acoustic filter Any sound-absorbing or transmitting arrangement, or combination of the two, that transmits sound waves of desired frequency while attenuating or eliminating others.
acoustic frequency response The sound frequency range as a function of sound intensity. A means of describing the performance of an acoustic device.
acoustic generator A device that produces sound waves of a desired frequency and/or intensity. Examples are electrical devices (headphones or loudspeakers operated from a suitable oscillator, buzzer, bell, or flame) and mechanical devices (tuning forks, bells, string, or whistles).
acoustic grating A set of bars or slits that are parallel to one another and arranged a fixed distance apart so that an interference pattern forms as sound passes through. Used to determine the wavelength of acoustic waves.
acoustic homing system 1. A system that uses a sound signal for guidance purposes. 2.A guidance method in which a missile homes in on noise generated by a target.
acoustic horn A tapered tube (round or rectangular, but generally funnel-shaped) that directs sound and, to some extent, amplifies it. So called to distinguish it from a microwave horn.
acoustic howl See ACOUSTIC FEEDBACK.
acoustician 1. A person skilled in acoustics (an acoustics technician). 2. An AUDIOLOGIST.
acoustic impedance Unit, ACOUSTIC OHM. The acoustic equivalent of electrical impedance. Like the latter, acoustic impedance is the total opposition encountered by acoustic force. Also like electrical impedance, acoustic impedance has resistive and reactive components: ACOUSTIC RESISTANCE and ACOUSTIC REACTANCE.
acoustic inductance Also called inertance. The acoustic equivalent of electrical inductance.
acoustic inertance See ACOUSTIC INDUCTANCE.
acoustic inhibition See AUDITORY INHIBITION.
acoustic intensity See SOUND INTENSITY.
acoustic interferometer An instrument that evaluates the frequency and velocity of sound waves in a liquid or gas, in terms of a standing wave set up by a transducer and reflector as the frequency or transducer-to-reflector distance varies.
acoustic labyrinth A loudspeaker enclosure whose internal partitions form a maze-like path or “tube”lined with sound-absorbing material. The tube effectively runs from the back of the speaker down to where it terminates in a MOUTH or PORT that opens at the front of the enclosure. The labyrinth provides an extremely efficient reproduction system because of its excellent acoustic impedance-matching capability.
acoustic lens A system of barriers that refracts sound waves the way that an optical lens does with light waves.
acoustic line Baffles or other such structures within a speaker that act as the mechanical equivalent of an electrical transmission line to enhance the reproduction of very low bass frequencies.
acoustic load A device that serves simultaneously as the output load of an amplifier and as a transducer of electrical energy into acoustic energy (e.g., headphones or a loudspeaker).
acoustic memory In a computer, a volatile memory element employing an acoustic delay line, often incorporating quartz or mercury as the transmission and delay element.
acoustic mirage A type of sound distortion in which the listener experiences the illusion of two sound sources when there is only one. The phenomenon is caused by the effect of a large temperature gradient in the air or water through which the sound passes.
acoustic mode Crystal-lattice vibration without producing an oscillating dipole.
acoustic noise Interferential (usually disagreeable) sounds carried by the air (or other propagation medium) to the ear or to an acoustic transducer. This is in contrast to electrical noise, which consists of extraneous current or voltage impulses and is inaudible until converted into sound.
acoustic ohm The unit of acoustic resistance, reactance, or impedance. One acoustic ohm equals the volume velocity of 1 cm/s produced by a sound pressure of 1 microbar (0.1 Pa). Also called acoustical ohm.
acoustic phase constant The imaginary-number component of the complex acoustic propagation constant expressed in radians per second or radians per unit distance.
acoustic phase inverter A bass reflex loudspeaker enclosure.
acoustic pressure 1. The acoustic equivalent of electromotive force, expressed in dynes per square centimeter; also called acoustical pressure. 2. Sound pressure level.
acoustic propagation The transmission of sound waves, or subaudible or ultrasonic waves, as a disturbance in a medium, rather than as an electric current or electromagnetic field.
acoustic radiator A device that emits sound waves. Examples are the cone of a loudspeaker, the diaphragm of a headphone, and the vibrating reed of a buzzer.
acoustic radiometer An instrument for measuring the intensity of a sound wave (see SOUND INTENSITY) in terms of the unidirectional steady state pressure exerted at a boundary as a result of absorption or reflection of the wave.
acoustic reactance Unit, ACOUSTIC OHM. The imaginary-number component of ACOUSTIC IMPEDANCE. It can take the form of ACOUSTIC CAPACITANCE or ACOUSTIC INDUCTANCE.
acoustic reflectivity The ratio Fr/Fi, where Fr is the rate of flow of sound energy reflected from a surface and Fi is the rate of flow of sound energy incident to the surface.
acoustic refraction The deflection of sound waves being transferred obliquely between media that transmit sound at different speeds.
acoustic regeneration See ACOUSTIC FEEDBACK.
acoustic resistance Unit, ACOUSTIC OHM. The real-number component of ACOUSTIC IMPEDANCE. The opposing force that causes acoustic energy to be dissipated in the form of heat. It is attributed to molecular friction in the medium through which sound passes. See ACOUSTIC OHM.
acoustic resonance In an enclosed chamber with walls that reflect sound waves, resonance that occurs at certain wavelengths because the echoes combine in and out of phase. Speaker enclosures almost always have resonance at certain frequencies. This effect can be used to an advantage when it is necessary to get good bass (low-frequency)
response from a relatively small speaker.
acoustic resonator 1. A chamber, such as a box, cylinder, or pipe, in which an air column resonates at a particular frequency. 2. A piezoelectric, magnetostrictive, or electrostrictive body that vibrates at a resonant audio frequency that is governed by the mechanical dimensions of the body when an audio voltage at that frequency is applied.
acoustics 1.The physics of sound. The study and applications of acoustic phenomena. 2.The qualities of an enclosure or sound chamber (room, auditorium, or box) that describe how sound waves behave in it.
acoustic scattering The spreading of a sound wave in many directions as a result of diffraction, reflection, or refraction.
acoustic suspension A loudspeaker design that allows exceptional low-frequency reproduction for a fairly small physical size. An airtight enclosure is used to increase the tension on the speaker cone.
acoustic system 1. A coordinated array of acoustic components (e.g., acoustic filters, resonators, etc.) that responds to sound energy in a predetermined manner. 2. An audio-frequency system in which sound energy is converted into electrical energy, processed, and then reconverted into sound energy for a clearly defined purpose.
acoustic telegraph A telegraph that gives audible signals, as opposed to visual signals or printed messages.
acoustic transducer 1. Any device, such as headphones or a loudspeaker, for converting audio frequency electrical signals into sound waves. 2. Any device, such as a microphone, for converting sound waves into alternating, pulsating, or fluctuating currents.
acoustic transmission The direct transmission of sound energy without the intermediary of electric currents.
acoustic transmission system A set of components designed to generate acoustic waves.
acoustic transmissivity Also called acoustic transmitivity. The ratio et/ei, where et is the
sound energy transmitted by a medium, and ei is the incident sound energy reaching the surface of the medium. Acoustic transmissivity is proportional to the angle of incidence.
acoustic treatment Application of sound-absorbing materials to the interior of an enclosure or room to control reverberation.
acoustic wave The traveling vibration, consisting of molecular motion, via which sound is transmitted through a gas, liquid or solid. Usually refers to sound waves in air.
acoustic wave filter See ACOUSTIC FILTER.
acoustoelectric effect The generation of a voltage across the faces of a crystal by sound waves traveling longitudinally through the crystal.
acoustoelectronics A branch of electronics concerned with the interaction of sound energy and electrical energy in devices, such as surface-wave filters and amplifiers. In such devices, electrically induced acoustic waves travel along the surface of a piezoelectric chip and generate electrical energy. Also called praetersonics and microwave acoustics.
ac plate current Symbol, IP(ac). The ac component of plate current in a vacuum tube.
ac plate resistance Symbol, RP(ac). The dynamic plate resistance of an electron tube. RP(ac)equals dEP/dIP, where EP is the plate voltage and IP is the plate current, for a constant value for grid voltage EG.
ac plate voltage Symbol, EP(ac). The ac component of plate voltage in an electron tube. The ac output-signal voltage in a common-cathode amplifier.
ac power Symbol, Pac. Unit, watt (W). The power acting in an ac circuit, Pac equals EI cos q, where E is in volts, I in amperes, and q is the phase angle. Compare DC POWER. Also see POWER.
ac power supply A power unit that supplies ac only (e.g., ac generator, vibrator-transformer, oscillator, or inverter). Compare DC POWER SUPPLY.
acquisition 1.The gathering of data from transducers or a computer. 2. Locating the path of an orbiting body for purposes of collecting telemetered data. 3. Orienting an antenna for optimum pickup of telemetered data.
acquisition and tracking radar An airborne or ground radar, which locks in on a strong signal and tracks the body that reflects (or transmits) the signal.
acquisition radar A radar that spots an oncoming target and supplies position data regarding the target to a fire-control or missile-guidance radar, which then tracks the target.
ac reclosing relay The controlling component in an alternating-current circuit breaker. It causes the breaker to reset after a specified period of time.
ac relay A relay designed to operate on alternating current without chattering or vibrating.
ac resistance Pure resistance in an ac circuit. Unlike reactance and impedance, which are also forms of opposition to the flow of current, ac resistance introduces no phase shift.
acrylic resin A synthetic resin used as a dielectric and in electronic encapsulations. It is made from acrylic acid or one of its derivatives.
ACS Abbreviation of automatic control system.
ac source current Symbol, IS(ac). The ac component of source current in a field-effect transistor.
ac source resistance Symbol, RS(ac). The dynamic source resistance in a field-effect transistor; RS(ac) equals dVS/dIS for a constant value of VG.
ac source voltage Symbol, VS(ac). The ac component of source voltage in a field-effect transistor. The ac output-signal voltage in a source-follower (grounded-drain) FET amplifier.
ac time overcurrent relay A device with a certain time characteristic, which breaks a circuit when the current exceeds a certain level.
actinic rays Short-wavelength light rays in the violet and ultraviolet portion of the spectrum that give conspicuous photochemical action.
actinism The property whereby radiant energy (such as visible and ultraviolet light, X-rays, etc.) causes chemical reactions.
actinium Symbol, Ac. A radioactive metallic element. Atomic number, 89. Atomic weight, 227.
actinodielectric Exhibiting a temporary rise in electrical conductivity during exposure to light.
actinoelectric effect The property whereby certain materials (such as selenium, cadmium sulfide, germanium, and silicon) change their electrical resistance or generate a voltage on exposure to light. Also see ACTINODIELECTRIC.
actinometer An instrument for measuring the direct heating power of the sun’s rays or the actinic power of a light source.
action current A small transient current that flows in a nerve in the human body as a result of stimulation.
activate To start an operation, usually by applying an appropriate enabling signal.
activation 1.Supplying electrolyte to a battery cell to prepare the cell for operation. 2. Causing the acceleration of a chemical reaction.
activation time In the activation of a battery cell (see ACTIVATION, 1), the interval between addition of the electrolyte and attainment of full cell voltage.
activator A substance added to an accelerator (see ACCELERATOR, 3) to speed the action of the accelerator.
active Pertaining to a circuit or device that requires a power supply for its operation. This differs from a passive circuit or device, which operates with no external source of power.
active antenna An antenna that uses a small whip, loop, or ferrite loopstick with a high-gain amplifier for receiving at very-low, low, medium, and high radio frequencies (approximately 9 kHz to 30 MHz).
active area The forward-current-carrying portion of the rectifying junction of a metallic rectifier.
active arm See ACTIVE LEG.
active balance In telephone repeater operation, the sum of return currents at a terminal network balanced against the local circuit or drop resistance.
active chord mechanism Abbreviation, ACM. In robots, an electromechanical gripper capable of conforming to irregular objects. It has a structure similar to the human spine, with numerous small, rigid links connected by hinges.
active communications satellite A satellite containing receivers (which pick up beamed electromagnetic signals from a ground point and amplify them) and transmitters (which send signals back to the surface of the earth). Also called active comsat. Compare PASSIVE COMMUNICATIONS SATELLITE.
active component 1. A device capable of some dynamic function (such as amplification, oscillation, or signal control) that usually requires a power supply for its operation. Examples include bipolar transistors, field-effect transistors, and integrated circuits. Compare PASSIVE COMPONENT. 2. In an ac circuit, a quantity that contains no reactance so that the current is in phase with the voltage.
active component of current See ACTIVE CURRENT.
active computer A computer in an installation or network that is processing data.
active comsat See ACTIVE COMMUNICATIONS SATELLITE.
active control system A device or circuit that compensates for irregularities in the operating environment.
active current In an ac circuit, the current component that is in phase with the voltage. This is in contrast to reactive current, which is not in phase with the voltage, and is “inactive,”with respect to power in the circuit. The active current is equal to the average power divided by the effective voltage.
active decoder An automatic ground-station device that gives the number or letter designation of a received radio beacon reply code.
active device 1. An electronic component, such as a transistor that needs a power supply, and/or that is capable of amplifying. 2.Broadly, any device (including electromechanical relays) that can switch (or amplify) by application of low-level signals.
active electric network A network containing one or more active devices or components, usually amplifiers or generators, in addition to passive devices or components.
active element The driven or RF-excited element in a multielement antenna or antenna array
active file A computer file in use (i.e., one that is being updated or referred to).
active filter A bandpass, bandstop, highpass or lowpass filter, consisting of resistors, capacitors, and operational amplifiers, arranged to pass a desired frequency response. Commonly used at audio frequencies.
active infrared detection Detection of infrared rays reflected from a target to which they were beamed.
active jamming Transmission or retransmission of signals for the purpose of disrupting communications.
active junction A pn junction in a semiconductor device that has been created by a diffusion process.
active leg An element within a transducer that changes one or more of its electrical characteristics in response to the input signal of the transducer. Also called active arm.
active lines In a U.S. television picture, the lines (approximately 488) that make up the picture. The remaining 37 of the 525 available lines are blanked and are called INACTIVE LINES.
active material 1. In a storage cell, the chemical material in the plates that provides the electrical action of the cell, as distinguished from the supporting material of the plates themselves. 2. A radioactive substance. 3.The phosphor coating of a cathode-ray tube screen. 4. The material used to coat an electron-tube cathode.
active mixer A signal mixer using one or more active components, such as transistors or integrated circuits. An active circuit provides amplification, input-output isolation, and high
input impedance, in addition to the mixing action. Compare PASSIVE MIXER.
active modulator A modulator using one or more active components, such as transistors or integrated circuits. An active circuit provides gain, input-output isolation, and high input impedance, in addition to modulation. Compare PASSIVE MODULATOR.
active network See ACTIVE ELECTRIC NETWORK.
active pressure The electromotive pressure that produces a current in an ac circuit.
active pull-up An arrangement using a transistor as a pull-up resistor replacement in an integrated circuit, providing low output impedance and low power consumption.
active RC network 1. A resistance-capacitance (RC) circuit that contains active components (transistors or integrated circuits), as well as passive components (capacitors and resistors). 2. An RC network in which some or all of the resistors and capacitors are simulated by the action of active components.
active repair time The time during which maintenance is done on a system and the system is out of operation.
active satellite See ACTIVE COMMUNICATIONS SATELLITE.
active sensor In an electronic security system, a transducer that generates an electromagnetic field or acoustic-wave field, and detects changes in the field resulting from the presence or movement of objects in the vicinity.
active substrate In an integrated circuit, a substrate consisting of single-crystal semiconductor material into which the components are formed; it acts as some or all of the components. This is in contrast to a substrate consisting of a dielectric, where the components are deposited on the surface.
active system A radio and/or radar system that requires transmitting equipment to be carried in a vehicle.
active tracking system A system in which a transponder or responder on board a vehicle retransmits information to tracking equipment (e.g., azusa, secor).
active transducer 1. A transducer that contains an active device, such as a transistor or integrated circuit, for immediate amplification of the sensed quantity. 2.A transducer that is itself an active device.
active wire In the armature of a generator, a wire experiencing induction and, therefore, is delivering voltage.
activity 1. Intensity of, as well as readiness for, oscillation in a piezoelectric crystal. 2. Radioactive intensity. 3. Intensity of thermal agitation. 4. Thermionic emission of electrons.
activity ratio The ratio of active to inactive records in a computer file.
ac transducer A transducer that either requires an ac supply voltage or delivers an ac output signal—even when operated from a dc supply.
ac transmission The use of an alternating voltage to transfer power from one point to another, usually from generators to a distribution center, and generally over a considerable distance.
actual ground The ground as “seen” by an antenna. The actual ground surface is not necessarily in the same physical location as the true ground surface (i.e., the earth itself ). An actual ground can be an artificial ground plane, such as that provided in some antenna structures. Actual ground can also be modified by nearby rooftops, buildings, guy wiring, and utility wiring.
actual height The highest altitude where radio wave refraction actually occurs.
actual power Also called active or AVERAGE POWER. Symbol, Pavg. In a resistive circuit under sine-wave conditions, average power is the product of the rms voltage and the rms current. It is also equal to half the product of the maximum current and maximum voltage.
actuating device A device or component that operates electrical contacts to affect signal transmission.
actuating system 1. An automatic or manually operated system that starts, modifies, or stops an operation. 2. A system that supplies energy for ACTUATION.
actuating time Also called actuation time. The time interval between generation of a control signal, or the mechanical operation of a control device, and the resulting ACTUATION.
actuation 1.The starting, modification, or termination of an operation or process. 2. Activation of a mechanical or electromechanical switching device.
actuator An electromechanical device that uses electromagnetism to produce a longitudinal or rotary thrust for mechanical work. It is often the end (load) device of a servosystem.
ACU Abbreviation of automatic calling unit.
ac voltage A voltage, the average value of which is zero, that periodically changes its polarity. In one cycle, an ac voltage starts at zero, rises to a maximum positive value, returns to zero, rises to a maximum negative value, and finally returns to zero. The number of such cycles per second is termed the ac frequency.
ac voltmeter See AC METER.
acyclic machine Also called ACYCLIC GENERATOR. A dc generator in which voltage induced in the active wires of the armature is always of the same polarity.
Ada A microcomputer language designed primarily for use in multi-computer systems, where each small computer communicates with the others, providing some of the advantages of a larger computer.
Adam A communications code word sometimes used for phonetic verbalizing of the letter A. More commonly, ALPHA is used.
adapter 1. A fitting used to change either the terminal scheme or the size of a jack, plug, or socket to that of another. 2. A fitting used to provide a transition from one type or style of conductor to another (e.g., waveguide to coaxial line). 3. An auxiliary system or unit used to extend the operation of another system (e.g., a citizens-band adapter for a broadcast receiver).
adaptive communication A method of communication that adjusts itself according to the particular requirements of a given time. adaptive suspension vehicle Abbreviation, ASV.
A specialized robot that moves on mechanical legs, rather than on wheels. It generally has six legs and resembles an insect. It is designed to move over extremely irregular or rocky terrain, and to carry a human passenger.
adaptivity The ability of a system to respond to its environment by changing its performance characteristics.
Adcock antenna A directional antenna system consisting of two vertical antennas, spaced in such a way that the whole array behaves like a loop antenna. Its members are connected and positioned so that it discriminates against horizontally polarized waves, and delivers output that is proportional to the vector difference of signal voltages induced in the two vertical arms.
Adcock direction finder A radio direction-finding system based on the directivity of the ADCOCK ANTENNA.
Adcock radio range A radio range system with four ADCOCK ANTENNAS situated at the corners of a square, and a fifth antenna at the center of the square.
add-and-subtract relay A stepping relay that can be switched either uprange (add) or downrange(subtract).
addend In a calculation, any number to be added to another. Compare AUGEND
addend register In a digital computer, the register that stores the addend.
adder 1. In a digital computer, the device or circuit that performs binary addition. A HALF ADDER is a two-input circuit that can produce a sum output and a carry output, but it cannot accommodate a carry signal from another adder. A FULL ADDER can accommodate a carry input, as well as two binary signals to be added. Also see ANALOG ADDER. 2. A circuit in a color TV receiver that amplifies the receiver primary matrix signal.
additive 1. The character or characters added to a code to encipher it. 2.In a calculation, an item that is to be added. 3.An ingredient, usually in a small quantity, added to another material to improve the latter in quality or performance.
additive color A color formed by combining the rays from two or three primary-colored lights onto a single neutral surface. For example, by projecting a red and a green beam onto a neutral screen, a yellow additive color results.
additive primaries Primary colors that form other colors in a mixing of light (see ADDITIVE COLOR), but are not themselves formed by mixing other additive primaries. For example, red, green, and blue are the additive primaries used in color television. Through appropriate mixing, these colors can be used to generate an unlimited variety of
other colors. Compare SUBTRACTIVE PRIMARIES, which form the color spectrum by mixing pigments rather than lights. In additive systems, each superimposed primary color increases the total light output from the reflecting (viewing) surface; in subtractive systems, each superimposed primary decreases the total reflectivity. Thus, equal combination of additive primaries produces gray or white, and equal combination of subtractive primaries produces gray or black.
addition record An extra data store created in a computer during processing.
address 1. In computer operations, a usually numerical expression designating the location of material within the memory or the destination of such material. 2.The accurately stated location of information within a computer; a data point within a grid, matrix, or table; a station within a network. 3. In computer operations, to select the location of stored information.
address comparator A device that ensures that the address being read is correct.
address computation In digital computer operations, the technique of producing or modifying only the address part of an instruction.
address field In a computer, the part of the instruction that gives the address of a bit of data (or a word) in the memory.
address generation The programmed generation of numbers or symbols used to retrieve records from a randomly stored direct-access file.
address indirect An address that specifies a storage location that contains another address.
address memory The memory sections in a digital computer that contain each individual register.
address modification In computer operations, altering only the address portion of an instruction; if the command or instruction routine is then repeated, the computer will go to the new address.
address part In a digital computer instruction, the part of an expression that specifies the location. Also called ADDRESS FIELD.
address register In a computer, a register in which an address is stored.
add/subtract time In a computer, the time required to perform addition or subtraction, excluding the time required to get the quantities from storage and to enter the sum or difference into storage.
add time In computer operations, the time required to perform addition, excluding the time required to get the quantities from storage and to enter the sum into storage.
a/d converter A device that changes an analog quantity into a digital signal. See ANALOG-TODIGITAL CONVERSION.
ADF Abbreviation of AUTOMATIC DIRECTION FINDER.
ADI Abbreviation of ALTERNATE DIGIT INVERSION.
adiabatic damping In an accelerator (see ACCELERATOR, 1), reduction of beam size as beam energy is increased.
adiabatic demagnetization A technique using a magnetic field to keep a substance at a low temperature, sometimes within a fraction of a degree of absolute zero.
adjacency A character-recognition condition in which the spacing reference lines of two characters printed consecutively in line are closer than specified.
adjacent- and alternate-channel selectivity The selectivity of a receiver or radio-frequency (RF) amplifier, with respect to adjacent-channel and alternate-channel signals. That is, the extent to which a desired signal is passed, and nearby unwanted signals are rejected.
adjacent audio channel See ADJACENT SOUND CHANNEL.
adjacent channel The channel (frequency band) immediately above or below the channel of interest.
adjacent-channel attenuation The reciprocal of the selectivity ratio of a radio receiver. The selectivity ratio is the ratio of the sensitivity of a receiver (tuned to a given channel) to its sensitivity in an adjacent channel or on a specified number of channels removed from the original.
adjacent-channel interference In television or radio reception, the interference from stations on adjacent channels. A common form arises from the picture signal in the next higher channel and the sound signal in the next lower channel.
adjacent-channel selectivity The extent to which a receiver or tuned circuit can receive on one channel and reject signals from the nearest outlying channels.
adjacent sound channel In television, the radiofrequency (RF) channel containing the sound modulation of the next lower channel.
adjacent video carrier In television, the radiofrequency (RF) carrier containing the picture
modulation of the next higher channel.
adjustable component Any circuit component whose main electrical value can be varied at will (e.g., a variable capacitor, inductor, resistor, or load).
adjustable instrument 1. An instrument whose sensitivity, range, or response can be varied at will (e.g., multirange meter or wideband generator). 2.An instrument that requires adjustment or manipulation to measure a quantity (e.g., bridge, potentiometer, or attenuator).
adjustable motor tuning An arrangement that allows the motor tuning of a receiver to be confined to a portion of the frequency spectrum.
adjustable resistor A wirewound resistor in which the resistance wire is partially exposed to allow varying the component’s value.
adjustable voltage divider A wirewound resistor with terminals that slide on exposed resistance wire to produce various voltage values.
adjusted circuit A circuit in which leads that are normally connected to a circuit breaker are shunted so that current can be measured under short-circuit conditions without breaker tripping.
adjusted decibels Noise level (in decibels) above a reference noise level (designated arbitrarily as zero decibels) measured at any point in a system with a noise meter that has previously been adjusted for zero (at reference), according to specifications.
admittance Symbol, Y. Unit, siemens (formerly mho). The property denoting the comparative ease with which an alternating current flows through a circuit or device. Admittance is the reciprocal of impedance (Z): Y = 1/Z.
adp 1. Abbreviation of AMMONIUM DIHYDROGEN PHOSPHATE, a piezoelectric compound used for sonar crystals. 2. Abbreviation of AUTOMATIC DATA PROCESSING.
adsorption Adhesion of a thin layer of molecules of one substance to the surface of another without absorption. An example is adsorption of water to the surface of a dielectric. This term is often confused with ABSORPTION because the spellings of the two words are almost identical. Compare ABSORPTION.
adu Abbreviation of automatic dialing unit.
advanced-class license An amateur-radio license conveying all operating privileges, except for a few small bands that are allocated to extra-class licensees. The second-highest class of amateur license.
advance information Data published prior to the actual production or availability of a manufactured component, circuit, or system. Advance information is often only an approximate reflection of the expected characteristics of a device.
advance wire A resistance wire used in thermocouples and precision applications. It is an alloy of copper and nickel, which has high resistivity and a negligible temperature coefficient of resistance.
aeolight A glow lamp using a cold cathode and a mixture of inert gases. Because its illumination can be regulated with an applied signal voltage, it is sometimes used as a modulation indicator for motion-picture sound recording.
aerial See ANTENNA.
aerial cable A wire or cable run through the air, using support structures, such as towers or poles.
aerodiscone antenna A miniature discone antenna designed for use on aircraft.
aerodynamics The science dealing with forces exerted by air and other gases in motion—especially upon bodies (such as aircraft) moving through these gases.
aerogram See RADIOGRAM.
aeromagnetic Pertaining to terrestrial magnetism, as surveyed from a flying aircraft.
aeronautical advisory station A civil defense and advisory communications station in service for the use of private aircraft stations.
aeronautical broadcasting service The special service that broadcasts information regarding air navigation and meteorological data pertinent to aircraft operation.
aeronautical broadcast station A station of the aeronautical broadcasting service.
aeronautical fixed service A fixed radio service that transmits information regarding air navigation and flight safety.
aeronautical fixed service station A station that operates in the aeronautical fixed service.
aeronautical ground station A land station that provides communication between aircraft and ground stations.
aeronautical marker-beacon signal A distinctive signal that designates a small area above a beacon transmitting station for aircraft navigation.
aeronautical marker-beacon station A land station that transmits an aeronautical marker beacon signal.
aeronautical mobile service A radio service consisting of communications between aircraft, and between aircraft and ground stations.
aeronautical radio-beacon station An aeronautical radio-navigation land station that transmits signals used by aircraft and other vehicles to determine their position.
aeronautical radionavigation services Services provided by stations transmitting signals used in the navigation of aircraft.
aeronautical radio service A service that encompasses aircraft-to-aircraft, aircraft-to-ground, and ground-to-aircraft communications important to the operation of aircraft.
aeronautical station A station on land, and occasionally aboard ship, operating in the aeronautical mobile service.
Aeronautical Telecommunication Agency The agency that administers the operation of stations in the aeronautical radio service.
aeronautical telecommunications Collectively, all of the electronic and nonelectronic communications used in the aeronautical service.
aeronautical utility land station A ground station in an airport control tower that provides
communications having to do with the control of aircraft and other vehicles on the ground.
aeronautical utility mobile station At an airport, a mobile station that communicates with aeronautical utility land stations and with aircraft and other vehicles on the ground.
aerophare See RADIO BEACON.
aerospace 1. The region encompassing the earth’s atmosphere and extraterrestrial space. 2.Pertaining to transport and travel in the earth’s atmosphere and in outer space. This includes aircraft, orbiting space vessels, and interplanetary spacecraft.
AES Abbreviation for Audio Engineering Society.
AEW Abbreviation of airborne (or aircraft) early warning.
aF Abbreviation of ATTOFARAD.
AF Abbreviation of AUDIO FREQUENCY.
AFC 1.Abbreviation of AUTOMATIC FREQUENCY CONTROL. 2. Abbreviation of AUDIO-FREQUENCY CHOKE.
affirmative In voice communications, a word often used for “yes”—especially when interference is present or signals are weak.