18WuD2x

558 promethium cell • protective gap
to glow. Self-generating photocells then convert
this light into electricity.
PROM programmer An electronic device that can
store a computer program in a PROGRAM-
MABLE READ-ONLY MEMORY (PROM). It uses a
built-in keyboard.
prompt In computer operations, a message received
by an operator from an operating system
or an individual program. For example, in disk
operating system (DOS), it could be the statement
“Bad command or file name.”
prompting In computer or programmed-calculator
operations, the entry of a special, required variable
when the machine halts and awaits such entry.
prong See PIN.
prony brake An arrangement for measuring the
mechanical power output of a rotating machine.
It is a special form of friction brake consisting of
a band passed around a pulley on the rotating
shaft of the machine under test and held at each
end by a spring balance.
propagation 1. The extension of energy into and
through space. Thus, radiant energy is propagated
from and by its source. 2. A phenomenon
resulting from the extension of energy into and
through space. Thus, radio waves can be spoken
of as a propagation.
propagation constant For waves transmitted
along a line, a number showing the effect the line
has on the wave. This is a complex figure [i.e., one
containing a real-number component (the attenuation
constant) and an imaginary-number component
(the phase constant)].
propagation delay 1. Symbol, t pd . In an integrated-circuit
logic gate, the time taken for a logic
signal to be propagated across the gate. 2. In digital-circuit
operation, the time required for a
logic-level change to be transmitted through one
or more elements.
propagation delay-power product See DELAY-
POWER PRODUCT.
propagation factor The ratio E/E o , where E is the
complex electric-field strength at a point to which
a wave has been propagated, and E o is the complex
electric-field strength at the point of origin.
Also called propagation ratio.
propagation loss The path loss of an electromagnetic
disturbance between the transmitting and
receiving antennas.
propagation mode See WAVEGUIDE MODE.
propagation ratio See PROPAGATION FACTOR.
propagation time In digital-circuit operation, the
time required for a binary bit to be transferred
from one point to another in the system.
propagation velocity See VELOCITY OF PROPA-
GATION.
proportional action An action, such as amplification
or conversion, that produces an output
signal proportional to the input signal.
proportional amplifier An amplifier in which the
instantaneous output amplitude is proportional
to the instantaneous input amplitude.
proportional control A voltage-regulation system
in which the feedback correction voltage is proportional
to the output-voltage error.
proportional counter A Geiger tube having a
pointed-wire (or ball-tipped-wire) anode. The voltage
developed across the load resistor is proportional
to the number of ions created by the
radioactive particles entering the tube.
proprioceptor A set of transducers and associated
circuitry that allows a computerized robot to constantly
sense the positions of its end effectors, and
use this data in carrying out programmed tasks.
prosodic features Variations in voice tone and emphasis
that lend meaning and implication to
spoken statements. It is important in advanced
computer speech recognition and speech synthesis
systems. Two sentences with identical wording
can have greatly different meanings, depending on
these factors (e.g., “You are!” versus “You are?”).
prosthesis An electromechanical artificial human
limb or body part. Examples: artificial legs, artificial
hands, and artificial respirators. Some such
devices are computer-controlled; others can be
manipulated by nerve impulses.
protactinium Symbol, Pa. A relatively short-lived
radioactive metallic element. Atomic number, 91.
Atomic weight, 231.04 Formerly called protoactinium.
protected area A region to which access is restricted,
and that is secured by an alarm system,
surveillance cameras, or other intrusion-prevention
systems.
protected location In computer storage, a location
whose contents are protected from mutilation
or erasure by making the location usable
only by following a special procedure (e.g., using
a password).
protection In a multiple processing computer system,
preventing interference between data or programs.
protective bias In the final power amplifier of a radio
transmitter, external direct-current bias applied
to the base, gate, or grid. Prevents runaway
in collector, drain, or plate current when the bias
caused by the driving signal is lost.
protective capacitor A power-line bypass capacitor.
protective device 1. A component that breaks a
circuit in the event of excessive voltage or current
from the power supply. 2. A device that prevents
excessive power from being delivered to a load by
a driving circuit.
protective gap 1. A spark gap connected in parallel
with a component, or between a line and
ground as protection against high-voltage transients
and surges. 2. A spark-gap-type lightning
arrester.