18WuD2x

folded pattern • fork oscillator 295
screen than the preceding one. The foldedpattern
technique provides a time axis several
times longer than the screen width.
folding frequency In a system where sampling is
made at uniform frequency increments, the frequency
corresponding to half the sampling rate in
hertz.
foldover Distortion characterized by the horizontal
or vertical overlapping of a television picture.
follower A single-stage, active circuit characterized
by zero phase reversal, and voltage gain less than
unity. The emitter follower is also called a common-collector
circuit; the source follower is also
known as a common-drain circuit. Characterized
by moderate to high input impedance, and low
output impedance over a wide band of frequencies.
follower drive In a servo system, the drive that
mechanically follows the master drive.
following blacks In a television picture, the effect
in which a moving white object has a black border
following it.
following whites In a TV picture, the effect in
which a moving dark object has a white border
following it.
follow-up motor See SERVOMOTOR.
font The physical shape and size of the letters and
numbers in an alphanumeric system.
font reticle In optical character recognition, an
overlay reference pattern of lines used to check
the size and configuration of an input character,
the size of punctuation marks, and spacing between
lines and characters.
food-service robot Any robot that is used for the
purpose of packaging, preparing, and/or serving
food.
foot Abbreviation, ft. A unit of linear measure in
the English system equal to 0.3048 meter.
foot-candle Abbreviation, fc. A unit of illuminance;
1 fc is the amount of direct light emitted by 1 candela
(see CANDLEPOWER) that falls on 1 square
foot of a surface on which every point is 1 foot
away from the source. In the International System
of Units, the unit is lux (lumens per square
meter). Compare METER-CANDLE.
foot-candle meter A light meter whose scale reads
directly in foot-candles.
foot-lambert Abbreviation, fL. A unit of luminance;
the average brightness of a surface that
emits or reflects 1 lumen per square foot. The
Standard International (S.I.) unit is the candela
per square meter (cd/m 2 ); 1 fL = 3.426 cd/m 2 .
foot-pound
Abbreviation, ft-lb. In the English system,
a unit of energy equal to 1 pound displaced
through 1 foot in the direction of the exerting
force. The Standard International (S.I.) unit is the
joule (j); 1 ft-lb = 1.356 j.
foot-pound-second system See FPS SYSTEM OF
UNITS.
foot switch A switch operated by the foot, generally
used for the purpose of turning a playback or
recording system on and off. Often used for taking
dictation.
forbidden band See ENERGY GAP.
forbidden character code An error-finding code
using forbidden characters: combinations of prohibited
bits. Also called forbidden combination.
forbidden energy band See ENERGY GAP.
force 1. Symbol, F. Units: newton, dyne, poundal.
The agency or influence that accomplishes work.
2. An operator interjection made during a program
run that causes the computer to execute a
branch instruction; it is usually necessary when
a condition responsible for halting a program
must be bypassed.
forced coding Programming that minimizes the
time required to retrieve information from storage.
Also called minimum latency programming or
minimum access programming.
forced oscillations Oscillations in a circuit, such as
in an inductance-capacitance (LC) tank, that result
from continuously applied alternating-current
(ac) excitation. Compare FREE OSCILLATIONS.
foreground job A relatively high-priority, shortrunning
program that is carried out by interrupting
a low priority, long-running program.
Compare BACKGROUND JOB.
force pump In a multistage vacuum system, the
first pump that reduces the pressure considerably
below atmospheric pressure. Also see DIF-
FUSION PUMP and VACUUM PUMP.
force summing device A transducer element that
is physically moved by a force being transduced.
foreshortened addressing In control computers,
the mixing of available storage by using simplified
addressing instructions.
fork oscillator An audio-frequency oscillator controlled
by a tuning fork. The dimensions of the
fork determine its vibration frequency and, accordingly,
the frequency of the oscillator.
Tuning
fork
fork oscillator
B+
B−
AF output