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connected to the earth ground, and the socket for the common wire (the white wire) is wider than the socket
for the “hot” wire (the black wire).
pole shoes (electrical machine component). Inward extensions from the field frame of an electric motor
or generator around which the field coils are mounted. The pole shoes are fastened to the field frame to
form a very low reluctance path for the lines of magnetic flux. The pole shoes are shaped in such a way that
the density of the flux across the air gap between the pole shoe and the armature is uniform.
poles of a magnet. The locations on a magnet where the lines of magnetic flux leave and where they enter.
In a ferromagnetic metal, groups of atoms whose electrons have their spin axes in alignment are called
magnetic domains. When all of the domains are aligned in the same direction, the material is magnetized.
All magnets, regardless of their size, have two poles, a north pole and a south pole. Lines of
magnetic flux leave the magnet at its north pole and return through its south pole. All the lines of flux leave
and enter the poles at right angles to the surface.
polonium. A naturally radioactive, metallic chemical element produced by bombarding bismuth or lead
with neutrons. Polonium’s symbol is Po, its atomic number is 84, and the mass number of its most stable
isotope is 209.
polyconic projection (map projection). A projection used for laying out the grid of latitude and longitude
for a chart or map. In a polyconic projection, the parallels of latitude are represented by arcs of a circle
whose center is the north or south pole. The meridians of longitude are straight lines radiating from the
poles.
polyester fibers. A synthetic fiber made by the polymerization process, in which tiny molecules are united
to form a long chain of molecules. Polyester fibers are woven into fabrics that are known by their trade
names of Dacron, Fortrel, and Kodel. Polyester film and sheet are known as Mylar and Celenar.
Printed from Summit Aviation's Computerized Aviation Reference Library, 2/7/2007
Page 448
Cloth woven of polyester fibers for use as an aircraft covering material is sold under such trade
names as Poly Fiber, Eonnex, and Ceconite. This cloth is shrunk on the structure with heat, rather than with
shrinking dope.
polyester resin (reinforced plastic component). A thermosetting resin serving as the matrix for much of
the fiberglass cloth and mat used to make parts for aircraft composite construction.
polyethylene plastic material. A lightweight, chemical-resistant, thermoplastic material. Polyethylene
resins are used for making sheets of protective covering material and containers for liquids.
polymerization. A chemical reaction in which several small molecules join together to form larger
molecules. The larger molecules are made up of repeating structural units of the original molecules.
Polymerization often causes liquid materials to change into a jelly.
polymer paint. A water-based paint that contains vinyl or acrylic resins. When the water evaporates, it
leaves a waterproof film of the plastic resin.
polyphase alternating current. Alternating current electricity produced by more than one set of generator
windings. The most commonly used polyphase electricity has three phases.
polyphase electric motor. An induction motor designed to operate on two-phase or three-phase alternating
current.
polyurethane enamel. A two-part, chemically cured, finishing material that contains a high percentage of
solids. Polyurethane enamel requires a long time to flow out, and this long flowing-out time gives the finish
a “wet” look. Polyurethane enamel is highly resistant to mechanical abrasion and chemical action.
polyvinyl chloride (thermoplastic resin). A thermoplastic resin used to make transparent tubing for
insulating electrical wires. Polyvinyl chloride (PVC) tubing is used for carrying low-pressure air to aircraft
instruments.
pontoon (aircraft landing gear component). A float that can be attached to a land airplane to allow it to
operate from water. Two pontoons are often installed on a land plane to convert it into a float plane.
poppet valve (reciprocating engine component). A T-shaped valve with a circular head. Poppet valves
are used to cover the intake and exhaust openings in the cylinder head of a reciprocating engine. The valves
are held closed with one or more coil springs and are opened by a cam lobe or rocker arm pushing on the
end of their stem.
porcelain. A high-grade, translucent ceramic material having a hard, glazed finish. Porcelain is used for
dishes, vases, and high-strength electrical insulators. It was used as the insulator in some early aircraft
spark plugs.
porosity. The condition of a material that has many tiny openings, or pores, in its surface. These pores
　
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