Front 4/9/01 4:38 PM Page iii The Illustrated Dictionary of Electronics Eighth Edition Stan Gibilisco Editor-in-Chief McGraw-Hill New York Chicago San Francisco Lisbon London Madrid Mexico City Milan New Delhi San Juan Seoul Singapore Sydney Toronto
5059F-pA_1-55 4/9/01 4:41 PM Page 2 2 abhenry • absolute error and is the inductance across which a current that changes at the rate of 1 abampere per sec- ond induces a potential of 1 abvolt. ABL Abbreviation of Automated Biology Laboratory (NASA). abmho The obsolete unit of conductance and of conductivity in the cgs electromagnetic system. Replaced with ABSIEMENS. abnormal dissipation Power dissipation higher or lower than the customary level, usually an over- load. abnormal oscillation 1. Oscillation where none is desired or expected, as in an amplifier. 2. Oscilla- tion at two or more frequencies simultaneously when single-frequency operation is expected. 3. Oscillation at an incorrect frequency. 4. Parasitic oscillation. abnormal propagation 1. The chance shifting of absence-of-ground searching selector A rotary the normal path of a radio wave, as by displace- switch that searches for an ungrounded contact ments in the ionosphere, so that reception is de- in a dial telephone system. graded. 2. Unintentional radiation of energy from absiemens The unit of conductance or conductiv- some point other than the transmitting antenna. ity in the cgs electromagnetic system. One 3. Propagation over a path or in a direction not absiemens equals 109 siemens and is the expected. conductance through which a potential of 1 ab- abnormal reflections Sharp, intense reflections at volt forces a current of 1 abampere. frequencies higher than the critical frequency of absolute 1. A temperature scale in which zero repre- the ionosphere’s ionized layer. sents the complete absence of heat. Units of mea- abnormal termination The shutdown of a running sure are same as units on Celsius and Fahrenheit computer program or other process. Caused by scales. See ABSOLUTE SCALE. 2. Independent of the detection of an error by the associated hard- any arbitrarily assigned units of measure or value. ware that indicates that some ongoing series of absolute accuracy The full-scale accuracy of a me- actions cannot be executed correctly. ter with respect to a primary (absolute) standard. abnormal triggering The false triggering or switch- absolute address In a digital computer program, ing of a circuit or device, such as a flip-flop, by the location of a word in memory, as opposed to some undesirable source instead of the true trigger location of the word in the program. signal. Electrical noise pulses often cause abnor- absolute code A computer code in which the exact mal triggering. address is given for storing or locating the refer- abohm The unit of resistance and of resistivity in ence operand. the cgs electromagnetic system. One abohm absolute coding In computer practice, coding that equals 10–9 ohms and is the resistance across uses absolute addresses. which a steady current of 1 abampere produces a absolute constant A mathematical constant that potential difference of 1 abvolt. has the same value wherever it is used. abort To deliberately terminate an operation, ex- absolute delay The time elapsing between the periment, process, or project before it has run its transmission of two synchronized signals from normal course. the same station or from different stations, as in AB power pack 1. A portable dry-cell or wet-cell radio, radar, or loran. By extension, the time in- array containing both A and B batteries in one terval between two such signals from any source, package. 2. An ac-operated unit in one package as from a generator. for supplying A and B voltages to equipment nor- absolute digital position transducer A digital po- mally operated from batteries. sition transducer whose output signal indicates abrasion machine An instrument for determining absolute position. (See ENCODER.) the abrasive resistance of a wire or cable. absolute efficiency The ratio Xx/Xs, where Xx is abrasion resistance A measure of the ability of a the output of a given device, and Xs is the output wire or wire covering to resist mechanical dam- of an ideal device of the same kind under the age. same operating conditions. ABS A basic programming abbreviation for the ab- absolute encoder system A system that permits solute value (of a number, variable, or expres- the encoding of any function (linear, nonlinear, sion). continuous, step, and so on) and supplies a non- abscissa 1. The independent variable in a function. ambiguous output. 2. The axis (usually horizontal) on the graph of a absolute error The difference indicated by the ap- function that indicates the independent variable. proximate value of a quantity minus the actual
5059F-pA_1-55 4/9/01 4:41 PM Page 3 absolute error • absolute tolerance 3 value. This difference is positive when the ap- proximate value is higher than the exact value, and it is negative when the approximate value is lower than the exact value. Compare RELATIVE ERROR. absolute gain Antenna gain for a given orientation when the reference antenna is isolated in space and has no main axis of propagation. absolute humidity The mass of water vapor per unit volume of air. Compare RELATIVE HUMID- ITY. absolute instruction A computer instruction that absolute Peltier coefficient The product of the states explicitly and causes the execution of a absolute Seebeck coefficient and absolute tem- specific operation. perature of a material. absolute magnitude For a complex number quan- absolute pitch A tone in a standard scale, deter- tity, the vector sum of the real and imaginary mined according to the rate of vibration, indepen- components (i.e., the square root of the sum of dent of other tones in the range of pitch. the squares of those components). Also see AB- absolute pressure Pressure (force per unit area) of SOLUTE VALUE and IMPEDANCE. a gas or liquid determined with respect to that of absolute maximum rating The highest value a a vacuum (taken as zero). quantity can have before malfunction or damage absolute-pressure transducer A transducer actu- occurs. ated by pressure from the outputs of two different absolute maximum supply voltage The highest pressure sources, and whose own output is pro- supply voltage that can be applied to a circuit portional to the difference between the two ap- without permanently altering its characteristics. plied pressures. absolute measurement of current Measurement absolute scale 1. A scale in which the zero value of a current directly in terms of defining quan- indicates the lowest physically possible value that tities. 1. TANGENT GALVANOMETER method: a parameter can attain. 2. A standard scale Current is proportional to the tangent of the an- for measurement of a quantity. 3. A universally gle of deflection of the needle of this instrument. agreed-upon scale for the determination of a vari- Deflection depends on torque, resulting from the able quantity. 4. The Kelvin temperature scale. magnetic field produced by current in the gal- 5. The Rankine temperature scale. vanometer coil acting against the horizontal absolute Seebeck coefficient The quotient, as an component of the earth’s magnetic field. integral from absolute zero to the given tempera- 2. ELECTRODYNAMOMETER method: With this ture, of the Thomson coefficient of a material di- 2-coil instrument, current is determined from vided by its absolute temperature. the observed deflection, the torque of the sus- absolute spectral response The frequency output pension fiber of the movable coil, and the coil di- or response of a device in absolute power units mensions. (such as milliwatts) as opposed to relative units absolute measurement of voltage Measurement (such as decibels). of a voltage directly in terms of defining quan- absolute system of units A system of units in tities. 1. CALORIMETRIC method: A current- which the fundamental (ABSOLUTE) units are carrying coil immersed in water raises the those expressing length (l), mass (m), charge (q), temperature of the water. The difference of and time (t). All other physical units, including potential that forces the current through the coil practical ones, are then derived from these abso- then is determined in terms of the equivalent heat lute units. energy. 2. Disk-electrometer method: In this absolute temperature Temperature measured on setup, a metal disk attached to one end of a either the Kelvin or Rankine scales, where zero balance beam is attracted by a stationary disk represents the total absence of heat energy. mounted below it, the voltage being applied to the absolute temperature scale 1. The Kelvin temper- two disks. The other end of the beam carries a ature scale, in which the divisions are equal in pan into which accurate weights are placed. At size to 1° Celsius, and the zero point is absolute balance, the voltage is determined in terms of the zero, the coldest possible temperature, approxi- weight required to restore balance, the upper-disk mately –273.16° Celsius. 2. The Rankine temper- area, and the disk separation. ature scale, in which the divisions are equal in absolute minimum resistance The resistance be- size to 1° Fahrenheit, and the zero point is abso- tween the wiper and the nearer terminal of a po- lute zero or approximately –459.7° Fahrenheit. tentiometer, when the wiper is as close to that absolute tolerance The value of a component as it terminal as physically possible. All potentiome- deviates from the specified or nominal value. It is ters have two such specifications, one for each usually expressed as a percentage of the specified end terminal. value.
5059F-pA_1-55 4/9/01 4:41 PM Page 4 4 absolute units • A-B test absolute units Fundamental physical units (see into heat or other forms of energy. 2. Loss of all or ABSOLUTE SYSTEM OF UNITS) from which all part of a skywave because of absorption by the others are derived. See, for example, AMPERE, ionosphere. Also called ionospheric absorption or OHM, VOLT, and WATT. atmospheric absorption. absolute value The magnitude of a quantity with- absorption marker A small blip introduced onto out regard to sign or direction. The absolute value an oscilloscope trace to indicate a frequency of a is written |a|. The absolute value of a posi- point. It is so called because it is produced by the tive number is the number itself; thus, |10| action of a frequency-calibrated tuned trap, simi- equals 10. The absolute value of a negative num- lar to an absorption wavemeter. ber is the number with its sign changed: |-10| absorption modulation Amplitude modulation of a equals 10. transmitter or oscillator by means of an audio- absolute-value circuit A circuit that produces a frequency-actuated absorber circuit. In its simplest unipolar signal in response to a bipolar input and form, the modulator consists of a few turns of wire in proportion to the absolute value of the magni- coupled to the transmitter tank coil and con- tude of the input. nected to a carbon microphone. The arrangement absolute-value computer A computer in which absorbs energy from the transmitter at a varying data is processed in its absolute form; i.e., every rate as the microphone changes its resistance in variable maintains its full value. (Compare to accordance with the sound waves it receives. INCREMENTAL COMPUTER.) absolute-value device In computer practice, a de- vice that delivers a constant-polarity output signal equal in amplitude to that of the input signal. Thus, the output signal always has the same sign. absolute zero The temperature –273.16°C (Ϫ459.7°F and 0 Kelvin). The coldest possible temperature, representing the complete absence of heat energy. absorbed wave A radio wave that dissipates in the ionosphere as a result of molecular agitation. This effect is most pronounced at low and medium frequencies. absorptance The amount of radiant energy ab- absorption spectrum For electromagnetic waves, a sorbed in a material; equal to 1 minus the trans- plot of absorption coefficient (of the medium of mittance. propagation) versus frequency. Also called EMIS- absorption The taking up of one material or me- SION SPECTRUM. dium by another into itself, as by sucking or absorption trap See WAVETRAP. soaking up. Also, the retention of one medium (or absorption wavemeter A resonant-frequency indi- a part of it) by another medium, through which cating instrument that is inductively coupled to the first one attempts to pass. See, for example, the device under test. ABSORBED WAVE, ABSORPTION COEFFI- CIENT, DIELECTRIC ABSORPTION. Compare ADSORPTION. absorption band See ABSORPTION SPECTRUM. absorption circuit A circuit that absorbs energy from another circuit or from a signal source—es- pecially a resonant circuit, such as a wavemeter or wavetrap. absorption current In a capacitor, the current re- sulting from absorption of energy by the dielectric material. absorption dynamometer A power-measuring in- strument in which a brake absorbs energy from a revolving shaft or wheel. absorption fading Fading of a radio wave, result- ing from (usually) slow changes in the absorption of the wave in the line of propagation. absorptivity In audio and microwave technologies, absorption frequency meter See WAVEMETER. a measure of the energy absorbed by a given vol- absorption line See ABSORPTION SPECTRUM. ume of material. absorption loss 1. Transmission loss caused by A-B test Comparison of two sounds by reproduc- dissipation of electrical energy, or conversion of it ing them in alternating succession.
5059F-pA_1-55 4/9/01 4:41 PM Page 5 abvolt • accentuation 5 abvolt The unit of potential difference in the cgs acceleration at stall The angular acceleration of a electromagnetic system. One abvolt equals 10–8 V servomotor at stall, determined from the stall and is the difference of potential between any two torque and the moment of inertia of the motor’s points when 1 erg of work is required to move 1 rotor. abcoulomb of electricity between them. acceleration derivative Acceleration (a) expressed abwatt The unit of power in the cgs electromagnetic as the second derivative of distance (s) with re- system. One abwatt equals 10Ϫ7 W and is the spect to time (t): a equals d 2s/dt 2. power corresponding to 1 erg of work per second. acceleration potential See ACCELERATING ac 1. Abbreviation of ALTERNATING CURRENT. VOLTAGE. 2. Abbreviation of ATTITUDE CONTROL. 3. Ab- acceleration switch A switch that operates auto- breviation of AERODYNAMIC CENTER. 4. A suf- matically when the acceleration of a body to fix meaning AUTOMATIC CALCULATOR or which it is attached exceeds a predetermined rate AUTOMATIC COMPUTER. in a given direction. a/c 1. Abbreviation of AIRCRAFT. 2. Abbreviation acceleration time The time required by a com- of AIR CONDITIONING. puter to take in or deliver information after inter- Ac Symbol for ACTINIUM. preting instructions. Compare ACCESS TIME. ACA Abbreviation of automatic circuit analyzer. acceleration torque During the accelerating pe- ac base current Symbol, IB(ac). The ac component of riod of a motor, the difference between the torque base current in a bipolar transistor. demanded and the torque actually produced by ac base resistance Symbol, RB (ac). The dynamic the motor. base resistance in a bipolar transistor. acceleration voltage The potential between accel- ac base voltage Symbol, VB(ac). The ac component erating elements in a vacuum tube, the value of of base voltage in a bipolar transistor. It is the ac which determines average electron velocity. input signal voltage in a common-emitter ampli- accelerometer A transducer whose output voltage fier or emitter-follower amplifier. is proportional to the acceleration of the moving ac bias In a tape recorder, the high-frequency cur- body to which it is attached. rent that passes through the recording head to accentuation The emphasis of a desired band of linearize operation. frequencies, usually in the audio-frequency spec- acc 1. Abbreviation of AUTOMATIC CHROMI- trum. NANCE CONTROL. 2. Abbreviation of AUTO- MATIC COLOR COMPENSATION. 3. Abbreviation of ACCELERATION. ac cathode current Symbol, IK(ac). The ac compo- nent of cathode current in an electron tube. ac cathode resistance Symbol, RK(ac). The dynamic cathode resistance in an electron tube. RK(ac) equals dVK/dIK for a constant value of VG. ac cathode voltage Symbol, VK(ac). The ac compo- nent of cathode voltage in an electron tube. It is the ac output signal voltage in cathode-follower and grounded-grid amplifiers. accelerated life test A test program that simu- lates the effects of time on devices or apparatus, by artificially speeding up the aging process. accelerated service test A service or bench test in which equipment or a circuit is subjected to an extreme condition in an attempt to simulate the effects of average use over a long time. accelerating conductor or relay A conductor or relay that prompts the operation of a succeeding device in a starting mode according to established conditions. accelerating electrode In a cathode-ray tube or klystron, the electrode to which the accelerating voltage is applied. accelerating time The elapsed time that starts when voltage is applied to a motor, and ends when the motor shaft reaches maximum speed. accelerating voltage A positive high voltage applied to the accelerating electrode of a cathode-ray tube to increase the velocity of electrons in the beam.
5059F-pA_1-55 4/9/01 4:41 PM Page 6 6 accentuator • accuracy rating accentuator A circuit or device, such as a filter, RC(ac) equals dVC/dIC for a constant value of base tone control, or equalizer, used to emphasize a current IB (in a common-emitter circuit) or emit- band of frequencies, usually in the audio- ter current IE (in a common-base circuit). frequency spectrum. Also see ACCENTUATION. ac collector voltage Symbol, VC(ac). The ac compo- acceptable-environmental-range test A test to nent of collector voltage in a bipolar transistor. disclose the environmental conditions that equip- The ac output signal voltage in a common-emitter ment can endure while maintaining at least the or common-base amplifier. minimum desired reliability. accompanying audio channel The RF signal that acceptable quality level Abbreviation, AQL. A per- supplies television sound. Also called Cochannnel centage that represents an acceptable average of sound frequency. defective components allowable for a process, or ac component In a complex wave (i.e., one con- the lowest quality that a supplier is permitted to taining both ac and dc), the alternating, fluctu- regularly present for acceptance. ating, or pulsating part of the combination. acceptance sampling plan A probabilistic method Compare DC COMPONENT. of sampling a quantity of units from a lot, and de- accordion A printed-circuit connector contact with termining from the sample whether to accept the a Z-shaped spring that allows high deflection lot, reject the lot, or perform another sampling. with low fatigue. acceptance test A test performed on incoming ac-coupled flip-flop A flip-flop that is operated by equipment or on submitted samples to determine the rise or fall of a clock pulse. if they meet tester’s or supplier’s specifications. ac coupling Transformer coupling or capacitive acceptor 1. Any device or circuit, such as a series- coupling, which transmit ac, but not dc. Compare resonant circuit, that provides relatively easy DIRECT COUPLING. transmission of a signal, in effect accepting the signal. 2. A hole-rich impurity added to a semi- conductor to make the latter p-type. It is so called because its holes can accept electrons. Compare DONOR. acceptor circuit See ACCEPTOR, 1. acceptor impurity See ACCEPTOR, 2. access 1. To gain entrance to something, such as the interior of the cabinet of a high-fidelity ampli- fier. 2. In a computer, the action of going to a spe- cific memory location for the purpose of data retrieval. 3. A port or opening into a piece of equipment, placed there to make the equipment easy to maintain and repair. access arm A mechanical device that positions the read/write mechanism in a computer storage unit. access control register A register that is part of a computer protection system that prevents inter- ference between different software modules. access method A method of transferring informa- tion or data from main storage to an input/out- put unit. access right The access status given to computer system users that indicates the method of access permitted (e.g., read a file only or write to a file). access time The time required by a computer to begin delivering information after the memory or accumulator 1. In a digital computer, a circuit or storage has been interrogated. register device that receives numbers, totals accidental error An unintentional error commit- them, and stores them. 2. Storage battery. ted by a person making measurements and accuracy 1. Precision in the measurement of recording data. quantities and in the statement of physical char- accidental triggering The undesired chance- acteristics. 2. Degree of precision. Usually ex- operation of a flip-flop or other switching circuit pressed, in terms of error, as a percentage of the caused by a noise pulse or other extraneous sig- specified value (e.g., 10 V plus or minus 1%), as a nal. percentage of a range (e.g., 2% of full scale), or as ac collector current Symbol, IC(ac). The ac compo- parts (e.g., 100 parts per million). nent of collector current in a bipolar transistor. accuracy rating The maximum error in an instru- ac collector resistance Symbol, RC(ac). The dy- ment, given as a percentage of the full-scale namic collector resistance of a bipolar transistor. value.
5059F-pA_1-55 4/9/01 4:41 PM Page 7 accw • ac magnetic bias 7 accw Abbreviation of ALTERNATING-CURRENT ac generator 1. A rotating electromagnetic ma- CONTINUOUS WAVE. chine that produces alternating current (e.g., a ac/dc Abbreviation of ALTERNATING CURRENT/ dynamo or alternator). 2. An oscillator or com- DIRECT CURRENT. Pertains to equipment that bination of an oscillator and an output ampli- will operate from either ac utility power or a dc fier. power source. A notebook computer is a good ex- ac grid voltage Symbol, VG(ac). The ac component ample. of control grid voltage in an electron tube. The ac ac directional overcurrent relay A relay that input signal voltage in a common-cathode ampli- works on a specific value of alternating overcur- fier or cathode follower. rent that is rectified for a desired polarity. A channel The left channel of a two-channel stereo ac drain current Symbol, ID(ac). The ac component system. of drain current in a field-effect transistor. achieved reliability A statement of reliability based ac drain resistance Symbol, RD(ac). The dynamic on the performance of mass-produced parts or drain resistance in a field-effect transistor; RD(ac) systems under similar environmental conditions. equals dVD/dID for a constant value of gate volt- Also called OPERATIONAL RELIABILITY. age VG. achromatic 1. Without color. In a TV image, the ac drain voltage Symbol, VD(ac). The ac component tones from black through gray to white. The term of drain voltage in a field-effect transistor. The ac occasionally refers to black-and-white television, output signal voltage in a common-source FET although MONOCHROMATIC is more often used amplifier. in this sense. ac dump The removal of all ac power from a system achromatic locus Also called achromatic region. or component. An area on a chromaticity diagram that contains ac emitter current Symbol, IE(ac). The ac compo- all points, representing acceptable reference nent of emitter current in a bipolar transistor. white standards. ac emitter resistance Symbol, RE(ac). The dynamic achromatic scale A musical scale without acci- emitter resistance of a bipolar transistor; RE(ac) dentals. equals dVE/dIE for a constant value of base cur- ACIA Abbreviation of asynchronous communica- rent IB (in an emitter-follower circuit) or collector tions interface adapter. voltage VCC (in a common-base circuit). acicular Pertaining to the shape of magnetic parti- ac emitter voltage Symbol, VE(ac). The ac compo- cles on recording tape. Under magnification, nent of emitter voltage in a bipolar transistor. The these particles look like thin rods. ac input signal voltage in a common-base ampli- acid A substance that dissociates in water solution fier; the ac output signal voltage in an emitter- and forms hydrogen (H) ions (e.g., sulfuric acid). follower amplifier. Compare BASE, 2. ac equipment An apparatus designed for opera- acid depolarizer Also called acidic depolarizer. tion from an ac power source only. Compare DC An acid, in addition to the electrolyte, used in EQUIPMENT and AC/DC. some primary cells to slow the process of polar- ac erasing In tape recording, the technique of us- ization. ing an alternating magnetic field to erase material ac line A power line that delivers alternating cur- already recorded on the tape. rent only. ac erasing head Also called ac erase head. In tape ac line filter A filter designed to remove extrane- and wire recording, a head that carries alternat- ous signals or electrical noise from an ac power ing current to erase material already recorded on line, while causing virtually no reduction of the the tape or wire. Also see AC ERASING. power-line voltage or power. acetate Cellulose acetate, a tough thermoplastic ac line voltage The voltage commonly delivered material that is an acetic acid ester of cellulose. It by the commercial power line to consumers. In is used as a dielectric and in the manufacture of the United States, the two standards are 117 V photographic films. and 234 V (~ about 5 percent). The lower voltage acetate base 1. The cellulose acetate film that is used by most appliances; the higher voltage is served as the base for the magnetic oxide coating intended for appliances and equipment that in early recording tape. Most such tapes today draws high power, such as electric ovens, cook- are of polyester base. 2. The cellulose acetate ing ranges, clothes dryers, and amateur-radio substrate onto which certain photosensitive ma- amplifiers. In Europe, 220 V is the common terials are deposited for lithographic reproduc- standard. tion. Also see ACETATE and ANCHORAGE. aclinic line Also called magnetic equator. An imag- acetate tape Recording tape consisting of a mag- inary line drawn on a map of the world or of an netic oxide coating on a cellulose acetate film. area that connects points of zero inclination (dip) Also see ACETATE BASE. of the needle of a magnetic compass. ac gate voltage Symbol, VG(ac). The ac component ACM Abbreviation for Association for Computing of gate voltage in a field-effect transistor. The ac Machinery. input signal voltage. ac magnetic bias See AC BIAS.
5059F-pA_1-55 4/9/01 4:41 PM Page 8 8 ac meter • acoustic feedback ac meter A meter that is intended to work only on quencies. If one fork is struck and then brought alternating current or voltage. Such meters in- near the other, the second fork will begin vibrating. clude iron-vane and rectifier types. If the second fork has a fundamental frequency that is a harmonic of the frequency of the first fork, the second fork will vibrate at its own resonant frequency. See HARMONIC, RESONANCE. acoustic coupling Data transfer via a sound link between a telephone and a pickup/reproducer. Was once common in computer terminals and facsimile machines. This scheme has been largely replaced by hard wiring and optical coupling. acoustic damping The deadening or reduction of the vibration of a body to eliminate (or cause to die out quickly) sound waves arising from it. acoustic delay line Any equivalent of a special transmission line that introduces a useful time delay between input and output signals. In one ac noise 1. Electromagnetic interference originat- form, it consists of a crystal block or bar with an ing in the ac power lines. 2. Electrical noise of a input transducer at one end and an output trans- rapidly alternating or pulsating nature. ducer at the other. An electrical input signal in ac noise immunity In computer practice, the abil- the first transducer sets up sound waves that ity of a logic circuit to maintain its state, despite travel through the interior of the crystal; the excitation by ac noise. piezoelectric reaction of the crystal to sound vi- acous Abbreviation for ACOUSTIC. brations sets up an output voltage in the second acoustic Pertaining to audible sound distur- transducer. The delay is caused by the time re- bances, usually in air (versus audio-frequency quired for the acoustic energy to travel the length currents or voltages). of the crystal bar. acoustic absorption The assimilation of energy from sound waves passing through or reflected by a given medium. acoustic absorption loss That portion of sound energy lost (as by dissipation in the form of heat) because of ACOUSTIC ABSORPTION. acoustic absorptivity The ratio of sound energy absorbed by a material to sound energy striking the surface of the material. acoustic attenuation constant The real-number component of the complex acoustical propagation constant, expressed in nepers per unit distance. acoustic burglar alarm An alarm that receives the noise made by an intruder. The alarm device re- acoustic depth finder A direct-reading device for sponds to the impulses from concealed micro- determining the depth of a body of water, or for phones. locating underwater objects via sonic or ultra- acoustic capacitance The acoustic equivalent of sonic waves transmitted downward and reflected electrical capacitance. back to the instrument. acoustic clarifier In a loudspeaker system, a set of acoustic dispersion Variation of the velocity of cones attached to the baffle that vibrate to absorb sound waves, depending on their frequency. and suppress sound energy during loud bursts. acoustic elasticity 1. In a loudspeaker enclosure, acoustic communication Communications by the compressibility of air behind the vibrating means of sound waves. This can be through the cone of the speaker. 2. In general, the compress- atmosphere, or it can be through solids or liq- ibility of any medium through which sound uids, such as a taut wire, a body of water, or the passes. earth. acoustic electric transducer A transducer, such acoustic compliance COMPLIANCE in acoustic as a microphone or hydrophone, that converts transducers, especially loudspeakers. It is equiv- sound energy into electrical energy. Compare alent to electrical capacitive reactance. ELECTRICAL/ACOUSTIC TRANSDUCER. Also acoustic consonance An effect that occurs when see ACOUSTIC TRANSDUCER. two objects are near each other but not in physical acoustic feedback A usually undesirable effect contact, and both have identical or harmonically that occurs when sound waves from a loud- related resonant frequencies. An example is shown speaker (or other reproducer) reach a microphone by two tuning forks with identical fundamental fre- (or other input transducer) in the same system.
5059F-pA_1-55 4/9/01 4:41 PM Page 9 acoustic feedback • acoustic radiator 9 This can cause an amplifier to oscillate, with a re- sultant rumbling, howling, or whistling. acoustic filter Any sound-absorbing or transmit- ting 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 acoustic line Baffles or other such structures loudspeakers operated from a suitable oscillator, within a speaker that act as the mechanical equiv- buzzer, bell, or flame) and mechanical devices alent of an electrical transmission line to enhance (tuning forks, bells, string, or whistles). the reproduction of very low bass frequencies. acoustic grating A set of bars or slits that are par- acoustic load A device that serves simultaneously allel to one another and arranged a fixed distance as the output load of an amplifier and as a trans- apart so that an interference pattern forms as ducer of electrical energy into acoustic energy sound passes through. Used to determine the (e.g., headphones or a loudspeaker). wavelength of acoustic waves. acoustic memory In a computer, a volatile mem- acoustic homing system 1. A system that uses a ory element employing an acoustic delay line, of- sound signal for guidance purposes. 2. A guid- ten incorporating quartz or mercury as the ance method in which a missile homes in on transmission and delay element. noise generated by a target. acoustic mirage A type of sound distortion in acoustic horn A tapered tube (round or rectangu- which the listener experiences the illusion of two lar, but generally funnel-shaped) that directs sound sources when there is only one. The phe- sound and, to some extent, amplifies it. So called nomenon is caused by the effect of a large tem- to distinguish it from a microwave horn. perature gradient in the air or water through acoustic howl See ACOUSTIC FEEDBACK. which the sound passes. acoustician 1. A person skilled in acoustics (an acoustic mode Crystal-lattice vibration without acoustics technician). 2. An AUDIOLOGIST. producing an oscillating dipole. acoustic impedance Unit, ACOUSTIC OHM. The acoustic noise Interferential (usually disagreeable) acoustic equivalent of electrical impedance. Like sounds carried by the air (or other propagation the latter, acoustic impedance is the total opposi- medium) to the ear or to an acoustic transducer. tion encountered by acoustic force. Also like elec- This is in contrast to electrical noise, which con- trical impedance, acoustic impedance has sists of extraneous current or voltage impulses resistive and reactive components: ACOUSTIC and is inaudible until converted into sound. RESISTANCE and ACOUSTIC REACTANCE. acoustic ohm The unit of acoustic resistance, re- acoustic inductance Also called inertance. The actance, or impedance. One acoustic ohm equals acoustic equivalent of electrical inductance. the volume velocity of 1 cm/s produced by a acoustic inertance See ACOUSTIC INDUCTANCE. sound pressure of 1 microbar (0.1 Pa). Also called acoustic inhibition See AUDITORY INHIBITION. acoustical ohm. acoustic intensity See SOUND INTENSITY. acoustic phase constant The imaginary-number acoustic interferometer An instrument that eval- component of the complex acoustic propagation uates the frequency and velocity of sound waves constant expressed in radians per second or radi- in a liquid or gas, in terms of a standing wave set ans per unit distance. up by a transducer and reflector as the frequency acoustic phase inverter A bass reflex loudspeaker or transducer-to-reflector distance varies. enclosure. acoustic labyrinth A loudspeaker enclosure acoustic pressure 1. The acoustic equivalent of whose internal partitions form a maze-like path electromotive force, expressed in dynes per or “tube” lined with sound-absorbing material. square centimeter; also called acoustical pres- The tube effectively runs from the back of the sure. 2. Sound pressure level. speaker down to where it terminates in a MOUTH acoustic propagation The transmission of sound or PORT that opens at the front of the enclosure. waves, or subaudible or ultrasonic waves, as a The labyrinth provides an extremely efficient re- disturbance in a medium, rather than as an elec- production system because of its excellent acous- tric current or electromagnetic field. tic impedance-matching capability. acoustic radiator A device that emits sound acoustic lens A system of barriers that refracts waves. Examples are the cone of a loudspeaker, sound waves the way that an optical lens does the diaphragm of a headphone, and the vibrating with light waves. reed of a buzzer.
5059F-pA_1-55 4/9/01 4:41 PM Page 10 10 acoustic radiometer • ac plate resistance acoustic radiometer An instrument for measuring acoustic scattering The spreading of a sound the intensity of a sound wave (see SOUND IN- wave in many directions as a result of diffraction, TENSITY) in terms of the unidirectional steady- reflection, or refraction. state pressure exerted at a boundary as a result acoustic suspension A loudspeaker design that of absorption or reflection of the wave. allows exceptional low-frequency reproduction acoustic reactance Unit, ACOUSTIC OHM. The for a fairly small physical size. An airtight enclo- imaginary-number component of ACOUSTIC sure is used to increase the tension on the IMPEDANCE. It can take the form of ACOUSTIC speaker cone. CAPACITANCE or ACOUSTIC INDUCTANCE. acoustic system 1. A coordinated array of acous- acoustic reflectivity The ratio Fr/Fi, where Fr is tic components (e.g., acoustic filters, resonators, the rate of flow of sound energy reflected from a etc.) that responds to sound energy in a predeter- surface and Fi is the rate of flow of sound energy mined manner. 2. An audio-frequency system in incident to the surface. which sound energy is converted into electrical acoustic refraction The deflection of sound waves energy, processed, and then reconverted into being transferred obliquely between media that sound energy for a clearly defined purpose. transmit sound at different speeds. acoustic telegraph A telegraph that gives audible acoustic regeneration See ACOUSTIC FEEDBACK. signals, as opposed to visual signals or printed acoustic resistance Unit, ACOUSTIC OHM. The messages. real-number component of ACOUSTIC IMPE- acoustic transducer 1. Any device, such as head- DANCE. The opposing force that causes acoustic phones or a loudspeaker, for converting audio- energy to be dissipated in the form of heat. It is frequency electrical signals into sound waves. 2. attributed to molecular friction in the medium Any device, such as a microphone, for converting through which sound passes. See ACOUSTIC sound waves into alternating, pulsating, or fluc- OHM. tuating currents. acoustic resonance In an enclosed chamber with acoustic transmission The direct transmission of walls that reflect sound waves, resonance that oc- sound energy without the intermediary of electric curs at certain wavelengths because the echoes currents. combine in and out of phase. Speaker enclosures acoustic transmission system A set of compo- almost always have resonance at certain frequen- nents designed to generate acoustic waves. cies. This effect can be used to an advantage when acoustic transmissivity Also called acoustic it is necessary to get good bass (low-frequency) transmitivity. The ratio et/ei, where et is the response from a relatively small speaker. sound energy transmitted by a medium, and ei is acoustic resonator 1. A chamber, such as a box, the incident sound energy reaching the surface of cylinder, or pipe, in which an air column resonates the medium. Acoustic transmissivity is propor- at a particular frequency. 2. A piezoelectric, mag- tional to the angle of incidence. netostrictive, or electrostrictive body that vibrates acoustic treatment Application of sound-absorb- at a resonant audio frequency that is governed by ing materials to the interior of an enclosure or the mechanical dimensions of the body when an room to control reverberation. audio voltage at that frequency is applied. acoustic wave The traveling vibration, consisting of molecular motion, via which sound is trans- mitted 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 trav- eling longitudinally through the crystal. acoustoelectronics A branch of electronics con- cerned 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 en- ergy. Also called praetersonics and microwave acoustics. ac plate current Symbol, IP(ac). The ac component of plate current in a vacuum tube. acoustics 1. The physics of sound. The study and ac plate resistance Symbol, RP(ac). The dynamic applications of acoustic phenomena. 2. The qual- plate resistance of an electron tube. RP(ac) equals ities of an enclosure or sound chamber (room, dEP/dIP, where EP is the plate voltage and IP is the auditorium, or box) that describe how sound plate current, for a constant value for grid volt- waves behave in it. age EG.
5059F-pA_1-55 4/9/01 4:41 PM Page 11 ac plate voltage • active chord mechanism 11 ac plate voltage Symbol, EP(ac). The ac component ac source voltage Symbol, VS(ac). The ac compo- of plate voltage in an electron tube. The ac out- nent of source voltage in a field-effect transistor. put-signal voltage in a common-cathode ampli- The ac output-signal voltage in a source-follower fier. (grounded-drain) FET amplifier. ac power Symbol, Pac. Unit, watt (W). The power acss Abbreviation of analog computer subsystem. acting in an ac circuit, Pac equals EI cos q, where ac time overcurrent relay A device with a certain E is in volts, I in amperes, and q is the phase an- time characteristic, which breaks a circuit when gle. Compare DC POWER. Also see POWER. the current exceeds a certain level. ac power supply A power unit that supplies ac actinic rays Short-wavelength light rays in the vi- only (e.g., ac generator, vibrator-transformer, os- olet and ultraviolet portion of the spectrum that cillator, or inverter). Compare DC POWER give conspicuous photochemical action. SUPPLY. actinism The property whereby radiant energy acquisition 1. The gathering of data from trans- (such as visible and ultraviolet light, X-rays, etc.) ducers or a computer. 2. Locating the path of an causes chemical reactions. orbiting body for purposes of collecting teleme- actinium Symbol, Ac. A radioactive metallic ele- tered data. 3. Orienting an antenna for optimum ment. Atomic number, 89. Atomic weight, 227. pickup of telemetered data. actinodielectric Exhibiting a temporary rise in acquisition and tracking radar An airborne or electrical conductivity during exposure to light. ground radar, which locks in on a strong signal actinoelectric effect The property whereby cer- and tracks the body that reflects (or transmits) tain materials (such as selenium, cadmium sul- the signal. fide, germanium, and silicon) change their acquisition radar A radar that spots an oncoming electrical resistance or generate a voltage on ex- target and supplies position data regarding the posure to light. Also see ACTINODIELECTRIC. target to a fire-control or missile-guidance radar, actinometer An instrument for measuring the di- which then tracks the target. rect heating power of the sun’s rays or the actinic acr 1. Abbreviation of AUDIO CASSETTE RE- power of a light source. CORDER. 2. Abbreviation of AUDIO CASSETTE action current A small transient current that RECORDING SYSTEM. flows in a nerve in the human body as a result of ac reclosing relay The controlling component in stimulation. an alternating-current circuit breaker. It causes activate To start an operation, usually by applying the breaker to reset after a specified period of an appropriate enabling signal. time. activation 1. Supplying electrolyte to a battery cell ac relay A relay designed to operate on alternating to prepare the cell for operation. 2. Causing the current without chattering or vibrating. acceleration of a chemical reaction. ac resistance Pure resistance in an ac circuit. Un- activation time In the activation of a battery cell like reactance and impedance, which are also (see ACTIVATION, 1), the interval between addi- forms of opposition to the flow of current, ac re- tion of the electrolyte and attainment of full cell sistance introduces no phase shift. voltage. acronym A word formed from letters or syllables activator A substance added to an accelerator (see taken from other applicable words of a multiword ACCELERATOR, 3) to speed the action of the ac- term. Acronyms are convenient for naming new celerator. devices and processes in electronics. Usually, a active Pertaining to a circuit or device that re- term is considered an acronym only when it is quires a power supply for its operation. This dif- spelled in all-capital letters; once the term is ac- fers from a passive circuit or device, which cepted and popularized, it is written as a conven- operates with no external source of power. tional word and is no longer thought of as an active antenna An antenna that uses a small acronym. For example, LASER was once an whip, loop, or ferrite loopstick with a high-gain acronym for light amplification by the stimulated amplifier for receiving at very-low, low, medium, emission of radiation. By the popularization pro- and high radio frequencies (approximately 9 kHz cess, the acronym became a conventional word to 30 MHz). from which other terms (such as the verb “lase”) active area The forward-current-carrying portion were derived. of the rectifying junction of a metallic rectifier. acrylic resin A synthetic resin used as a dielectric active arm See ACTIVE LEG. and in electronic encapsulations. It is made from active balance In telephone repeater operation, the acrylic acid or one of its derivatives. sum of return currents at a terminal network bal- ACS Abbreviation of automatic control system. anced against the local circuit or drop resistance. ac source current Symbol, IS(ac). The ac component active chord mechanism Abbreviation, ACM. In of source current in a field-effect transistor. robots, an electromechanical gripper capable of ac source resistance Symbol, RS(ac). The dynamic conforming to irregular objects. It has a structure source resistance in a field-effect transistor; RS(ac) similar to the human spine, with numerous equals dVS/dIS for a constant value of VG. small, rigid links connected by hinges.
5059F-pA_1-55 4/9/01 4:41 PM Page 12 12 active communications satellite • active repair time active communications satellite A satellite con- active file A computer file in use (i.e., one that is taining receivers (which pick up beamed electro- being updated or referred to). magnetic signals from a ground point and amplify active filter A bandpass, bandstop, highpass or them) and transmitters (which send signals back lowpass filter, consisting of resistors, capacitors, to the surface of the earth). Also called active and operational amplifiers, arranged to pass a de- comsat. Compare PASSIVE COMMUNICATIONS sired frequency response. Commonly used at au- SATELLITE. dio frequencies. active component 1. A device capable of some dy- active infrared detection Detection of infrared namic function (such as amplification, oscilla- rays reflected from a target to which they were tion, or signal control) that usually requires a beamed. power supply for its operation. Examples include active jamming Transmission or retransmission bipolar transistors, field-effect transistors, and of signals for the purpose of disrupting communi- integrated circuits. Compare PASSIVE COMPO- cations. NENT. 2. In an ac circuit, a quantity that con- active junction A pn junction in a semiconductor tains no reactance so that the current is in phase device that has been created by a diffusion pro- with the voltage. cess. active component of current See ACTIVE CUR- active leg An element within a transducer that RENT. changes one or more of its electrical characteris- active computer A computer in an installation or tics in response to the input signal of the trans- network that is processing data. ducer. Also called active arm. active comsat See ACTIVE COMMUNICATIONS active lines In a U.S. television picture, the lines SATELLITE. (approximately 488) that make up the picture. active control system A device or circuit that The remaining 37 of the 525 available lines are compensates for irregularities in the operating blanked and are called INACTIVE LINES. environment. active material 1. In a storage cell, the chemical active current In an ac circuit, the current compo- material in the plates that provides the electrical nent that is in phase with the voltage. This is in action of the cell, as distinguished from the sup- contrast to reactive current, which is not in phase porting material of the plates themselves. 2. A ra- with the voltage, and is “inactive,” with respect to dioactive substance. 3. The phosphor coating of a power in the circuit. The active current is equal to cathode-ray tube screen. 4. The material used to the average power divided by the effective voltage. coat an electron-tube cathode. active decoder An automatic ground-station de- active mixer A signal mixer using one or more ac- vice that gives the number or letter designation of tive components, such as transistors or in- a received radio beacon reply code. tegrated circuits. An active circuit provides active device 1. An electronic component, such as a amplification, input-output isolation, and high transistor that needs a power supply, and/or that input impedance, in addition to the mixing ac- is capable of amplifying. 2. Broadly, any device (in- tion. Compare PASSIVE MIXER. cluding electromechanical relays) that can switch active modulator A modulator using one or more (or amplify) by application of low-level signals. active components, such as transistors or inte- active electric network A network containing one grated circuits. An active circuit provides gain, or more active devices or components, usually input-output isolation, and high input impe- amplifiers or generators, in addition to passive dance, in addition to modulation. Compare PAS- devices or components. SIVE MODULATOR. active element The driven or RF-excited element active network See ACTIVE ELECTRIC NET- in a multielement antenna or antenna array. WORK. 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 pas- sive components (capacitors and resistors). 2. An RC network in which some or all of the resistors and capacitors are simulated by the action of ac- tive components. active repair time The time during which mainte- nance is done on a system and the system is out of operation.
5059F-pA_1-55 4/9/01 4:41 PM Page 13 active satellite • adapter 13 ily 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 prod- uct 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 oper- ates electrical contacts to affect signal transmis- sion. active satellite See ACTIVE COMMUNICATIONS actuating system 1. An automatic or manually SATELLITE. operated system that starts, modifies, or stops an active sensor In an electronic security system, a operation. 2. A system that supplies energy for transducer that generates an electromagnetic ACTUATION. field or acoustic-wave field, and detects changes actuating time Also called actuation time. The in the field resulting from the presence or move- time interval between generation of a control sig- ment of objects in the vicinity. nal, or the mechanical operation of a control de- active substrate In an integrated circuit, a sub- vice, and the resulting ACTUATION. strate consisting of single-crystal semiconductor actuation 1. The starting, modification, or termi- material into which the components are formed; nation of an operation or process. 2. Activation of it acts as some or all of the components. This is in a mechanical or electromechanical switching de- contrast to a substrate consisting of a dielectric, vice. where the components are deposited on the sur- actuator An electromechanical device that uses face. electromagnetism to produce a longitudinal or ro- active system A radio and/or radar system that tary thrust for mechanical work. It is often the requires transmitting equipment to be carried in end (load) device of a servosystem. a vehicle. ACU Abbreviation of automatic calling unit. active tracking system A system in which a ac voltage A voltage, the average value of which is transponder or responder on board a vehicle re- zero, that periodically changes its polarity. In one transmits information to tracking equipment cycle, an ac voltage starts at zero, rises to a max- (e.g., azusa, secor). imum positive value, returns to zero, rises to a active transducer 1. A transducer that contains maximum negative value, and finally returns to an active device, such as a transistor or inte- zero. The number of such cycles per second is grated circuit, for immediate amplification of the termed the ac frequency. sensed quantity. 2. A transducer that is itself an ac voltmeter See AC METER. active device. acyclic machine Also called ACYCLIC GENERA- active wire In the armature of a generator, a wire TOR. A dc generator in which voltage induced in experiencing induction and, therefore, is deliver- the active wires of the armature is always of the ing voltage. same polarity. activity 1. Intensity of, as well as readiness for, os- A/D Abbreviation for ANALOG-TO-DIGITAL. See cillation in a piezoelectric crystal. 2. Radioactive ANALOG-TO-DIGITAL CONVERSION. intensity. 3. Intensity of thermal agitation. 4. Ada A microcomputer language designed primarily Thermionic emission of electrons. for use in multi-computer systems, where each activity ratio The ratio of active to inactive records small computer communicates with the others, in a computer file. providing some of the advantages of a larger com- ac transducer A transducer that either requires an puter. ac supply voltage or delivers an ac output sig- Adam A communications code word sometimes nal—even when operated from a dc supply. used for phonetic verbalizing of the letter A. More ac transmission The use of an alternating voltage commonly, ALPHA is used. to transfer power from one point to another, usu- adapter 1. A fitting used to change either the ter- ally from generators to a distribution center, and minal scheme or the size of a jack, plug, or socket generally over a considerable distance. to that of another. 2. A fitting used to provide a actual ground The ground as “seen” by an an- transition from one type or style of conductor to tenna. The actual ground surface is not necessar- another (e.g., waveguide to coaxial line). 3. An
5059F-pA_1-55 4/9/01 4:41 PM Page 14 14 adapter • address generation 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 out- put and a carry output, but it cannot accommo- date 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 ANA- LOG ADDER. 2. A circuit in a color TV receiver auxiliary system or unit used to extend the oper- that amplifies the receiver primary matrix signal. ation of another system (e.g., a citizens-band additive 1. The character or characters added to a adapter for a broadcast receiver). code to encipher it. 2. In a calculation, an item adaptive communication A method of communi- that is to be added. 3. An ingredient, usually in a cation that adjusts itself according to the particu- small quantity, added to another material to im- lar requirements of a given time. prove the latter in quality or performance. adaptive suspension vehicle Abbreviation, ASV. additive color A color formed by combining the A specialized robot that moves on mechanical rays from two or three primary-colored lights legs, rather than on wheels. It generally has six onto a single neutral surface. For example, by legs and resembles an insect. It is designed to projecting a red and a green beam onto a neutral move over extremely irregular or rocky terrain, screen, a yellow additive color results. and to carry a human passenger. additive primaries Primary colors that form other adaptivity The ability of a system to respond to its colors in a mixing of light (see ADDITIVE COLOR), environment by changing its performance char- but are not themselves formed by mixing other acteristics. additive primaries. For example, red, green, and adc Abbreviation of ANALOG-TO-DIGITAL CON- blue are the additive primaries used in color tele- VERTER. vision. Through appropriate mixing, these colors Adcock antenna A directional antenna system can be used to generate an unlimited variety of consisting of two vertical antennas, spaced in other colors. Compare SUBTRACTIVE PRI- such a way that the whole array behaves like a MARIES, which form the color spectrum by mix- loop antenna. Its members are connected and po- ing pigments rather than lights. In additive sitioned so that it discriminates against horizon- systems, each superimposed primary color in- tally polarized waves, and delivers output that is creases the total light output from the reflecting proportional to the vector difference of signal volt- (viewing) surface; in subtractive systems, each su- ages induced in the two vertical arms. perimposed primary decreases the total reflectiv- ity. 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 nu- merical 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 Adcock direction finder A radio direction-finding the address being read is correct. system based on the directivity of the ADCOCK address computation In digital computer opera- ANTENNA. tions, the technique of producing or modifying Adcock radio range A radio range system with only the address part of an instruction. four ADCOCK ANTENNAS situated at the corners address field In a computer, the part of the in- of a square, and a fifth antenna at the center of struction that gives the address of a bit of data (or the square. a word) in the memory. add-and-subtract relay A stepping relay that can address generation The programmed generation be switched either uprange (add) or downrange of numbers or symbols used to retrieve records (subtract). from a randomly stored direct-access file.