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dim and the second will be bright.
phase sequence, or phase rotation. The sequence with which the output phases of a three-phase generator
are connected to the load. Reversing the phase sequence of a generator from A-B-C to A-C-B prevents the
generator being synchronized with the others on the bus.
phase shift. The difference in time between similar points of an output and an input wave. If the input and
the output waves pass through zero at the same time, going in the same direction, the waves are in phase.
But if the circuit shifts the phase, the output will either lead or lag the input.
phase shifter (electronic circuit). A circuit that shifts the phase of an alternating current signal. A phase
shifter causes the output signal to either lead or lag the input signal.
phenol-formaldehyde resin. One of the oldest of thermosetting plastic resins. Phenol-formaldehyde resin,
called phenolic resin, has high strength and good chemical and electrical resistance and is low in cost. It
may be reinforced with cloth or paper to make molded plastic objects.
phenolic plastic. A plastic material made of a thermosetting phenol-formaldehyde resin, reinforced with
cloth or paper. Phenolic plastic materials are used for electrical insulators and control pulleys.
Phillips screw. A type of recessed-head screw designed to be driven with a power screwdriver. The recess
in the head of a Phillips screw is in the form of a cross, with the sides of the recess tapered and the bottom
of the recess nearly flat.
A Reed and Prince screw is similar to a Phillips screw, but the edges of the recess in a Reed and
Prince screw are straight, rather than tapered, and the bottom of the recess is sharp. See Reed and Prince
screw.
Printed from Summit Aviation's Computerized Aviation Reference Library, 2/7/2007
Page 434
phonetic alphabet. A list of standard words used for each letter in the alphabet. The use of a standard
phonetic alphabet assures that the letters are not misunderstood during radio transmission. See International
phonetic alphabet.
phosgene gas. A highly toxic, colorless gas, with an unpleasant odor. Phosgene is formed when certain
halogen compounds, such as Freon or carbon tetrachloride, are passed through a flame.
phosphate-ester hydraulic fluid. A synthetic hydraulic fluid used in many high-performance aircraft
hydraulic systems. Phosphate-ester hydraulic fluid is fire-resistant and is used in place of petroleum-base
hydraulic fluid. It is identified by the military specifications number MIL-H-8446.
phosphate film (corrosion protection). A dense, airtight film formed on the surface of aluminum or
magnesium alloy to prevent oxygen or electrolyte reaching the surface and causing oxidation or corrosion.
phosphorescent paint. Paint that absorbs energy from natural or ultraviolet light and radiates visible light.
Phosphorescent paint continues to glow for some time after the excitation is removed, and for this reason it
is used on dials and hands of clocks, watches, and aircraft instruments so they can be seen in the dark.
Phosphorescent paint has replaced radium paint for instrument marking because of the poisonous nature of
radium.
phosphorus. A highly reactive, poisonous, nonmetallic chemical element. The symbol for phosphorus is P,
its atomic number is 15, and its atomic weight is 30.98. Phosphorus is used in the manufacture of matches,
explosives, and fertilizer.
photocathode. An electrode in an electron tube that releases electrons when exposed to light.
Photocathodes are used in television cameras to convert light energy into electrical energy.
photochemistry. The branch of chemistry that deals with the effects of light on chemical reactions.
photoconductive cell. A photoelectric device that changes its resistance as the amount of light striking it
changes. When the cell is in total darkness, its resistance is maximum, and as light falls on it, its resistance
decreases.
Photoconductive cells using either cadmium sulfide or cadmium selenide as the active elements
are called light-dependent resistors (LDRs), and one of their uses is in photographic light meters.
photodiode. A semiconductor diode whose conduction characteristics change when its PN junction is
exposed to light. A photodiode in total darkness acts as an open circuit; no current can flow.
photoelectric characteristics. The characteristics of a material that change when the material is exposed to
light.
Printed from Summit Aviation's Computerized Aviation Reference Library, 2/7/2007
Page 435
Some materials (photovoltaic materials) produce a voltage when exposed to light, and others
(photoconductive materials) change their resistance when exposed to light.
photoelectricity. Electricity produced by light. When light strikes certain types of semiconductor materials
　
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