18WuD2x

522 perceived level • peripheral electron
perceived level The level of a disturbance, particularly
sound, as sensed by a person. It is generally
expressed in decibels, with respect to a
certain threshold value. The threshold is assigned
an intensity of 0 dB.
percent An expression of a fraction, in terms of
hundredths. A quantity of x percent indicates a
fraction of x/100. Percent is usually abbreviated
by the symbol %.
percentage error The amount by which a measured
value differs from the true value, expressed
as a percentage (the number of parts per 100 that
the measurement is in error).
percentage-modulation meter An instrument
that provides direct readings of the modulation
percentage of an amplitude-modulated signal.
The meter scale or dial is graduated in increments
from 0 to somewhat more than 100 percent.
percentage uncertainty The maximum possible
error in a measurement, expressed as a percentage
of the measured value. Also see UNCER-
TAINTY IN MEASUREMENT.
percent distortion Symbol, %D. In the determination
of harmonic distortion, the total harmonic
voltage expressed as a percentage of the fundamental
voltage, plus total harmonic voltage; %D =
100E h /E t , where E h is the total voltage of the harmonic
components, and E t is the total signal voltage
(fundamental plus harmonics).
percent modulation See MODULATION PER-
CENTAGE.
percent modulation meter See PERCENTAGE-
MODULATION METER.
percent ripple The amount of ripple voltage in the
direct-current (dc) output of a rectifier or generator,
expressed as a percentage of the nominal dc
output voltage.
perfect crystal A crystal without defects or impurities.
The atoms are arranged in a regular pattern
with no faults.
perforated board A plastic panel provided with a
number of small holes in orderly columns and
rows for the insertion of the pigtails of components,
or of push-in terminals to facilitate quick
assembly of prototype circuits. Also called perfboard.
performance curve A curve depicting the behavior
of a component or circuit under specified conditions
of operation. Such a curve, for example,
might display the variation of output power with
input power, the variation of frequency with voltage,
etc. Compare CHARACTERISTIC CURVE.
performance test A test made primarily to ascertain
how a system behaves. The test is concerned
with normal operation, whereas a diagnostic test
is a troubleshooting procedure. Compare TROU-
BLESHOOTING TEST.
perigee 1. The point at which an earth-orbiting
satellite attains its lowest altitude. It occurs once
for every complete orbit. At this point, the satellite
travels faster than at any other point in the orbit.
2. The altitude, measured from the earth’s surface
or the earth’s center, of an earth-orbiting
satellite at its closest approach.
perihelion 1. The point at which a solar-orbiting
satellite attains its lowest altitude. It occurs once
for every complete orbit. At this point, the satellite
travels faster than at any other point in the
orbit. 2. The altitude, measured from the sun’s
surface or the sun’s center, of a solar-orbiting
satellite at its closest approach.
perimeter protection The use of a security system
to restrict or prevent access to a designated area,
using sensors and/or barriers around the boundaries
of the area.
period Symbol, T. Unit, second. The duration of a
complete alternating-current cycle or of any
cyclic event; T = 1/f, where f is the frequency
in Hertz. Also see CYCLE, FREQUENCY, and
HERTZ.
periodic and random deviation Abbreviation,
PARD. In the direct-current output of a rectifier,
the combined PERIODIC DEVIATION, including
ripple, noise, hum, and transient spikes.
periodic curve A curve that repeats its shape in
each period (e.g., a sine curve).
periodic deviation Repetitive deviation of a quantity
from its normal value (e.g., ripple in the
direct-current output of a rectifier).
periodic function A mathematical function that is
represented by a periodic curve (e.g., the sine
function y = sin x).
periodicity In a transmission line, the tendency
for power to be reflected at a point or points
where the diameter of the line changes.
periodic law The observation that when the chemical
elements (see ELEMENT, 3) are arranged in
increasing order of atomic number, their physical
and chemical properties recur periodically. Also
see PERIODIC TABLE.
periodic table A table in which the chemical elements
(see ELEMENT, 3) are arranged according
to the periodic law. The vertical columns in the
table, labeled groups, contain elements possessing
related properties (e.g., silicon and germanium
in group IV). The rows, labeled periods,
depict the periodic shift in the properties of the elements.
peripheral
1. Pertaining to equipment accessory to
a central system (e.g., peripheral input/output
devices online or offline to computers, data
recorders, and indicators). Also see ANCILLARY
EQUIPMENT. 2. Peripheral equipment in a computer
system (e.g., printers, modems, external
disk drives, tape drives, etc.).
peripheral buffer As part of a peripheral in a computer
system, a storage unit in which data
temporarily resides on its way to or from the
central processing unit. Also called INPUT/
OUTPUT BUFFER.
peripheral electron
See VALENCE ELECTRON.