilot. An automatic flight control system for an aircraft that controls the movement
of the aircraft about all three of its axes.
three-dimensional cam. A drum-shaped cam in a hydromechanical fuel control whose outer surface is
precision ground in such a way that followers riding over it as it moves up and down and rotates move
mechanical linkages to control the fuel according to a preprogrammed schedule.
three-dimensional object. An object that has length, width, and depth. A cube, for example, is a
three-dimensional object. A flat, or plane, surface on the other hand, has only length and width and is
two-dimensional.
three-phase alternating current. The form of alternating current produced by a generator with three sets
of windings. The three windings are arranged in such a way that the current and voltage in each winding is
120 electrical degrees out of phase with the current and voltage in either of the other two windings.
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The three windings can be connected in either a delta or a Y-arrangement. In a delta connection,
the three windings are connected in series. The voltage across any one of the three windings is the same as
that across the other windings. The total current is 1.73 times the current through any of the individual
windings.
In a Y-connected three-phase circuit, one end of each of the windings is connected to a common
neutral point. The voltage across any two windings is 1.73 times the voltage between any single winding
and the neutral point. The current through any two windings is the same as the current through any single
winding.
three-point landing (aircraft operation). A full-stall landing of an airplane equipped with a tail wheel. In
a three-point landing, the main wheels and the tail wheel touch the ground at the same time. A three-point
landing with a tail wheel airplane is desirable, but landing an airplane with a nose wheel so the main wheels
and the nose wheel touch the ground at the same time is not normally considered a good operating
procedure.
three-pole, single-throw switch. An electrical switch that has three sets of contacts, controlled by a single
operating handle. A three-pole, single-throw switch can select two conditions, such as on and off, in three
separate circuits.
three-state buffer. A digital logic device used as a switch in an integrated circuit chip. When there is a
logic low on the control line, the device acts as a closed switch with the in and out lines connected, a low
on the in line produces a low on the out line, and a high on the in line produces a high on the out line. When
there is a high on the control line, the device acts as an open switch, placing a high impedance between the
in and the out lines.
three-view drawing. An orthographic drawing showing three views of a part. There are six possible views,
but the three most commonly used for aircraft drawings are the front, right-side, and top views.
three-way light switch. A type of switch that allows an electrical light to be turned on or off at either of
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two locations. Three-way light switches are used at the top and the bottom of stairs and at either end of a
long hall.
threshold of pain (intensity of sound). The intensity at which sound begins to cause pain. A sound
intensity of 150 decibels which is often produced near an operating jet engine is normally considered to be
the threshold of pain.
throatless shears (sheet metal tool). A floor- or bench-mounted metalworking tool used to cut across
large sheets of thin metal. Short cutting blades, operated by a lever, cut across the sheet in much the same
　
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