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between the stud and the plate. The arc melts the end of the stud and the area of the surface of the plate
where the stud is to be attached. The stud is pressed into the molten metal to complete the weld.
perfect dielectric. A dielectric (insulator) that, when an electric field is removed, returns to the circuit all
the energy used to establish the field. A perfect dielectric has absolutely no conductivity — its resistance is
infinite. The only known perfect dielectric is a vacuum.
perforate. To punch a series of holes in a material. Paper is often perforated with a series of tiny holes in a
straight line, so it will tear evenly along the perforations.
performance number (reciprocating engine fuel specification). The antidetonation rating of a fuel which
has a higher critical pressure and temperature than iso-octane (a rating of 100). Iso-octane which has been
treated with varying amounts of tetraethyl lead is used as the reference. Aviation gasoline antidetonation
ratings above 100 are called performance numbers.
period (oscillation). The length of time needed for one complete cycle of oscillation to take place.
periodic. Repeated at regular time intervals. Any event repeated regularly, with the same amount of time
between the events, is said to be a periodic event.
periodic inspection (aircraft inspection). A maintenance inspection that must be performed on a regular
or recurring basis. Annual and 100-hour inspections are both periodic inspections.
periodic table of chemical elements. A table listing all the chemical elements, arranged in horizontal rows
and vertical columns to show the similarities of the properties of the elements. See Appendix 2.
periodic vibration. A vibration having a regularly recurring waveform. Vibrations in the form of sine
waves, square waves, or sawtooth waves are examples of periodic vibrations.
peripheral equipment (computer equipment). Components in a computer system that may be attached to
the computer. Peripherals consist of display units, storage devices, and such input/output devices as
keyboards and printers.
Permalloy. The registered trade name of an alloy of iron and nickel. Permalloy has a high magnetic
permeability and is used in the manufacture of permanent magnets.
permanent ballast (aircraft balance). A weight permanently installed in an aircraft to bring its center of
gravity into allowable limits.
permanent echo (radar return). Radar signals reflected from fixed objects on the earth’s surface; objects
such as buildings, towers, and terrain that under certain conditions may be used to check radar alignment.
Permanent echoes are distinguished from ground clutter by being definable locations rather than large
areas.
permanent magnet. A piece of magnetizable material, such as hard steel, that has been exposed to a strong
magnetizing force. This force aligns the spin axes of the electrons surrounding the atoms of the material,
and thus the magnetic domains within it. When the material is removed from the magnetic field, it is
magnetized and the domains remain in alignment until a strong demagnetizing force is applied.
permanent magnet speaker. The type of speaker used in small radios and tape players. The magnetic field
in which the voice coil moves is produced by a small permanent magnet.
permanent mold. A reusable metal mold, used for making complex castings. Castings made in permanent
molds can be made thinner than those made by sand casting.
permanent-mold casting. A casting made in a reusable metal mold. Many aircraft engine crankcases are
Printed from Summit Aviation's Computerized Aviation Reference Library, 2/7/2007
Page 432
cast in permanent-molds because their walls can be made thinner than similar walls made by sand casting.
permanent set (mechanical deformation). A permanent deformation in a material that has been strained
beyond its elastic limit. According to Hooke’s law, a material deforms, or strains, in proportion to the
amount of stress put on it, and when the stress is removed, the material will return to its original shape and
size. This is true until the elastic limit of the material is reached. If the elastic limit of the material is
exceeded, the material will not return to its original condition; rather it will take a permanent set.
permeability (μ). A measure of the ease with which lines of magnetic flux can pass through a material.
The permeability of a material is found by dividing the flux density (B), in lines per square inch, by the
magnetizing force (H), in ampere-turns per inch, needed to produce this flux density.
Air is used as the reference, and it is assigned a permeability of one. Pure iron has a permeability
of about 200,000.
persistence (cathode-ray oscilloscope characteristic). A measure of the length of time the phosphor dot
　
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