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formed on a substrate of N-material. See field effect transistor.
peak current (alternating current value). The greatest amount of current flowing in one alternation of
alternating current. Peak current can be measured with an oscilloscope by measuring the distance between
the zero-current level and the point of the greatest deviation (the greatest distance from the zero-current
line).
Printed from Summit Aviation's Computerized Aviation Reference Library, 2/7/2007
Page 430
peak inverse voltage (PIV). The highest peak value of reverse voltage a rectifying diode can withstand
before it breaks down and conducts. A semiconductor diode does not normally conduct when it is
reverse-biased (the anode negative and the cathode positive). But if too high a reverse voltage is placed
across the diode, it will break down and conduct.
peak-to-peak voltage (alternating current value). Voltage measured with a cathode-ray oscilloscope
between the peak negative value and the peak positive value.
peak voltage (alternating current value). The greatest momentary voltage in an alternating current circuit.
In sine wave AC, the peak voltage is 1.414 times the effective, or rms, value of the voltage. See effective
value of AC.
peak voltage. The AC voltage measured from zero voltage to the peak of either alternation.
pedestal grinder (machine tool). A type of electric motor-driven grinding wheel mounted on a pedestal,
rather than on a bench.
peel ply (composite materials fabrication). A layer of porous fabric used in the lay-up of a composite
material that vents the excess resin and prevents the bagging material from sticking to the part. The peel ply
is removed after the resin has cured.
peen. To flatten or round-over the end of a rivet or shaft by a series of light hammer blows around its edge.
pencil compass (drawing instrument). A drawing instrument made in the shape of a pair of dividers. One
leg has a sharp point that forms the center of a circle or arc, and the other leg holds a pencil or a piece of
pencil lead that marks the circumference of the circle or the arc.
pendulum. A weight suspended on a rod or cable from a fixed point, free to swing back and forth. The
period of a pendulum, the time required for one complete swing, is determined by its length. A pendulum is
used as the regulating device in a clock because the period of a fixed-length pendulum is always the same.
The clock spring or weights add a tiny bit of energy to the pendulum each time it swings, and by
adding as much energy as the pendulum loses because of friction, the pendulum can be kept swinging.
pendulum valves (pneumatic gyro horizon component). Gravity-operated air valves mounted on the
bottom of the gyro housing in a gyro horizon instrument. Air used to spin the gyro leaves the housing
through these valves. If the gyro should tilt, the size of the holes covered by the valves changes, and the
exhausting air produces a force that causes the gyro to erect.
penetrant dwell time. The length of time a part is left in the penetrant when preparing it for inspection by
the fluorescent or dye penetrant method. The hotter the part and the longer the penetrant dwell time, the
smaller the fault that will be detected.
penguin trainer (early training aircraft). A type of training aircraft used by the French in the early stages
of World War I. The engines in these aircraft were not powerful enough to produce flight, but the student
was able to develop enough feel of the controls to allow an effective transition to a flyable aircraft.
pentagrid converter (electron tube). A mixer tube used in a superheterodyne radio receiver. A pentagrid
converter tube has five grids, a cathode, a plate, and a filament.
The incoming radio-frequency signal is applied to the signal grid, and a signal from a local
oscillator is applied to the oscillator grid. The two signals mix and form two additional signals. The
frequency of one is the sum of the two, and the frequency of the other is the difference between the two.
The other three grids in the pentagrid tube are: two screen grids and one suppressor grid.
pentavalent element. A chemical element with five electrons in its valence shell. Nitrogen, arsenic,
antimony, and bismuth are pentavalent elements.
pentode (electron tube). A vacuum tube with five active electrodes. A pentode tube has a cathode, a
control grid, a screen grid, a suppressor grid, and a plate, or anode.
Printed from Summit Aviation's Computerized Aviation Reference Library, 2/7/2007
Page 431
percussive welding. A method of electric arc welding, used to attach studs to metal plates. A large amount
of electrical energy is stored in capacitors and released through the stud. This produces a high current
density between the stud and the plate. The high current density ionizes the air and allows an arc to form
　
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