. 3
( 42)


earth™s surface or the earth™s center, of an earth- usefulness.
orbiting satellite at its most distant point. application factor A factor involved in determin-
A power supply A term sometimes used to denote ing the failure rate of a circuit or system affected
the unit that supplies energy to a vacuum-tube by unusual operating conditions.
filament. Compare B POWER SUPPLY. application schematic diagram A diagram of pic-
apparent bearing In radio-direction finding, the torial symbols and lines that illustrate the inter-
uncorrected direction from which a signal ap- relationship of functional circuit blocks in a
pears to arrive. specific program mode.
apparent power In an ac circuit, the power value applicators 1. In dielectric heating, the electrodes
obtained by multiplying the current by voltage (P between which the dielectric body is placed and
equals IE), with no consideration of the effects of the electrostatic field developed. 2. In medical
phase angle. Compare TRUE POWER. electronics, the electrodes applied to a patient
apparent power loss The loss in an ammeter or undergoing diathermy or ultrasonic therapy.
voltmeter, caused by the imperfection of the in- applied voltage The voltage presented to a circuit
strument. At full scale, the ammeter has a certain point or system input, as opposed to the voltage
voltage across its terminals; the apparent power drop resulting from current flow through an ele-
loss is the current multiplied by this voltage. A ment.
voltmeter carries a small current; the apparent applique circuit A circuit for adapting equipment
power loss is the product of the current and the to a specialized job.
indicated voltage. approach-control radar A radar installation serv-
appearance potential The potential through ing a ground-controlled approach (GCA) system.
which an electron must move to produce a cer- approximate data 1. Data obtained through phys-
tain ion from the atom with which it is associated. ical measurements. Such data can never be ex-
applause meter An instrument consisting essen- act; all measurements are subject to error. 2.
tially of a microphone, audio amplifier, and indi- Loosely estimated data or imprecise calculations.
cating meter (reading directly in sound level). It is AQL Abbreviation of ACCEPTABLE QUALITY
so called because of its familiar use in measuring LEVEL. A statistically defined quality level, de-
audience response, as indicated by loudness of fined in terms of percent defective, accepted on
applause. an average of 95 percent of the time.
Aquadag A tradename for a material that consists
of a slurry of fine particles of graphite. Aquadag
forms a conductive coating on the inside and out-
side walls of some cathode-ray tubes.
aqua pura Pure water; in most instances, distilled
water. Formula, H2O. Pure water is a nonconduc-
tor with a dielectric constant of about 81.
Ar Symbol for ARGON.
arbitrary function fitter A circuit or device, such
as a potentiometer, curve changer, or analog
computer element, providing an output current
or voltage that is some preselected function of the
input current or voltage.
arc 1. A luminous sustained discharge between
two electrodes. Because it is sustained, rather
than intermittent, an arc is distinguished from a
spark discharge, the latter being a series of dis-
charges (sparks)”even when it appears continu-
ous. 2. In graphical presentations, a section of
Applegate diagram For a velocity-modulated tube,
curved line, as of a circle.
a plot of the positions of electron bunches in the
arc angle The angle in degrees traced out by a cir-
drift space versus time.
cular arc if the center point of the circle is con-
Appleton layer Collectively, the F1 and F2 layers
sidered to be the vertex of an angle formed by
of the ionosphere, at a height between 150 and
two rays intersecting the arc at designated
400 kilometers above the surface of the earth.
apple tube A color picture tube, used in television,
arc cosecant Abbreviated arc csc or csc“1. 1. The
with the red, blue, and green phosphor in vertical
inverse of the cosecant function. 2. The angle, in
38 arc cosecant • area code

radians or degrees, corresponding to a given
arc cosine Abbreviated arc cos or cos“1. 1. The in- arc length The length along a given arc, usually a
part of the circumference of a circle. If the circle
verse of the cosine function. 2. The angle, in radi-
has circumference C and the arc measures x de-
ans or degrees, corresponding to a given cosine.
arc cotangent Abbreviated arc cot or cot“1. 1. The grees, then the arc length is Cx/360 units.
arc minute See MINUTE.
inverse of the cotangent function. 2. The angle, in
arc oscillation Oscillations that can occur when
radians or degrees, corresponding to a given
opening relay contacts arc.
arcover The occurrence of an electrical ARC be-
arc failure 1. Damage to, and/or failure of, insula-
tween electrodes, contacts, or capacitive plates.
tion or a dielectric as a result of ARCOVER. 2.
arcover voltage The voltage at which disruptive
Failure of make-and-break contacts through
discharge occurs, typically accompanied by an
damage caused by arcover.
arc function An inverse trigonometric function. See
arc resistance The ability of a material, usually a
dielectric, to resist damage from arcing. This
property is commonly expressed as the length of
time between the start of the arc and the estab-
arc furnace A high-temperature electric furnace in
lishment of a conductive path through the mate-
which heat is produced by one or more electrical
arc secant Abbreviated arc sec or sec“1. 1. The in-
architecture The functional design elements of a
verse of the secant function. 2. The angle, in ra-
computer”especially the components of the cen-
dians or degrees, that corresponds to a given
tral processing unit (CPU) and the manner in
which these elements interact.
arc second See SECOND.
archived file A computer file stored on some
arc sensor A device for detecting visible arcs and
backup medium, such as magnetic tape, disk-
excessive reflected power in microwave systems.
ette, or CD-ROM (compact disk, read-only mem-
arc sine Abbreviated arc sin or sin“1. 1. The inverse
ory), rather than being held on the hard disk.
of the sine function. 2. The angle, in radians or
Such a file will be apart from the operating sys-
degrees, that corresponds to a given sine.
tem™s catalog of current files, but can be reconsti-
arc suppression Extinguishing an arc discharge.
tuted as needed.
Disruptive arcs in electronic circuits are sup-
archives A complete, periodically updated set of
pressed by means of auxiliary diodes or resistor-
capacitor networks.
arcing See ARCOVER.
arc-suppressor diode A semiconductor diode used
arcing contacts Make-and-break contacts be-
to prevent arcing between make-and-break con-
tween which an arc occurs when they are sepa-
arc tangent Abbreviated arc tan or tan“1. 1. The
arcing ring A metal ring placed around an insula-
inverse of the tangent function. 2. The angle, in
tor in a high-voltage electrical system. This keeps
radians or degrees, corresponding to a given tan-
an arc from charring or breaking the insulator.
arcing time The elapsed time between the break-
arcthrough The puncturing of a material by an
ing of contacts and the end of the arc between the
area code In the United States, a three-digit
arc lamp An electric lamp in which a brilliant arc
number that indicates the location, according to
jumps between the tips of two rods (originally car-
specified assigned districts, of a telephone sub-
area code • arithmetic symmetry

scriber. When making a long-distance call, the argon Symbol, Ar. An inert gaseous element.
area code of the desired station must be given in Atomic number, 18. Atomic weight, 39.94. Argon,
addition to the seven-digit telephone number. present in small amounts in the earth™s atmo-
area protection Coverage of a defined region, in sphere, is used in various specialized devices,
terms of area or volume, by an alarm system. such as lasers.
area redistribution A scheme to determine the ef- argon laser A laser whose tube is filled with argon
fective duration of an irregularly shaped pulse. A gas. It generates coherent light at specific wave-
rectangle is constructed whose height is equal to lengths that are characteristic of elemental ar-
the peak height of the pulse, as displayed on an gon.
oscilloscope. The rectangle width is adjusted un- argument 1. The direction angle of a polar vector.
til the area of the rectangle is the same as the 2. An independent variable whose value deter-
area under the curve representing the pulse. The mines the value of a function.
width of the rectangle then represents the effec- arithmetic address An address obtained by per-
tive duration of the pulse. forming an arithmetic operation on another ad-
area search The scanning of a large group of com- dress.
puter records for those of a major category or arithmetic and logic unit Abbreviation, ALU. The
class. part of a digital computer containing the circuits
area sensor A transducer, used with an alarm sys- that perform calculations and logic operations;
tem, that protects a defined region or volume, distinguished from mass storage, input/output,
such as an office or bedroom. and peripheral units.
Argand diagram Named after Jean Robert Argand, arithmetic circuit Also called arithmetic element.
(1768-1822) of Geneva, for his work on the In a digital computer, a circuit that is involved in
graphical representation of complex numbers. A the execution of calculations. Included are
graphical illustration of a complex number in the adders, storage registers, accumulators, sub-
form A + jB, where the real-number (A) axis is tracters, and multipliers.
perpendicular to the imaginary-number ( jB) axis. arithmetic mean The average of a group of quan-
The value j is the square root of -1, the unit imag- tities, obtained by dividing their sum by the num-
inary number. The axes are perpendicular, usu- ber of quantities.
ally with the A axis horizontal. The length of the arithmetic operation In digital computer prac-
line from the point (0,0) to the point (A,jB) is the tice, a numerical process performed: addition,
amplitude of the vector X = A + jB. The direction subtraction, multiplication, division, compari-
is specified as the angle, in degrees or radians, of son.
the vector measured counterclockwise from the A arithmetic progression A mathematical series in
axis. which each term following the first is obtained by
adding a constant quantity to the preceding one.
For example, S = 1, 2, 3, 4, . . . n. Compare GEO-
arithmetic shift In a digital computer, the multi-
plication or division of a quantity by a power of
the base used in the notation.
arithmetic sum The sum of two or more quantities
disregarding their signs. Compare ALGEBRAIC
arithmetic symmetry A filter response that is ex-
actly symmetrical about the center frequency
when the frequency scale is linear.
40 arm • arsenic

arm 1. Any of the distinct branches of a circuit or
network. Also called leg. 2. A movable element in a
device, usually containing a contact for switching.
armature 1. The rotating member of a motor. 2.
The rotating member of some types of electro-me-
chanical generator. 3. The movable member of a
relay, bell, buzzer, or gong. 4. The movable mem-
ber of an actuator. 5. The soft-iron keeper placed
across the poles of a permanent magnet to con-
serve power.
armature coil A coil of insulated wire wound on a
ferromagnetic core to provide the electromagnetic
properties of an armature. In a motor or genera-
tor, the armature coil is distinguished from the
armature core The ferromagnetic core upon which
the armature coil of a motor or generator is
armature gap 1. In a motor or generator, the space
between an armature core and the pole of a field
magnet. 2. In a relay, the space between the ar-
mature and the relay-coil core.
armature hesitation A momentary delay in the
movement of a relay.
armature-hesitation contact chatter Undesired
(usually rapid, repetitive) making and breaking of
relay contacts. Generally caused by armature
armature-impact contact chatter Undesired ARPA Acronym for Advanced Research Projects
(usually rapid, repetitive) making and breaking of Agency, a subsidiary of the U.S. Department of
relay contacts, caused by contact bounce when Defense.
the armature strikes the relay core (closure) or array 1. A directive antenna that consists of an as-
backstop (opening). sembly of properly dimensioned and spaced ele-
armature relay A relay that uses an electromagnet ments, such as radiators, directors, and
to pull a lever toward or away from a set of fixed reflectors. 2. A coordinated group or matrix of
contacts. components, such as diodes, resistors, memory
armature travel The distance traveled by an arma- cells, etc., often enclosed in one capsule. 3. Sub-
ture during relay operation. scripted variables representing data arranged so
armor A protective metal cable covering. that a program can examine the array and extract
Armstrong FM system (Edwin H. Armstrong, data relevant to a particular subscript.
1890 “1954). A phase-shift method of frequency array device A group of similar or identical compo-
modulation. See PHASE MODULATION. nents that are connected together in a certain
armature voltage control A means of controlling fashion, to perform a specific task.
motor speed by changing the applied armature arrester 1. A device used to protect an installation
winding voltage. from lightning. It consists of a varistor or an air
armchair copy An amateur radio term for recep- gap connected between an antenna or power line
tion of exceptionally clear signals. and an earth ground. The device passes little or
arming the oscilloscope sweep Enabling an oscil- no current under ordinary conditions, but passes
loscope to trigger on the next pulse by closing a heavy current to ground during a lightning
switch. stroke. Also called LIGHTNING ARRESTER. 2. A
Armstrong oscillator (Edwin H. Armstrong, self-restoring protective device used to reduce
1890“1954). An oscillator circuit that uses in- voltage surges on power lines.
ductive feedback between the output and input. ARRL Abbreviation for American Radio Relay
Either the output coil or the input coil can be League.
tuned to set the oscillator frequency. The amount arrowhead A wideband, log-periodic antenna with
of positive feedback is controlled by varying the linear polarization.
coupling between the coils. ARS Abbreviation of Amateur Radio Service.
Armstrong superheterodyne circuit See SUPER- arsenic Symbol, As. A metalloidal element. Atomic
HETERODYNE CIRCUIT. number, 33. Atomic weight, 74.91. Arsenic is fa-
Armstrong superregenerative circuit See SU- miliar as an n-type dopant in semiconductor pro-
ARSR • assemble

ARSR Abbreviation of air route surveillance radar. those of the more bulky transmission line it re-
articulation A measure of the effectiveness of voice places in tests and measurements. It also serves
communications, expressed as the percentage of as a time-delay or phase-shift device and as a
speech units understood by the listener when the pulse-forming network.
effect of context is negligible. artificial voice A device used to test and calibrate
artificial antenna See DUMMY ANTENNA. noise-canceling microphones, consisting essen-
artificial ear A microphone-type sensor, equiva- tially of a small loudspeaker that has a baffle
lent to the human ear, used to measure sound whose acoustical properties simulate those of the
pressures. human head.
artificial echo 1. In radar practice, the reflections artos stripper A machine that cuts and strips wire
of a transmitted pulse returned by an artificial for the fabrication of multiconductor cables.
target. 2. A signal from a pulsed radio-frequency artwork 1. In the manufacture of printed circuits,
(RF) generator, delayed to simulate an echo. the scaled drawings from which the mask or etch
artificial ground The effective ground provided by pattern is obtained photographically. 2. Collec-
the radials or disk of a ground-plane antenna, as tively, the illustrations depicting an electronic cir-
opposed to actual ground (the earth itself ). Com- cuit, device, or system.
pare TRUE GROUND. As Symbol for ARSENIC.
artificial horizon In aircraft instrumentation, a ASA Abbreviation of AMERICAN STANDARDS AS-
device that displays lines showing the position of SOCIATION.
the aircraft in flight, with reference to the hori- asbestos A nonflammable fibrous material consist-
zon. ing of calcium and magnesium silicates that is
artificial intelligence Abbreviation, AI. 1. A spe- used for high-temperature insulation.
cialized field of computer science overlapping A-scan A radar-screen presentation in which the
with electronics, biology, physiology, and other horizontal time axis displays distance or range,
sciences, concerning attempts to develop ad- and the vertical axis displays the amplitude of
vanced computer systems that can emulate the signal pulse and echo pulses.
processes of the human mind. 2. The ability of a ascending node For a satellite orbiting the earth or
computer to learn from its mistakes, refine its another planet, any point at which the ground-
own processes, and perhaps ultimately reason in track crosses the equator as it moves from the
a humanlike manner. southern hemisphere into the northern hemi-
artificial ionization An artificial reflecting layer sphere. This node generally changes for each
that is created in the atmosphere to provide a succeeding orbit, because the earth or planet ro-
skip condition. tates underneath the orbit of the satellite. Com-
artificial language A language that is not com- pare DESCENDING NODE.
monly used, but has been devised for efficiency in ascending pass For a specific point on the earth™s
a particular situation”especially in a computer surface, the time during which an artificial com-
system. munications satellite is accessible when its lati-
artificial life 1. The ultimate endpoint of ARTIFI- tude is moving northward. The duration of
CIAL INTELLIGENCE, wherein machines acquire accessibility depends on the altitude of the satel-
qualities, such as wisdom and the capability to lite, and on how close its groundtrack comes to
feel emotions. The state of the art is currently the earth-based point. Compare DESCENDING
nowhere near this point. 2. A hypothetical ma- PASS.
chine or set of machines with lifelike qualities, in- ASCII Acronym (pronounced “ask-ee”) for Ameri-
cluding human-level intelligence, wisdom, and can Standard Code for Information Interchange.
emotion. ASI Abbreviation for American Standards Institute.
artificial stimulus An electronic method of robot A-scope A radar system that displays an A-SCAN.
guidance and navigation using radar, sonar, vi- Askarel A synthetic, nonflammable liquid dielec-
sion systems, edge detection, and/or beacons. tric.
artificial transmission line A network of capaci- aspect ratio The width-to-height ratio of a video
tors and inductors with characteristics similar to image, generally three units high by four units
asperities On the surface of an electrode, tiny
points at which the electric field is intensified and
from which discharge is highly probable.
ASR 1. Abbreviation of AIRBORNE (or AIRPORT)
surveillance radar. 2. Abbreviation of AUTO-
ASRA Acronym for automatic stereophonic record-
ing amplifier.
assemble 1. To gather subprograms into a com-
plete digital computer program. 2. To translate a
42 assemble • astrionics

Symbols for ASCII teleprinter code assembly program The program that operates on
a symbolic-language program to produce a ma-
chine language program in the process of assem-
First Last three signals bly. Also called assembler.
four assembly robot A form of industrial robot that puts
hardware together. Such a robot is generally a
signals 000 001 010 011 100 101 110 111
component of an automated integrated manufac-
turing system (AIMS). The robot can do repetitive
0000 NUL DLE SPC 0 P / p
work at high speed and precision for long periods
0001 SOH DC1 ! 1 A Q a q
of time.
0010 STX DC2 " 2 B R b r
assign To reserve part of a computing system for a
0011 ETX DC3 # 3 C S c s specific purpose, normally for the duration of a
0100 EOT DC4 $ 4 D T d t program run.
0101 ENQ NAK % 5 E U e u assigned frequency The radio carrier frequency or
0110 ACK SYN & 6 F V f v band of frequencies designated for a transmitting
station by a licensing authority. Also see RADIO
0111 BEL ETB ' 7 G W g w
1000 BS CAN ( 8 H X h x
associative memory Computer memory in which
1001 HT EM ) 9 I Y i y
locations are identified by content, rather than by
1010 LF SUB * : J Z j z
specific address.
1011 VT ESC + ; K [ k {
assumed decimal point A decimal point that does
1100 FF FS , < L / l / not occupy an actual computer storage space,
1101 CR GS “ = M ] m } but is used by the computer to align values for
1110 SO RS . > N n ˜ calculation; the decimal point is assumed to be at
1111 SI US | ? O - o DEL the right unless otherwise specified.
astable Having two temporary states; BISTABLE.
astable circuit A circuit that has two unstable
ACK: acknowledge FF: form feed
states, and whose operation is characterized by
BEL: bell FS: file separator
alternation between those states at a frequency
BS: back space GS: group separator determined by the circuit constants.
CAN: cancel HT: horizontal tab astable multivibrator A free-running multivibra-
CR: carriage return LF: line feed tor. The common circuit uses two bipolar or field-
DC1: device control no. 1 NAK: do not acknowledge effect transistors, their inputs and outputs being
cross coupled. Conduction switches alternately
DC2: device control no. 2 NUL: null
between the two.
DC3: device control no. 3 RS: record separator
astatic 1. Without fixed position or direction. 2. In
DC4: device control no. 4 SI: shift in
a state of neutral equilibrium.
DEL: delete SO: shift out
astatic galvanometer A galvanometer with a mov-
DLE: data link escape SOH: start of heading
able element consisting of two identical magne-
ENQ: enquiry SPC: space tized needles mounted nonparallel on the same
EM: end of medium STX: start of text suspension. Each needle is surrounded by a coil.
EOT: end of transmission SUB: substitute The coils are wound in opposite directions, and
ESC: escape SYN: synchronous idle are connected in series to the current source. A
large permanent magnet provides the field
ETB: end of transmission US: unit separator
against which the needle assembly rotates. The
block VT: vertical tab
instrument functions independently of the geo-
ETX: end of text
magnetic field.
astatine Symbol, At. A radioactive elemental halo-
gen produced from radioactive decay. Atomic
symbolic program language into a machine (bi- number, 85. Atomic weight, 210. Formerly called
nary) language program by substituting opera- alabamine.
tion codes and addresses. A station One of the two stations in the transmit-
assembly 1. A finished unit that can be either a ting system of LORAN (long-range navigation).
practical working model or a dummy, a proto- astigmatism A focusing fault in a cathode-ray
type, or a final model; an integrated aggregation tube (CRT), in which electrons in different axial
of subunits. 2. A low-level computer source-code planes focus at different points.
language that uses crude mnemonics that are ASTM Abbreviation for American Society for Testing
easier to remember than the machine-language and Materials.
equivalents. astrionics The design, production, and application
assembly language A source code that uses of electronic devices and systems for use in space
mnemonic instructions. (See ASSEMBLY, 2.) vehicles and space navigation.
astronomical unit • atmospheric absorption noise

astronomical unit Abbreviation, AU. A unit of dis-
tance equal to 1.496 — 108 kilometers (9.296 — 107
miles). Approximately equal to the mean distance
between the earth and the sun.
A supply See A POWER SUPPLY.
asymmetrical cell A photocell exhibiting ASYM-
asymmetrical communications 1. Two-way com-
munications in which the volume of transmitted
data is much greater in one direction than in the
other. 2. Two-way communications in which the
speed of transmitted data is much greater in one
direction than in the other. Compare SYMMET-
asymmetrical conductivity A condition in which
a device conducts well in one direction, but poorly
in the other direction. A rectifier diode is a com-
mon example of a component that exhibits this
asymmetrical distortion In a binary system,
lengthening or shortening of one of the states, by
comparison to the theoretical or ideal duration.
asymmetrical FET A FIELD-EFFECT TRANSIS- asynchronous input In digital circuitry, any flip-
TOR in which the source and drain cannot be in- flop input at which a pulse can affect the output
terchanged without degrading performance. independently of the clock.
asymmetrical multivibrator An unbalanced mul- asychronous motor An ac motor whose speed is
tivibrator (i.e., one in which the circuit halves are not proportional to the supply frequency.
not identical). If the time constants of the halves asynchronous transmission Data transmission
are different, the output pulses will be short and in which each character or symbol begins with a
widely separated. start signal and ends with a stop signal. This
asymmetrical sideband See VESTIGIAL SIDE- eliminates the need for the data to be sent at a
BAND. uniform speed.
asymmetrical sideband transmission See VESTI- asynchronous vibrator In a vibrator-type portable
GIAL SIDEBAND TRANSMISSION. power supply, a vibrator that only makes and
asymmetrical wave A wave whose upper (positive breaks the primary circuit of the step-up trans-
half-cycle) and lower (negative half-cycle) por- former. This is in contrast to the synchronous vi-
tions have different amplitudes or shapes. Also brator, which also makes and breaks the
called asymmetric wave. secondary circuit in synchronism with the pri-
asymmetry control An adjustment in a device in- mary. Also called NONSYNCHRONOUS VIBRA-
tended for measuring the pH (acidity/alkalinity). TOR.
This corrects the inaccuracies that results from AT A quartz crystal cut wherein the angle between
the differences between the electrodes. the x-axis and the crystal face is 35 degrees.
asymptote In analytical geometry, a fixed straight At Symbol for astatine.
line or ray L with a special relationship to a AT-cut crystal A piezoelectric crystal cut at a 35-
curve or part of a curve K that recedes to infin- degree angle, with respect to the optical axis of
ity. As the distance from the origin (0,0) in- the quartz. The frequency of such a crystal does
creases without limit, the separation between K not appreciably change with variations in tem-
and L approaches zero, but K and L never actu- perature.
ally meet. atmosphere 1. The gas surrounding a planet, par-
asymptotic breakdown voltage A voltage that will ticularly the air sheathing the earth. 2. Abbrevia-
tion, atm. A unit of pressure equal to 1.013 — 106
cause dielectric breakdown if applied continu-
ously for a sufficiently long time. dynes per square centimeter (about 14.7 pounds
asymptotic expression An expression having a per square inch).
very small error in terms of percentage. atmospheric absorption 1. The conversion of
asynchronous 1. Not synchronous, i.e., nonrecur- electromagnetic energy into heat, with resulting
rent (as in out-of-phase waves). 2. A mode of loss, as the energy passes through the earth™s at-
computer operation in which the completion of mosphere. The extent of this effect depends on
one operation starts another. the wavelength. 2. See ABSORPTION LOSS, 2.
asynchronous device A device not regulated by atmospheric absorption noise Noise, principally
the system in which it is used, as far as its oper- above 1 GHz, resulting from atmospheric absorp-
ating frequency or rate is concerned. tion (see ABSORPTION LOSS, 2).
44 atmospheric bending • atomic radiation

atmospheric bending The refraction or reflection
of electromagnetic waves by the troposphere or
atmospheric duct A tropospheric stratum, often
associated with temperature inversions, lake ef-
fects, or weather fronts, through which electro-
magnetic energy at ultra-high and microwave
frequencies is efficiently propagated for long dis-

bending of radio waves in the ionosphere, result-
ing in long-range propagation at high frequencies.
atmospheric scatter 1. The scattering of very-high
frequency ( VHF) and ultra-high frequency (UHF)
radio waves by the lower atmosphere. 2. Commu-
nication via scattering of VHF and UHF radio
waves in the lower atmosphere.
atmospherics See ATMOSPHERIC NOISE.
atom 1. The smallest material particle that dis-
plays the unique characteristics and properties of
an element. Atoms consist of a dense, positively
charged central nucleus, around which less-mas-
sive, negatively charged electrons “swarm” at def-
atmospheric electricity Static electricity present inite levels called shells. Also see BOHR ATOM
in the atmosphere, which evidences itself in dis- and RUTHERFORD ATOM. 2. In a computer-
turbance of radio communications and in dis- compiling operation, an operator or operand.
plays of lightning. atomechanics The physics of electron movement.
atmospheric noise Receiver noise resulting from atomic battery A battery in which atomic energy is
ATMOSPHERIC ELECTRICITY. Also called sferics converted into electrical energy.
or static. atomic charge The electrification (i.e., the electron
atmospheric pressure Abbreviation, atm press. 1. charge) exhibited by an ion.
The pressure exerted by the earth™s atmosphere, atomic clock Also called atomic time standard. A
as indicated by a barometer at sea level; normally highly accurate electronic clock, driven by the
between 29 and 31 inches of mercury. 2. A pres- characteristic oscillations of certain atoms.
sure of 1.013 — 106 dynes per square centimeter. atomic energy Energy released by the FUSION or
See ATMOSPHERE, 2. FISSION of atomic nuclei. Also see ATOMIC
atmospheric radio wave See SKYWAVE. POWER.
atmospheric radio window The band of frequen- atomic fission See FISSION.
cies (approximately 10 MHz to 10 GHz), including atomic frequency The natural vibration frequency
radio waves that can penetrate the earth™s tropo- of an atom.
sphere and ionosphere. atomic fusion See FUSION.
atmospheric reflection The return of a radio wave atomic pile See REACTOR, 2.
to earth, resulting from reflection by an ionized atomic mass unit Abbreviated amu. A unit that
portion of the atmosphere. expresses the relative mass of an elemental iso-
tope. One amu is equal to 1„12 of the atomic mass
of carbon 12 (C12). A neutron has a mass of
roughly one amu.
atomic migration The transfer or “wandering” of a
valence electron between or among atoms in a
single molecule.
atomic number The number of protons in the nu-
cleus of an atom. Also, the number of electrons if
the atom is electrically neutral. For example, the
atomic number for copper is 29, indicating 29
protons in the nucleus. An electrically neutral
atom of copper has 29 electrons. The atomic
atmospheric refraction 1. Downward bending of number uniquely identifies an element.
radio waves as a result of variations in the dielec- atomic radiation The emission of radiant energy
tric constant of the troposphere. 2. Downward by radioactive substances.
atomic reactor • attracted-disk electrometer

atomic reactor See REACTOR, 2. attenuate To reduce in amplitude.
atomic theory The scientific theory that all matter is attenuation A reduction of signal amplitude.
composed ultimately of atoms, which are the small- attenuation characteristic Also called attenua-
est particles retaining the identity of an element. tion constant. 1. In an amplifier, network, or com-
Atoms combine to form molecules, the smallest ponent, the decrease in signal amplitude as a
particles that retain the identity of a compound. function of frequency, usually expressed in deci-
Atoms themselves contain minute subatomic parti- bels per octave. 2. In a transmission line, the de-
cles, some of which carry electric charges. See crease in signal amplitude per unit length.
BOHR ATOM and RUTHERFORD ATOM. Usually expressed in decibels per 100 feet, deci-
atomic time 1. A means of time determination bels per mile, or decibels per kilometer.
that makes use of the resonant vibrations of cer- attenuation constant See ATTENUATION
tain substances, such as cesium. 2. Synchro- CHARACTERISTIC.
nized astronomical time, as determined by an attenuation distortion A type of distortion char-
ATOMIC CLOCK. acterized by variation of attenuation with fre-
atomic unit of energy In a hydrogen atom, the po- quency within a given frequency range.
tential energy of the electron in the lowest-energy attenuation equalizer An equalizer that stabilizes
shell, as averaged over a certain length of time. the transfer impedance between two ports at all
The shell represents the mean energy of the elec- frequencies within a specified frequency band.
tron. attenuation-frequency distortion Distortion
atomic weight 1. The mass of a particular atom in characterized by the attenuation of the frequency
ATOMIC MASS UNITS (amu). 2. A number char- components in a complex waveform. Frequency-
acterizing the average mass of individual atoms sensitive RC networks (such as a Wien bridge) ex-
for a specific isotope of an element. Thus, carbon hibit this type of distortion when they attenuate a
12 (C12) has an atomic weight of 12, oxygen 16 fundamental and each harmonic unequally.
(O16) has an atomic weight of approximately 16, attenuation network A combination of compo-
and uranium 238 (U238) has an atomic weight of nents (R, C, or L singly or in any necessary
about 238. combination) that provide constant signal
atomistics The science of the atom and atomic en- attenuation with negligible phase shift through-
ergy. Also called atomics. out a frequency band.
attack 1. The rise of a pulse from zero to maximum attenuation ratio The ratio indicating a relative
amplitude. 2. The time required for a pulse to rise current, voltage, power or energy decrease. For
from zero to maximum amplitude. 3. The initial- example, for voltage, Einput/Eoutput = 6/2 = 3:1 = 3.
ization of a circuit voltage or current for a certain attenuator A device for reducing signal amplitude
purpose, such as an automatic gain control. 4. in precise, predetermined steps, or smoothly over
The rise of a musical note from zero to full volume. a continuous range. A network of resistors, ca-
pacitors, or both. The simplest attenuator con-
sists of one or more noninductive resistors.

attitude The position of an aircraft or space vehicle
relative to a (usually terrestrial) reference point,
often determined with electronic instruments.
atto- Abbreviated, a. A prefix meaning 10 “18 or
multiplication by 10“18.
attack time The time required for an applied sig-
nal that suddenly increases in amplitude to reach attofarad Abbreviation, aF. An extremely small
unit of low capacitance; 1 aF equals 10 “18 F.
63.2 percent of its final, stable value.
attemperator An automatic temperature-control- attracted-disk electrometer A device to measure
ling device; a thermostat. potential difference consisting of two parallel
attention display A computer-generated chart or metal disks”one of which is connected to a ten-
graph, displayed as an alert signal concerning a sion spring. The force between the disks indicates
particular situation. the magnitude of the electric field.
46 attraction • audio-frequency filter

attraction The drawing together or pulling toward, frequency section). 3. A radio channel of fixed fre-
as in the attraction between electric charges or quency that is reserved for voice communica-
magnetic poles. Dissimilar charges and poles at- tions.
tract each other (electric plus to minus, magnetic audio clipping Brute-force limiting of the ampli-
north to south). Compare REPULSION. tude of an audio signal, usually accomplished us-
ATV Abbreviation of amateur television, used in the ing semiconductor diodes to prevent the positive
Amateur Radio Service. and negative peak amplitudes from exceeding a
AU Abbreviation of ASTRONOMICAL UNIT. certain level.
Au Symbol for GOLD.
audibility The quality of being detectable by the
human ear. In a healthy listener, the threshold of
audibility is extremely low; at the threshold, the
pressure of a sound wave varies from normal by
approximately 10-4 pascal. The frequency range
of human audibility extends roughly from 20 Hz
to 20 kHz.

Sound Audibility (dB)
Threshold of hearing 0

Whisper 10 “ 20
Electric fan at 10 feet 30 “ 40
Running water at 10 feet 40 “ 60
Speech at 5 feet 60 “ 70
Vacuum cleaner at 10 feet 70 “ 80
Passing train at 50 feet 80 “ 90
Jet at 1000 feet altitude 90 “ 100
Rock band on stage 110 “ 120

Air hammer at 5 feet 130 “ 140

audibility table

audibility curve A graph (such as the Fletcher- audio component The audio-frequency portion of
Munson curve) that depicts the range of human any wave or signal.
hearing in terms of frequency versus the sound audio converter A circuit in which a received ra-
pressure at the threshold of AUDIBILITY. dio-frequency (RF) signal is heterodyned with a
audible Detectable by the human ear. local RF oscillator signal to produce an audio-
audible alarm device An ANNUNCIATOR that frequency (AF) beat-note output. The beat note is
produces an easily identifiable sound in re- then amplified by an AF amplifier. It is used es-
sponse to an ALARM CONDITION in a security pecially by amateur radio operators in the recep-
system. tion of continuous-wave (CW) radiotelegraphy,
audible frequency See AUDIO FREQUENCY. radioteletype, and packet radio at high frequen-
audible tone A vibration of air molecules that can cies.
be detected by the human ear, and with periodic audio frequency A frequency lying within the au-
properties, such as a sine-wave vibration. dible spectrum. Abbreviated AF. See AUDIO-
audio 1. Pertaining to the spectrum of frequencies FREQUENCY SPECTRUM.
corresponding to the human hearing range audio-frequency amplifier An amplifier that oper-
(about 20 Hz to 20 kHz), or to equipment or per- ates in part or all of the frequency range 20 Hz to
formance associated with that spectrum. 2. Any 20 kHz. High-fidelity amplifiers function over a
disturbance, such as a current or compression somewhat wider range (e.g., 10 Hz to 50 kHz).
wave, falling within the range of about 20 Hz to audio-frequency choke An inductor (usually hav-
20 kHz. 3. AUDIO FREQUENCY. ing a ferromagnetic core) that blocks audio-fre-
audio amplifier See AUDIO-FREQUENCY AMPLI- quency current, but passes direct current.
FIER. audio-frequency feedback 1. Electrical FEED-
audio band The range (band) of audio frequencies. BACK (positive and/or negative) that affects audio-
audio channel 1. The portion of a complex signal frequency circuits. 2. ACOUSTIC FEEDBACK.
or waveform used to convey audio information ex- audio-frequency filter A filter of any type that op-
clusively. 2. The audio-frequency section of a erates on any part of the frequency range 20 Hz to
transmitter or receiver (as opposed to the radio- 20 kHz.

audio-frequency meter • audio mixer

audio-frequency meter An instrument to measure type sound like a hiss or roar. Compare
frequencies in the audio-frequency spectrum (ap- FREQUENCY-SHIFT KEYING.
proximately 20 Hz to 20 kHz). Three types are audio-frequency-shift modulator A modulator for
commonly used: audio-frequency-shift keying of a signal.
audio-frequency spectrum The band of frequen-
• Analog Gives direct indications of frequency on
cies extending from roughly 20 Hz to 20 kHz.
the scale of a D™Arsonval meter; the usual range
High-fidelity component specifications extend
is 20 Hz to 100 kHz.
this range somewhat in both directions (e.g., from
• Digital Gives direct indications of frequency by
10 Hz to 50 kHz).
means of readout lamps; the usual range is 1 Hz
audio-frequency transformer Abbreviation, AF
to 15 MHz. This instrument is useful also as a
transformer. A device used for the purpose of
radio-frequency meter.
matching impedances at frequencies within the
• Bridge Consists of a frequency-sensitive bridge,
range of human hearing (up to approximately 20
such as a Wien bridge, with a null-indicating
kHz). This ensures the most efficient possible
meter. The operator balances the bridge and
transfer of power between stages of audio ampli-
reads the unknown frequency from the dial of
fication, between an amplifier and a speaker or
the balance control.
headset, or between a microphone and an audio
preamplifier. These transformers are available
with various power ratings and impedance-
matching ratios. Some devices are tailored to
have a certain attenuation-versus-frequency re-
sponse. At audio frequencies, transformers are
physically similar to the alternating-current
transformers used in power supplies. They are
wound on laminated or powdered-iron cores.
audio-frequency transistor A transistor that is
usually used only at audio frequencies.
audiogram A graph used to rate hearing, used by
audiologists and audiometrists.
audio image In a direct-conversion receiver, a re-
sponse to a signal on one side of (above or below)
the local-oscillator (LO) frequency, when the op-
erator is listening to a signal on the other side of
the LO frequency. These responses are reduced
or eliminated in single-signal receivers.
audio-level meter An ac meter for monitoring sig-
nal amplitude in an audio-frequency system. It
can indicate in volts, decibels, volume units (VU),
or arbitrary units, and is often permanently con-
nected in the circuit.
audio limiter A limiter or clipper operated in the
audio-frequency (AF) channel of a receiver or
audio-frequency noise Any electrical noise signal
transmitter to hold the output-signal amplitude
causing interference within the audio-frequency
constant, or to minimize the effect of noise peaks.
audiologist A person skilled in testing hearing (i.e.,
audio-frequency oscillator See AUDIO OSCILLA-
in using audiometers and other electronic instru-
ments) and evaluating their indications for the
audio-frequency peak limiter Any circuit or de-
fitting of hearing aids.
vice, such as a biased diode, that performs the
audiometer An instrument used for hearing tests,
function of audio limiting.
which consists of a specialized audio-frequency
audio-frequency-shift keying Abbreviation, AFSK.
(AF) amplifier with calibrated attenuators, output
Frequency-shift keying that is done at audio
meter, and signal source.
frequencies (below approximately 20 kHz) rather
audiometrist A person skilled in the use of au-
than at radio frequencies. There are two audio
diometers and other electronic instruments that
sine-wave signals, one for logic 1 (high or mark)
measure sound and human hearing, and who
pulses and the other for the logic 0 (low or space)
deals with attendant health and behavior prob-
pulses. This scheme is commonly used with
telephone modems where the signal bandwidth is
audio mixer An amplifier circuit for blending two
severely limited by circuit characteristics. At
or more audio-frequency (AF) signals, such as
typical data speeds in twisted-pair telephone
those delivered by microphones or receivers.
lines (usually 28.8 or 57.6 kbps), signals of this
48 audio oscillator • auroral propagation

audio oscillator 1. An oscillator that delivers an augend In a calculation, the number to which an-
output signal in the frequency range 20 Hz to 20 other is to be added. Compare ADDEND.
kHz. 2. An audio-frequency (AF) signal generator. augend register In a digital computer, the register
Some instruments of this type operate above and that stores the augend. Compare ADDEND REG-
below the limits of the common audio-frequency ISTER.
spectrum (e.g., 1 Hz to 1 MHz). aural Pertaining to sound actually heard, as op-
audio output The output of an audio-frequency posed to sound that exists only as audio-
oscillator or amplifier. It can be measured in frequency currents or waves.
terms of peak or rms volts, amperes, or watts. aurora A phenomenon sometimes called the north-
audiophile A sound-reproduction hobbyist. ern lights or southern lights, as seen in the night
audio power Alternating-current power at frequen- sky. In the Northern Hemisphere, it is known as
cies roughly between 20 Hz and 20 kHz. When Aurora Borealis; in the Southern Hemisphere, it
used in connection with transmitters and other is called Aurora Australis. It generally occurs a
modulated radio-frequency (RF) equipment, the few hours after a solar flare, when charged parti-
term refers to modulator power output. cles, emitted from the sun, arrive at the earth,
audio response unit A device that links digitized and are accelerated in the vicinity of the the geo-
responses, held in computer storage, to a tele- magnetic poles.
phone set or line to answer incoming calls and in- auroral absorption Radio wave absorption by an
quiries. aurora.
audio signal generator See AUDIO OSCILLA- auroral flutter Rapid fading of a signal at high or
TOR, 2. very high frequencies, so-called because it often
audio spectrum The range of sine-wave frequen- imparts a fluttering quality to the signal that is
cies detectable by the human ear when they oc- caused by phase distortion and Doppler shift
cur as acoustic vibrations. This range is about 20 when the waves are reflected from the aurora.
Hz to 20 kHz. auroral interference 1. Interference to high-
audio squelch A squelch circuit that operates only frequency radio propagation and also occasion-
on the audio channel of a receiver. ally to medium-frequency and low-frequency
audio system 1. The portion of any electronic as- propagation, caused by the activity of the aurora.
sembly that is used to process sound. 2. Special 2. Auroral flutter on a signal.
computer equipment capable of storing and pro- auroral opening A condition in which radio com-
cessing digitized audio-frequency (AF) data. munication becomes possible via AURORAL
audiotape Magnetic tape for the recording and re- PROPAGATION. It can occur when communica-
production of data in the audio-frequency (AF) tion between two points is normally impossible at
range. a certain frequency. Auroral openings allow long-
audio taper In potentiometers, a semilogarithmic distance communication well into the very-high-
variation of resistance versus rotation. Used in frequency (VHF) spectrum.
volume and tone controls for audio circuits. At auroral propagation Reflection of radio signals
midposition (the halfway point), the counter- from aurora that occur during geomagnetic
clockwise portion of the device has about 1„10 the storms. Theoretically, auroral propagation is pos-
resistance of the clockwise portion. A listener will sible when the aurora are active, between any two
hear sound at half-volume because of the loga- points on the earth™s surface from which the same
rithmic nature of the human audibility curve. part of the aurora lie on a line of sight. This type
audio-visual Pertaining to a combination of sound of propagation seldom occurs when one end of the
and sight (e.g., television and sound motion pic-
auditory backward inhibition A subjective phe-
nomenon, in which a sound is erased from the
memory of a listener by a second sound arriving
about 60 milliseconds later.
auditory inhibition The tendency of sound waves
to be partially or totally canceled by the
ears/mind of a listener, depending on the waves™
intensity, relative phase, and/or direction of im-
auditory mirage See ACOUSTIC MIRAGE.
audit trail A history of the processes relating to a
record, transaction, or file in a computer system.
Created during the routine processing of data,
the trail is stored as a file. The audit trail allows
auditing of the system or the subsequent recre-
ation of files.
auroral propagation • automatic chrominance control

circuit is at a latitude less than 35 degrees north automated integrated manufacturing system
or south of the equator. Auroral propagation can Acronym, AIMS. An assembly line or factory that
take place at frequencies well above 30 MHz. It is uses robots, often controlled by one or more com-
characterized by deep, rapid fading and random puters, to perform specific tasks that result in the
phase modulation of reflected signals. production of various hardware items.
auroral reflection The return of electromagnetic automatic Self-regulating, independent of human
waves that have been beamed toward an aurora. intervention. Some periodic adjustment might be
Most often observed between 15 MHz and 150 needed.
MHz. automatic base bias A method of obtaining base
authorized access switch A device that disables a bias in a bipolar transistor, where a resistor
security system in a defined region or volume so develops a voltage drop because of the current
that authorized personnel can enter without trig- flowing through it. The resistor is usually placed
gering an alarm condition. in the emitter circuit, raising the emitter above
authorized channel The carrier frequency or band ground potential.
assigned to a transmitting station by a licensing automatic bass compensation Also called bass
authority. Also see RADIO SPECTRUM. boost. In audio high-fidelity systems, a resistor-
autoalarm A device that is actuated from a re- capacitor (RC) network that increases the relative
ceived signal to alert a radio or computer network amplitude of the bass at low volume levels. This
operator to the existence of a message. compensates for the ear™s inefficiency at low fre-
autobaud 1. In digital communications, a function quencies. The function can be automatically ac-
that allows the equipment to adjust itself to the tuated by the setting of the volume control, or it
speed of the terminal. 2. Any digital communica- can be switched manually on and off.
tions equipment capable of automatically adjust- automatic bias In an amplifier, dc base/gate/
ing to the speed of the terminal. grid bias obtained from the voltage drop produced
autocondensation The application of radio- by collector/drain/plate current flowing through
frequency (RF) energy to the human body for a resistor common to the input and output. This
medical purposes. The living organs serve as an resistor is usually shunted by a capacitor and
impedance or load, across which the RF is ap- placed in the emitter/source/cathode circuit.
autoconduction The application of radio-
frequency (RF) currents into the body, by placing
the living organ inside a coil and supplying the
coil with RF. Used for medical purposes.
autocorrelation function A measure of the simi-
larity between delayed and undelayed versions of
a signal, expressed as a delay function.
autodyne reception Radio reception of cw signals
by means of an oscillating detector. This is in
contrast to heterodyne reception, in which a local
oscillator (LO) generates an audio beat note with
the cw signal in a separate detector.
autoionization A two-phase process of atomic ion-
ization. The atom is excited beyond its ionization
potential, and then it is allowed to deionize, caus-
ing the emission of an electron. The result is a
positively charged atom (positive ion).
automated communications The transfer of data
without the use of operating personnel; generally automatic brightness control A circuit that uses
done with computers connected to communica- the same principles used in AUTOMATIC GAIN
tions equipment. CONTROL (AGC) to hold steady the average
automated guided vehicle Abbreviation, AGV. A brightness of a television (TV) picture.
robot cart that runs without a driver. It uses an automatic carriage Typewriters, automatic key
electric engine and is guided by the magnetic field punches, and other devices that can control au-
produced by a current-carrying wire embedded in tomatically the spacing and feeding of paper,
the floor or pavement. Alternatively, the robot can cards, and forms.
run on a track. automatic check 1. In a digital computer, the au-
automated home A residence in which many, or tomatic inspection of operation and performance
most, of the routine chores are done by comput- by a self-contained subsystem. 2. The circuit or
ers and/or robots. Examples of such tasks are device for performing this inspection.
dishwashing, doing the laundry, mowing the automatic chrominance control In a color televi-
lawn, blowing snow, and vacuuming the floors. sion (TV) receiver, a subcircuit that controls the
50 automatic chrominance control • automatic intercept

gain of the chrominance bandpass amplifier by automatic error correction A technique of cor-
automatically adjusting its bias. recting transmission errors using error-detecting
automatic circuit breaker Any device that opens and error-correcting codes and, usually, auto-
a circuit automatically when the flow of current matic retransmission.
becomes excessive. The breaker generally resets automatic exchange A transmission exchange in
automatically after a specified length of time, or which interterminal communications are accom-
after power has been temporarily removed from plished without operators.
the circuit. automatic focusing A method of focusing a pic-
automatic coding The use of a computer to deter- ture tube automatically, in which a resistor con-
mine the steps for solving a problem, before the nects the focusing anode to the cathode; thus, no
actual program for the problem is written. This external focusing voltage is necessary.
can help software engineers develop long and/or automatic frequency control Abbreviation, AFC.
complex computer programs. A system that keeps a circuit automatically tuned
automatic contrast control A circuit that auto- to a desired signal frequency. A detector (such as
matically adjusts the gain of the video IF and RF a discriminator) operated from the tuned circuit
stages of a television (TV) receiver to preserve delivers a dc output voltage only when the circuit
good picture contrast. is operating above or below the signal frequency;
automatic controller In servo systems, any of sev- otherwise, it has zero dc output. The dc output,
eral circuits or devices that samples a variable when present, alters the capacitance of a varactor
signal, compares it with a standard (reference) in the tuned circuit to retune the stage to the de-
signal, and delivers a control or correction signal sired frequency.
to an actuator.
automatic crossover 1. Current limiting in a
power supply. 2. A device that switches a circuit
from one operating mode to another automati-
cally when conditions change in a predetermined
automatic current limiter A circuit or device for
holding the output current of a power supply to a
safe value during overload.
automatic current regulator A circuit or device
that holds the output current of a generator or
power supply to a predetermined value, in spite of
wide variations in load resistance. automatic gain control Abbreviated AGC. A sys-
automatic cutout A device that shuts down a cir- tem that holds the output of a receiver or ampli-
cuit or system when the safe limits of operation fier substantially constant despite input-signal
are exceeded. A circuit breaker is an example of amplitude fluctuations. A rectifier samples the ac
such a device, as is a thermostat in a power am- signal output and delivers a dc signal propor-
plifier. tional to that output. The dc signal is filtered, and
automatic data processing Abbreviation, ADP. the smoothed-out voltage is applied in correct po-
The use of computers and accessories for calcula- larity as bias to one or more preceding stages to
tions and tabulations using data gathered auto- reduce their gain. The stronger the signal enter-
matically by the system. ing the system, the greater the reduction in gain.
automatic degausser A system for automatically As a result, weak signals are amplified much
demagnetizing the picture tube in a color televi- more than strong ones. Various forms of this
sion (TV) receiver. scheme are used in many types of amplifiers and
automatic dialing unit Abbreviated, ADU. A de- communications systems.
vice that automatically generates dialing digits. automatic gate bias A method of obtaining gate
Many telephone sets have these devices, some of bias in a FET, where a resistor develops a volt-
which can be programmed for several different age drop because of the current flowing through
telephone numbers, including country codes and it. The resistor is usually placed in the source
area codes. circuit, raising the source above ground poten-
automatic dictionary A computer system compo- tial.
nent that substitutes codes for words and automatic height control In a television (TV) re-
phrases in information retrieval systems. In lan- ceiver, a system that automatically maintains the
guage-translating systems, it provides word- height of the picture, despite signal-amplitude
for-word substitutions. fluctuations, power-line voltage changes, and
automatic direction finder Abbreviated ADF. A gain variations.
specialized receiver/antenna combination for au- automatic intercept A telephone answering ma-
tomatically showing the direction from which a chine. It allows messages to be recorded when the
signal arrives. subscriber is not able to answer the telephone.
automatic interrupt • automatic secure voice communications

automatic phase control In a color television (TV)
receiver, a circuit that synchronizes the burst sig-
nal with the 3.58-MHz color oscillator.
automatic pilot An electronic device, often com-
puter-controlled, that automatically keeps a ship,
airplane, or space vehicle on course.
automatic polarity In an electronic metering de-
vice, a means of automatically switching the in-
put polarity of the instrument when the input
signal polarity is shifted. Also called bipolar oper-
automatic programming See AUTOMATIC COD-
automatic protective device A circuit or device
(such as a fuse, circuit breaker, limiter, or regu-
lator) that protects another circuit or device by
automatically removing, reducing, or increasing
the current or voltage during overload or under-
automatic radio compass See AUTOMATIC
automatic ranging In a metering device, the auto-
matic adjustment or optimization of the full-scale
range to compensate for large changes in the in-
put parameter.
automatic regulation 1. Voltage regulation. In a
power supply, the automatic holding of the output
voltage to a constant value, despite variations in
the input voltage or load resistance. 2. Current reg-
ulation. In a power supply, the automatic holding
of the output current to a constant value, despite
variations in the input voltage or load resistance.
automatic relay The relaying of messages auto-
automatic interrupt A program interruption
matically from one station to another via interme-
caused by hardware or software acting in re-
diate points, without the need for human
sponse to some event independent of the pro-
automatic repeater station A station that re-
automatic level compensation See AUTOMATIC
ceives signals and simultaneously retransmits
them, usually on a different frequency.
automatic level control Abbreviation, ALC. 1. A
automatic reset 1. The self-actuated restoration
circuit that adjusts the input gain of a magnetic-
of a circuit or device to a given state (e.g., the
tape recording device to compensate for changes
state of rest). 2. A circuit or device that restores
in the loudness of the sound reaching the micro-
another circuit or device to a given state.
phone. 2. A form of AUTOMATIC GAIN CONTROL
automatic scanning 1. The automatic (usually
used in single-sideband (SSB) radio transmitters
repetitive) tuning or adjustment of a circuit or
to maintain linearity while increasing the level of
system throughout a given frequency range. In a
the average power relative to the peak power.
radio receiver, the system can be programmed to
automatic line feed In the digital transmission of
pause or stop at occupied channels, passing over
printed matter, the automatic insertion of a line
vacant ones; or it can be programmed to pause or
feed (LF) character immediately following every
stop at vacant channels, passing over occupied
carriage return (CR) character.
ones. 2. The repetitive sweep of a cathode-ray-
automatic modulation control Abbreviation,
tube (CRT) electron beam.
AMC. In a frequency-modulated (FM) radio trans-
automatic scanning receiver Also called
mitter, a form of AUTOMATIC GAIN CONTROL
PANORAMIC RECEIVER. A radio receiver that is
that regulates the gain of the audio amplifiers to
automatically tuned (usually repetitively) over a
compensate for fluctuating audio input ampli-
frequency band. Such a receiver either homes in
tude. This prevents overdeviation while optimiz-
on a signal when one is found, or displays on a
ing signal intelligibility.
cathode-ray-tube (CRT) screen the distribution of
automatic noise limiter Abbreviation, ANL. Any
signals in the band.
of several circuits for clipping noise peaks ex-
automatic secure voice communications A wide-
ceeding a predetermined maximum received-
band and narrowband voice-digitizing application
signal amplitude.
52 automatic secure voice communications • auxiliary contacts

to a security network that provides encoded voice automatic zero In an electronic meter, a means of
communications. automatically setting the indicator to zero in the
automatic send/receive set A teletypewriter or absence of an input signal.
terminal that is capable of receiving and trans- automation 1. The control of machines or pro-
mitting. cesses by self-correcting electronic systems. See
automatic sensitivity control 1. A self-actuating ROBOT. 2. The use of robots and/or computers,
circuit using principles similar to those used in rather than human beings, to perform repetitive
AUTOMATIC GAIN CONTROL. It varies the sensi- tasks. 3. The use of robots and/or computers to
tivity of the radio-frequency (RF) and intermedi- assist human beings in industrial, office, govern-
ate-frequency (IF) sections of a receiver in inverse mental, and educational work.
proportion to the strength of a received signal. 2. automaton A simple robot that performs a task or
In a bridge null detector, a circuit similar to the set of tasks without artificial intelligence (AI).
one described in 1, which operates ahead of the These machines have existed for decades. Com-
detector, varying the sensitivity of the latter auto- pare ANDROID.
matically. automonitor In digital computer operations, to re-
automatic sequencing The ability of a digital com- quire the machine to supply a record of its infor-
puter to perform successive operations without mation-handling operations. Also, the program
additional instructions from the operator. for such instructions.
automatic short-circuiter A device that automat- automotive battery A set of four or eight
ically short-circuits the commutator bias in some rechargeable lead-acid cells connected in series
single-phase commutator motors. and housed in a common enclosure. The elec-
automatic short-circuit protection A circuit that trolyte is a free-flowing liquid acid. Provides ap-
allows the output of a power supply to be short- proximately 6 volts (four cells) or 12 volts (eight
circuited without damage to the components cells) under no-load conditions when fully
in the supply. It usually consists of a current- charged. The high mass results in large energy-
limiting device. storage capacity. These batteries must be han-
automatic shutoff A switching arrangement that dled with care and always kept in an upright
automatically shuts off a device or circuit under position to prevent spillage of the acid. See
certain specified conditions. LEAD-ACID BATTERY, LEAD-ACID CELL.
automatic switch center A telephone-switching autonomous robot A self-contained robot with an
network that routes calls to their destinations independent computerized control system. It
without the need for a human operator. moves under its own power, usually by rolling on
automatic target control For a vidicon television wheels or a track drive. Compare INSECT ROBOT.
camera tube, a circuit that automatically adjusts autopatch A remotely controllable device that in-
the target voltage in proportion to brightness of terconnects a radio-communications system into
the scene. a telephone network.
automatic telegraph reception Telegraph recep- autopilot A self-correcting control and guidance
tion providing a direct printout of the received in- device for the automatic management of an air-
formation, without intervention by an operator. craft or missile.
automatic telegraph transmission Telegraph autoranging See AUTOMATIC RANGING.
transmission originating from tapes, disks, or other autosyn A device or system that operates on the
records, rather than from a hand-operated key. principle of the synchronous ac motor, in which
automatic telegraphy Communications that uti- the position of the rotor in one motor (the trans-
lize automatic telegraph transmission and recep- mitter) is assumed by the rotor in a distant motor
tion. (the receiver) to which the first is connected.
automatic time switch A time-dependent circuit auto tracking A method of controlling the output
or device that opens or closes another circuit at voltages of many different power supplies simul-
the end of a predetermined time interval. taneously.
automatic tracking A method of keeping a radar autotransducter A type of magnetic amplifier
beam automatically fixed on a target. whose power windings serve also as control wind-
automatic trip A circuit breaker that automati- ings.
cally opens a circuit. autotransformer A single-winding transformer in
automatic tuning A process whereby a circuit which the primary coil is a fraction of the entire
tunes itself to a predetermined frequency upon winding for voltage step-up, or the secondary coil
receiving a command signal. is a fraction of the entire winding for voltage step-
automatic voltage regulator A circuit that keeps down.
the output of a power supply constant, despite auxiliary circuit A circuit that is supplementary to
the load resistance or input voltage to the supply. the main system.
automatic volume control Abbreviated AVC. The auxiliary contacts In switches and relays, con-
use of AUTOMATIC GAIN CONTROL in an audio tacts that are supplementary to the main con-
amplifier system. tacts and are usually actuated with them.
auxiliary equipment • avalanche breakdown

available gain The ratio Po/Pi, where Pi is the avail-
able power at the input of a circuit and Po is the
available power at the output.
available line The percentage of the length of a fac-
simile scanning line that is usable for picture sig-
available power The mean square of the open-
circuit terminal voltage of a linear source divided
by four, times the resistive component of the
source impedance. The available power is the
maximum power delivered to a load impedance,
equal to the conjugate of the internal impedance
of the power source.
available power gain In a power transistor, the ra-
tio of available transistor output power to the
power available from the generator. It depends on
the generator resistance, but not on the transis-
auxiliary equipment 1. Also known as peripher-
tor load resistance.
als. An apparatus not directly governed by the
available signal-to-noise ratio The ratio Ps/Pn,
central processing unit of a digital computer,
where Ps is the available signal power at a given
such as a printer or personal robot. 2. Peripheral
point in a system and Pn is the available random-
equipment in any system. 3. Backup equipment.
noise power at that point.
auxiliary memory In a digital computer, a unit
available time 1. The time during which a com-
that is supplementary to the main memory,
puter is available and ready for immediate use. 2.
which it augments.
The amount of time a computer is available to an
auxiliary receiver Also called standby receiver. In
a radio communications system, a receiver that is
avalanche The phenomenon in semiconductors
available for use if the main receiver fails.
operated at high reverse bias voltage, whereby
auxiliary relay 1. A standby relay. 2. A relay
carriers acquire sufficient energy to produce new
whose operation supports that of another relay.
electron-hole pairs as they collide with atoms.
3. A relay that is actuated by the operation of an-
The action causes the reverse current to increase
other relay.
auxiliary switch 1. A standby switch. 2. A switch
avalanche breakdown In a semiconductor P-N
wired in series or parallel with another switch. 3.
junction, a condition that occurs when the re-
A switch that is operated by another switch.
verse bias voltage exceeds a certain value. If the
electric field in the vicinity of the junction be-
comes strong enough, charge carriers are dis-
lodged from the atoms and the carriers (electrons
and holes) flow freely across the P-N junction in
the opposite direction from normal. The mini-

auxiliary transmitter Also called standby trans-
mitter. In a radio communications system, a
transmitter that is available for use if the main
transmitter fails.
a/v Abbreviation of AUDIO-VISUAL.
aV Abbreviation of attovolt.
availability The proportion of time during which
an apparatus is operating correctly. It is usually
given as a percentage.
available conversion gain The ratio of the input
power to the output power of a transducer or con-
verter. It is generally given in decibels.
54 avalanche breakdown • AX.25

mum reverse-bias voltage required to cause this
phenomenon varies among different kinds of
diodes. Some diodes are manufactured to have
precise avalanche voltages. See ZENER DIODE.
avalanche conduction In a semiconductor junc-
tion, the enhanced reverse-bias conduction
caused by a condition of AVALANCHE.
avalanche current The high current that flows
through a semiconductor junction when
avalanche diode See ZENER DIODE.
avalanche impedance The reduced impedance of
a diode during avalanche.
avalanche noise Electrical noise generated in a
junction diode operated at the point at which
avalanche just begins.
avalanche transistor A transistor that operates at
a high value of reverse-bias voltage, causing the average rectified current Abbreviation, Iavg. The
pn junction between the emitter and base to con- average value of rectifier output current before fil-
duct because of avalanche breakdown. tering. For a full-wave rectifier with a sine wave
avalanche voltage In a semiconductor P-N junc- input and a resistive load, Iavg is equal to the max-
tion, the minimum applied reverse-bias voltage imum current Im multiplied by 0.637.
that produces AVALANCHE BREAKDOWN. average rectified voltage Abbreviation, Eavg. The
AVC Abbreviation of automatic volume control. average value of rectifier output voltage before fil-
avdp Abbreviation for Avoirdupois, a weight- tering. For a full-wave rectifier with a sine-wave
measurement scheme that is used in English- input and a resistive load, Eavg is maximum volt-
speaking countries and is based on the pound. age Em multiplied by 0.637.
average absolute pulse amplitude The average average value 1. The arithmetic mean of two or more
(disregarding algebraic sign) of the absolute am- quantities. 2. The geometric mean of two or
plitudes of a pulse, taken over the duration of the more quantities. 3. The harmonic mean of two
pulse. or more quantities. 4. In ac operation, the aver-
average brightness The average brilliance of a age current, voltage, or power.
television (TV) picture, cathode-ray-tube (CRT) average voltage Abbreviation, Eavg. The average
computer display, or oscilloscope image. value of ac voltage in a circuit. Taking polarity into
average calculating operation The operating time account, this value is zero for a pure sine wave.
considered typical for a computer calculation For other waveforms, it can vary. When polarity is
(i.e., one that is longer than an addition and not considered, the sine-wave value of Eavg is equal
shorter than a multiplication); it is frequently to 0.637 times Epk, the peak value of voltage.
taken as the average of nine additions and one avg Abbreviation of average.
multiplication. aviation channels Frequency channels assigned
average current Abbreviation, Iavg. The average to the AVIATION SERVICES.
value of alternating current flowing in a circuit. Aviation services The radio-communication ser-
Taking polarity into account, this value is zero for vices used by aeronautical-mobile and radio nav-
a pure sine wave. For other waveforms, it can igation personnel.
vary. When polarity is not considered, the sine- avigation Acronym for aviation navigation. Aircraft
wave value of Iavg is equal to 0.637 times Ipk, the navigation by means of electronic equipment.
peak value of current; Iavg = 0.637 Ipk. avionics Acronym for aviation electronics. The de-
average life See MEAN LIFE. sign, production, and application of electronic de-
average noise figure The ratio of the total noise vices and systems for use in aviation, navigation,
output from a circuit to the thermal noise output and astronautics.
at 290 degrees Kelvin. It is usually expressed in Avogadro™s constant (Amedeo Avogadro, 1776“
decibels, with the noise taken at all frequencies. 1856.) Symbol, NA. The number of molecules in a
average power The average value of power in an ac kilogram-molecular weight of any substance; NA
equals 6.025 — 1026 (kg-mole)“1.
circuit. In a resistive circuit, it is the square of the
effective (rms) current times the resistance; Pavg = A voltage The filament voltage in a vacuum-tube
(Irms)2R (for sine waves). circuit.
average pulse amplitude Also called effective aW Abbreviation of attowatt.
pulse amplitude. The value obtained by integrat- AWG Abbreviation of AMERICAN WIRE GAUGE.
ing the pulse amplitude, with respect to time, AX.25 A signal format used in some digital com-
from the start of the pulse to its end, then divid- munications systems, notably amateur packet ra-
ing this integral by the pulse duration. dio.
axial leads • azusa

axial leads The centrally located leads emanating azel display A plan-position display that incor-
from the ends of cylindrical components, such as porates two different radar traces on a single
resistors and diodes. cathode-ray tube (CRT), one giving bearing, the
axial ratio The ratio of the minor to major axes of a other elevation.
waveguide™s polarization ellipse. azimuth Also called compass direction. Angular
axis 1. A coordinate in a graphical presentation or measurement in the horizontal plane, clockwise
display (e.g., horizontal and vertical axes in a from north. It is important in radio and television
rectangular coordinate system). 2. The real or communications, navigation, direction finding,
imaginary straight line around which a body ro- land surveying, and radar.
tates, or the line that passes through the center
of a symmetrical arrangement (line of symmetry).
axis of abscissas The horizontal axis (x-axis) of a
rectangular-coordinate graph or screen. Compare
axis of imaginaries The vertical axis of the com-
plex plane in which rectangular vectors lie. Com-
axis of ordinates The vertical (y-axis) of a rectan-
gular-coordinate graph or screen. Compare AXIS
axis of reals The horizontal axis of the complex
plane in which rectangular vectors lie. Compare
Ayrton-Mather galvanometer shunt A step-
adjustable universal shunt resistor for varying
the sensitivity of a galvanometer. It has the virtue
of keeping the galvanometer critically damped.
The shunt is also useful in multirange
milliammeters, microammeters, and ammeters.
The sensitive meter movement is never without a
shunting resistor during range switching.

azimuth alignment In a tape recorder, the align-
ment of record and playback head gaps so that
their centerlines are parallel.
azimuth blanking In a radar system, blacking-out
of the image as the antenna sweeps across a
specified range of azimuth angles. Effectively
eliminates nuisance echoes from stationary, per-
manent objects (such as tall buildings or commu-
nications towers).
azimuth resolution In a radar system, the mini-
mum azimuth separation of two targets whose
range (distance from the station) are equal that is
required for the system to show two echoes, rather
than one. It is generally measured in degrees.
azusa An electronic tracking system, in which a
Ayrton-Perry winding A noninductive winding single station provides slant range and two direc-
comprising two inductors conducting current in tion cosines for a distant airborne object. This ac-
opposite directions; the opposing flow cancels the curately defines the coordinates of the distant
magnetic field. object in three-dimensional space.
B 1. Symbol for SUSCEPTANCE. 2. Symbol for “walkie-talkie.” It is battery-powered and can be
FLUX DENSITY. 3. Abbreviation of BATTERY. 4. carried around. It has an inductively loaded, short
“rubber duckie” antenna similar to the antennas
Symbol for BORON. 5. Symbol for base of tran-

on cordless telephone sets. The receiver can pick
sistor (see BASE, 1). 6. Abbreviation of BASS. 7.
up signals from the transmitter at distances of up
Abbreviation of BEL. 8. Anode voltage or main op-
to about 200 feet. The radio-frequency signals
erating voltage in any circuit (when used with
pass easily through walls, ceilings, and floors.
sign). Also see B VOLTAGE.
back bias 1. A feedback signal (negative or posi-
b 1. Symbol for SUSCEPTANCE. 2. Symbol for
tive). 2. Reverse bias (also see BIAS). 3. A reverse
base of transistor (see BASE, 1). 3. Abbreviation
bias voltage, obtained from a voltage divider con-
of BASS. 4. Symbol for BARN.
nected between a voltage source and ground.
B5-cut crystal A piezoelectric plate cut from a
quartz crystal in such a way that the face of the
plate is at an angle, with respect to the z-axis of B+
the crystal. This type of crystal has good fre-
quency stability under conditions of changing
BA Abbreviation of BATTERY. Also see B and BAT.
Ba Symbol for BARIUM.
babbit A relatively soft, tin-base alloy of various
compositions. One composition contains 7.4%
antimony, 3.7% copper, and 88.9% tin. R1
babble Interference caused by crosstalk from a Out
number of channels.
babble signal A jamming signal containing babble
components. See BABBLE and JAMMING.
baby monitor A short-range radio transmitter and R2
receiver that can be used to listen at a distance to
the sounds in an infant™s room. The transmitter R1, R2
contains a sensitive microphone, a whip antenna,
and a power supply. The unit can be placed on a
table or desk, or even on the floor near the baby™s
crib. The receiver is similar to a handheld back bias, 3

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backbone • back resistance

cone feeds sound into the same area through a
backbone A form of transmission line with capaci-


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