الجزءالسابع من معجم الاجهزة G

G Thermocouple type for tungsten versus tungsten 26 percent rhenium. Also known as acceleration due to gravity; equal to 980 cm/sec 2 (32.2 ft/sec 2) on the surface of the earth s. 3. Specific gravity Gl In equipment corrosion specifications, the designation for one measure of mild environ­ .mental contamination, as defined in ISA standard ISA-71.04-1985 G1 represents less than 300 A per month of copper corrosion (for electrical terminals).

G2 In equipment corrosion specifications, the designation for one measure of moderate environmental contamination, as defined in ISA standard ISf-71.04-1985. G2 represents less than 1,000 A per monthpf copper corro sion (for electrical terminals).

G3 In equipment corrosion specifications, the designation for one measure of harsh envi­ ronmental contamination, as defined in ISA standard ISA-71.04-1985. G3 represents less than 2,000 A per month of copper corrosion (for electrical terminals). gage Variant spelling of gauge. See gauge. gain 1. The ratio of output signal magnitude to input signal magnitude. When gain is less than one this is usually called "attenuation."

[ISA-26-1968] See attenuation. 2. The relative degree of amplification in an electronic cir­ cuit. 3. The ratio of the change in output to the change in the input that caused the change. 4. In a controller, the reciprocal of proportional band. For example, if the pro­ portional band is set at 25 percent, the con­ troller gain is 4. Proportional band can be expressed as a dimensionless number (gain) or as a percentage. gain, antenna The difference in signal strengths between a given antenna and an isotropic antenna. gain, closed-loop In process instrumentation, the gain of a closed-loop system. It is expressed as the ratio of the output change to the input change at a specified frequency.

[ANSl/ISA-51.1-1979 (R1993)] gain, crossover frequency See frequency, gain

G

crossover. gain, derivative action (rate gain) The ratio of maximum gain that results from propor­ tional plus-derivative control action To the gain caused by proportional control action alone.

[ANSl/ISA-51.1-1979 (R1993)] gain, dynamic The magnitude ratio of the steady-state amplitude of the output signal

from an element or system to the amplitude of the input signal to that element or system, for a sinusoidal signal. [ANSl/ISA-51.1-1979 (R1993)] gain, loop In process instrumentation, the ratio of the absolute magnitude of the change in the feedback signal to the change in its corresponding error signal at a specified fre­ quency. Note: The gain of the loop elements is frequently measured by opening the loop, with appropriate term~Bation. The gain so measured is often called lhe "open-loop gain." [ANSIjISA-51.1-1979 (R1993)] gain, open-loop See gain, loop. gain, proportional The ratio of the change in output caused by proportional control action to the change in input. An example: Y = ±PX, where P = proportional gain, X = input trans­ form, Y = output transform. [ANSI/ISA-51.1­ 1979 (R1993)] gain (magnitude ratio) For a linear system or element, the ratio of the magnitude (ampli­ tude) of a steady-state sinusoidal output to the causal input. The length of a phasor (rotating vector) from the origin to a point of the transfer locus in a complex plane. Note: The quantity may De separated into two'fac­ tors: (1) a proportional amplification, often denoted as K, which is frequency indepen­ dent and associated with a dimensioned scale factor that relates to the units of input and output, and (2) a dimensionless factor, often denoted as G (ju), which is frequency dependent. Frequency, conditions of opera­ tion, and conditions of measurement must be specified. A loop-gain characteristic is a plot of log gain versus log frequency. In nonlinear systems, gains are often amplitude depen­ dent. [ANSI/ISA-51.1-1979 (R1993)] gain, static (zero-frequency gain) Of the gain of an element or the loop gain of a system, static gain is the value that is approached as a limit as frequency approaches zero. Note: Its value is the ratio of change of steady-state output to a step change in input, provided the output does not saturate. [ANSI/ISA­ 51.1-1979 (R1993)] gain, zero frequency See gain, static (zero fn:­

quency gain).

.

gain margin The reciprocal of the oP-fn-loop gain for a stable feedback system at the fre­ quency at which the phase angle reaches -180°. gal A unit of acceleration that is equal to 1 cm/s 2 . The milligal is frequently used because it is about 0.001 times the earth's gravity.

225

gaillng I gap scannIng

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galling Localized adhesive welding that subsequently exhibits spalling and roughening of the metal surfaces that rub together as a result of excessive friction and metal-to-metal contact at high spots. gallon A unit of capacity (volume) that usually refers to liquid measure in the British or U.s. customary system of units. The capacity defined by the British (Imperial) gallon equals 1.20095 U.s. gallon.s; one U.s. gallon equals four quarts or 3.785 x 10-3 m3.路 . galvanic corrosion Electrochemical corrosion associated with current in a galvanic cell. It is set up when two dissimilar metals (or the sa'me metal in two different metallurgical conditions) are in electrical contact and are immersed in an electrolytic solution. galvanic isolation The transfer of electrical power or signal fron;t one circuit to another by means that do not include a direct electrical connection, for example, through an isolating transformer or optical coupler. [ANSI/ ISA-RP12.6-1995] galvanizing The process of coating a metal with zinc by using any of several processes, the most common of which are hot dipping and electroplating. galvanometer An instrument for measuring small electric currents by using electromagnetic or electrodynamic forces to create mechanical motion, such as changing the position of a suspended moving coil. galvanometer recorder A sensitive moving-coil instrument in which a small mirror is mounted on the coil. A small signal voltage applied to the coil causes a light beam reflected from the mirror to move along the length of a slit. This produces a trace on a light-sensitive recording medium that moves transverse to the slit at constant speed. game theory A mathematical process for selecting an optimum strategy in the face of an opponent who has a strategy of his or her own. gamma 1. A measure of the contrast properties of a photographic or radiographic emulsion. It equals the slope of the straight-line portion of its Hand 0 curve. See Hand 0 curve. 2.ln digital graphic video, gamma measures the contrast that affects the mid tones of an image.

Adjusting gamma makes it possible to change the brightness values of middlerange gray tones without altering shadows and highlights. g.lmma counter An instrument for detecting gamma radiation either by measuring integrated intensity over a period of time or by detecting each photon separately.

gamma ray 1. Electromagnetic radiation that is emitted by the nucleus of an atom. Each photon results from the quantum transition between two energy levels of the nucleus. 2.

A term sometimes used to describe any high-energy electromagnetic radiation, such as X-rays that exceeds about 1 MeV or photons of annihilation radiation. gamma-ray spectrometer An instrument for measuring the energy distribution in a beam of gamma ray?

Gantt chart 路:A);tyle of bar chart that is used in production planning and control to display both work planned and work done in relation to time.

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Modified Gantt Chart of Unit Procedures gap 1. An interval of space or time that is used as an automatic sentinel to indicate the end of a word, record, or file of data on a tape. Examples include a word gap at the end of a word, a record or item gap at the end of a group of words, or a file gap at the end of a group of records or items. 2. The absence of information for a specified length of time, or space on a recording medium, as contrasted with marks and sentinels that indicate the presence of specific information for achieving a similar purpose. 3. The space between the reading or recording head and the recording medium, such as tape, drum, or disk. Related to head. 4. In a weldment, the space between members, prior to welding, at the point of closest approach for opposing faces. gap (diametral clearance) The distance between the corresponding surfaces of a flameproof joint. For cylindrical surfaces, the gap is the diametral c1earanci(the difference between the two diameters). [ANSI/ISA12.22.01-1998 (IEC 60079-1 Mod)] gap loss The loss resulting from the end separation of two axially aligned optical fibers. gap scanning In ultrasonic examination, projecting the sound beam through a short column of fluid that is produced by pumping

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garbage I gas plasma display

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couplant through a nozzle in the ultrasonic search unit. See couplant. garbage In data processing, meaningless or incorrect data. garter spring A closed ring that is made by welding the ends of a closely wound helical spring together. gas amplification A characteristic of a counter tube or ionization chamber. It is equal to the charge collected divided by the charge produced in the active volume by a given ionizing event. gas analysis The determination of the constit­ uents of a gasepus mixture. gas bearing A journal or thrust bearing that uses a film of gas to lubricate the running surfaces. Also known as "gas-lubricated bearing." gas burner A burner for u~e with gaseous fuel. gas carburizing A surface-hardening process in which steel or an alloy of suitable alterna­ tive composition is exposed at elevated tem­ perature to a gaseous atmosphere with a high carbon potential. The resulting car­ bon-rich surface layers are hardened by quenching the part from the carburizing tem­ perature or by reheating and quenching. gas chromatography A separation technique that involves passing a gaseous moving phase through a column that contains a fixed adsorbent phase. Gas chromatography is used principally as a quantitative technique for analyzing volatile compounds. gas counter A type of counter tube in which a gaseous sample whose radiation is to be measured is introduced directly into the counter tube itself. gas current A current of positive ions that flows to a negatively biased electrode. The positive ions are produced when electrons flowing between two other electrodes collide with residual gas molecules. gas-detection instrument 1. An assembly of electrical, mechanical, and (possibly) chemi­ cal components that senses and responds to the presence of gas in air mixtures. The _ instrument may be a single integrated unit or a system that is comprises of two or more phYSically separate but interconnected com­ ponent parts. The response of the instrument to the gas is to provide an indication, alarm, or other output ftmction. Note: For conve­ nience, the term instrument is used as an abbreviation for" gas-detection instrument" in the relevant standard: ISA-92.02, 03 and 06.01-1998. 2. An assembly of electrical, mechanical, and chemical components that

senses and responds to the presence of gas in air mixtures. gas dynamic pumping The production of a population inversion by a gas dynamic pro­ cess, in which a hot, dense gas is expanded into a near vacuum, causing the gas to cool rapidly. If the gas cools faster than energy can be redistributed, a population inversion is generated. gas etching Removing material from a semi­ conductor material by reacting it with a gas so as to form a.volatile compound. gasification The process of converting solid or liquid fuel i'1to a gaseous fuel such as the gasification of coal. gasket A sealing member, which usually made by the process of stamping from a sheet of cork, rubber, metal, or impregnated synthetic material. It is usually clamped

between two essentially flat surfaces to pre­

vent pressurized fluid from leaking through

the crevice. Typical applications include

flanged joints in piping, head seals in a recip­

rocating engine or compressor, casing seals

in a pump, or virtually anywhere a pres­

sure-tight joint is needed between stationary

members. Also known as "static seaL"

gas lift The technique of raising a liquid in a vertical flow line by injecting a gas below a portion of the liquid column, thereby causing upward flow. gas meter An instrument for measuring and recording the volume or mass of a gaseous fluid that flows past a given point in a piping system. gas-metal arc welding (GMAW) A form of electric arc welding in which the electrode is a continuous filler metal wire and the weld­ ing arc is shielded by supplying a gas such as argon, helium, or CO2 through a nozzle in the torch or welding head. The term GMAW includes the methods known as "MIG weld­ ing." gasometer A piece of apparatus that is typi­ cally used in analytical chemistry to hold and measure the quantity of gas evolved in a reaction. Similar equipment is used in some industrial applications. gas pass An arrangement in which the con­ vection banks of a boiler are sepa~ted by gas-tight baffles into two or more parallel gas paths. These paths isolate portions of the super heater and reheater surfaces. The pro­ portion of total gas flow through each gas pass may be varied by regulating dampers.

[ANSI/ISA-77.44.01-2000] gas plasma display A data display screen used on some laptop computers. Characters

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gas pliers I gauge on gas plasma displays are easier to read than those on liquid crystal display screens, but gas plasma units are more expensive. gas pliers A pinchers-type tool for grasping round objects such as pipes, tubes, and rods. gas pocket A cavity within a solid or liquid body that is filled with gas. gas recirculation A method by which gas from the boiler, economizer, or air heater out­ let is reintroduced into the furnace by means one or more fans, ducts, or both. [ANSl/ISA­ 77.44.01-2000] gas seal A type of shaft seal that prevents gas from leaking axially along a shaft where it penetrates a machine casing. gas-sensing element (sensor) 1. The primary element in the gas-detection system that responds to the presence of a combustible gas, including any referencli! or compensating unit (where applicable). [ANSI/ISA-12.13.01-2000] 2. The particular subassembly or element in the gas-detection instrument that produces a change in its electrical, chemical, or physical characteristics in the presence of gas. [ISA­ 92.02, 03, and 06.01-1998] gas-shielded arc welding An all-inclusive term for any arc-welding process that utilizes a gas stream to prevent direct contact between the ambient atmosphere and the welding arc and weld puddle. gassing 1. Absorption of gas by a material. 2.

The formation of gas pockets in a material. 3.

The evolution of gas during a process, for example, the evolution of hydrogen at the cathode during electroplating, gas evolution from a metal during melting or solidification, or the desorption of gas from internal sur­ faces during the evacuation of a vacuum sys­ tem. Cas desorption is sometimes referred to as outgassing. gas-specific gravity balance A weighing device that consists of a tall gas column with a floating bottom. A pointer mechanically linked to the floating bottom indicates den­ sity or specific gravity directly, depending on scale calibration. gas thermometer A temperature transducer that converts temperature into pressure of gas in a closed system. The relation between temperature and pressure is based on the gas laws at constant volume. gas tube An electron tube whose operating characteristics are substantially affected by the presence of gas or vapor within the tube envelope. gas-tungsten arc welding (GTAW) A form of metal arc welding in which the electrode is a nonconsumable pointed tungsten rod.

Shielding is provided by a stream of inert gas, usually helium or argon. Filler metal wire mayor may not be fed into the weld puddle, and pressure mayor may not be applied to the joint. The term GTAW includes the method known as "heliarc welding" or "TIC welding." gate 1. A flat or wedge-shaped sliding ele­ ment that modifies flow rate with linear motion across the flow path. [ANSl/ISA­ 75.05.01.-2000] 2. A movable barrier. 3. A dev'ic~ su~h as a"'Val-veor door that controls the rate at which materials are admitted into a conduit, pipe, Or conveyor. 4. A device for positioning film in a movie camera, printer, or projector. 5. The passage in a casting mold that connects the sprue to the mold cavity.

Also known as "in-gate." 6. An electronic component that allows only signals of prede­ termined amplitudes, frequencies, or phases to pass. gate array A circuit that consists of an array

of logic gates aligned on a substrate in a reg­

ular pattern.

gate circuit An electronic circuit that has one or more inputs and one output. It has the property that a pulse goes to the output line only if some specified combination of pulses occurs on the input lines. Cate circuits consti­ tute much of the hardware by means of which logical operations are built into a com­ pu ter. gate (logic) An electronic device that embod­

ies a logic function (AND, OR, NAND, NOR,

and XOR).

gate valve 1. A valve with a linear-motion

closure member that is a flat or wedge­

shaped gate. The gate may be moved in or

out of the flow stream. It has a straight­

through flow path, [ANSI/ISA-75.05.01­

2000] 2. A type of valve whose flow-control

element is a disk or plate that undergoes

translational motion in a plane transverse to

the flow passage through the valve body.

gateway A conceptual or logical network sta­

tion that serves to interconnect two other­

wise incompatible networks, network nodes,

subnetworks, or devices. A gateway per­

forms a protocol conversion operation acrosS

numerous communications la~rs. It is much

more complicated than a bridge. The ISO

defines a gateway as a device that traverses

all seven layers. A current, sloppier defini­

tion describes it as any mechanism that pro­

vides access to another system.

gauge (Also spelled gage) 1. The thickness of

metal sheet or the diameter of rod or wire. 2,

A device for determining dimensions such as

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thickness or length. 3. A visual inspection aid that helps an inspector to reliably determine whether the size or contour of a formed, stamped, or machined part meets tolerances. gauge block A rectangular chromium steel block with two flat parallel surfaces, that have a flatness and parallelism guaranteed to be within a few millionths of an inch. Gauge blocks are usually manufactured and sold in sets for use as standards in linear measure­ ment. Also known as "Johanssen block," "Jo block/':"precision block;" or "size block." gauge cock A valve that is attached to a water column'or drum for checking water level. gauge factor 1. A measure of the ratio of the relative change of resistance to the relative change in length of a resistive strain trans­ ducer (strain gauge). [ISN37.1-1975 (RI982)J 2. The ratio of resistance to relative change in length of a piezoresistive strain gauge. gauge glass A glass or plastic tube for mea­ suring the liquid level in a tank or pressure vessel, usually by direct sight. It is usually connected directly to the vessel through suit­ able fittings and shutoff valves. gauge length In materials testing, the origi­ nallength of an elongated specimen over which measurements of strain, thermal expansion, or other properties are taken. gauge point A specific location that is used to position a part in a jig, fixture, or qualifying gauge. , gauge pressure 1. Pressure measured relative to ambient pressure. 2. The difference between the local absolute pressure of the sys­ tem and the atmospheric pressure at the place of the measurement. 3. Static pressure as indi­ cated on a gauge. 4. The force per unit area exerted by fluid using atmospheric pressure as zero reference. Absolute pressure minus atmospheric pressure. gauss The CGS unit of magnetic flux density or magnetic induction. The SI unit, the tesla, is preferred.

Gaussian beam A laser beam in which the intensity has its peak at the center of the beam, then drops off gradually toward the edges. The intensity profile measured across the center of the beam is a classical Gaussian curve.

Gaussian curve The bell-shaped curve that corresponds to a population with a normal distribution. Also known as "normal curve." gaussmeter A magnetometer for measuring only the intensity of a magnetic field, but not usually the direction of the magnetic field. Its

scale is usually graduated in gauss or kilogauss. gauze 1. A sheer, loosely woven textile fabric.

One of its most common uses is for surgical dressings, but it also has some industrial uses such as for filter media. 2. Plastic or wire cloth of a fine to medium mesh size.

GAW Guided Acoustical Wave type of touch screen, which channels acoustical energy into th~ full volume ofscreen material; com­ pare with SAW.·~· .

GCR Gray component"replacement; color separation control technique in color screen displays and printing...

GOOM Graphical data display manager for the presentation of graphics.

GOT Gas discharge tube; used for overvolt­ age protection and energy transfer switches. gear 1. A toothed machine element for trans­ mitting power and motion betw~en rotating shafts whose axe~ p.re relatively close to each other or are intersecting. 2. A collective term for equipment that performs a specific func­ tions-lifting gear, for example. 3. A collec­ tive term for the portion of a machine that transmits motion from one mechanism to another. 4. A specific combination of gears in a transmission or adjustable gear train that determines mechanical advantage, speed, and direction of rotation. gear down To arrange a gear train so that the driven shaft rotates at a slower speed than the driving shaft. gear drive A mechanism for transmitting power (torque) and motion from one shaft to another by means of direct contact between toothed wheels. gear level To arrange a gear train so that the driving and driven shafts rotate at the same speed. gear-mesh frequency The speed of rotation multiplied by the number of teeth of the gear on that shaft. gear meter A positive-displacement fluid meter in which two meshing gearwheels pro­ vide the metering action. gearmotor A device that consists of an elec­ tric motor and a direct-coupled gear train.

The arrangement allows the motoi to run at optimum speed-usually 1,800 of'3,600 rpm-while delivering rotational motion at a substantially lower speed. gear pump 1. A pump in which fluid is fed to one side of a set of meshing gears, which entrain the fluid and discharge it on the other side. 2. A gear pump that is supplied with pressurized fluid that converts fluid flow into rotary motion.

229

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gear train / gewel hinge

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gear train A combination of two or more gears that are arranged to transmit power and motion between two rotating shafts or between a rotating shaft and a member that moves linearly. gear up To arrange a gear train so that the driven shaft rotates at a higher speed than the driving shaft. gear wheel A wheel with integral gear teeth that mesh with another gear, a rack, or a worm.

Geiger-Muller counter A radiation-measur­ ing instrument whose active element is a gas-filled chamber that usually consists of a hollow cathode with a fine-wire anode along its axis. In operation, the voltage between anode and cathode is high enough that the discharge caused by a primary ionizing event spreads over the entire anode until it is stopped when the space charge reduces the electric-field magnitude. Also known as "Geiger counter."

Geiger threshold The lowest voltage applied to a counter tube that results in output pulses of essentially equal amplitude, regardless of the magnitude of the ionizing event. gel coat A resin that is gelled on the internal surface of a plastics mold prior to filling it with a molding material. The finished part is a two-layer laminate, with the gel coat pro­ viding improved surface quality.

GEMS Global Enterprise wide Management System; beyond enterprise resource plan­ ning, (ERP) this includes connectivity from a corporation to the companies of suppliers as well as customers, often over the Internet, so as to manage the flow of business require­ ments and products among all of them. See ERP. general processor In numerical control, a computer program that carries out computa­ tions on the part program. It also prepares the cutter location data ("CL data") for a par­ ticular part without reference to the machine on which it might be made. general-purpose computer A computer that is designed to solve a large variety of prob­ lems. An example would be a stored pro­ gram computer that may be adapted to any of a very large class of applications.

General-Purpose Interface Bus (GPIB) An IEE£-488 standard bus. Used for interfacing computers to electronic instruments. general-purpose simulation system (GPSS) A generic class of discrete, transac­ tion-oriented simulation languages that are based on a block (diagramming) approach to problem statement.

general recipe A type of recipe that expresses equipment- and site-independent process­ ing requirements. [ANSI/ISA-88.01-1995] generating electric field meter An instru­ ment for measuring electric field strength. A flat conductor is alternately exposed to the field and shielded from it. The potential gra­ dient of the field is determined by measuring the rectified current through the conductor. generating magnetometer An instrument for nieasuring milgI;etic field strength by means of the electromotive force that is generated in a rotating coil immersed in the field being measured.

- ­ generator In EPA (U.5.) regulations, the per­ son, g.roup; orqrganization whose activities generate hazardous waste. genetic algorithm A reiterative computing method developed by John Holland in the 1970s to solve complex probll2ms more quickly l?Y using the-properties of natural selection foun'd in biological evolution so as to adjust to changing environments. Uses simple encoding and reproduction mecha­ nisms through the simple manipulation of chromosomes (strings of Is and Os). "New genes" (solutions) are developed through parent selection, mutation, and crossover so as to create children different than their par­ ents. By constantly comparing the multiple solutions with the desired result, a genetic algorithm will accept and reject various chunks of code and try new combinations from the chunks that remain, until it finds the optimum fit.

Geotechnology The application of science and engineering to problems involving the utilization of natural resources. gesso A mixture of chalk and either gelatine or casein glue. It is painted on panels to pro­ vide a suitable surface for tempera work or for polymer-based paints. getter A material that is exposed to the inte­ rior of a vacuum system in order to reduce via absorption or adsorption the concentra­ tion of residual gas. getter-ion pump A type of vacuum pump that produces and maintains high vacuum by continuously or intermitten!ly depositing chemically active metallayers:'bn the wall of the pump. There, they trap and hold inert gas atoms that have been ionized by an elec­ tric discharge and drawn to the activated pump wall. Also known as "sputter-ion pump." gewel hinge A hinge that consists of a hook inserted in a loop.

230

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GFCI I glaze

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GFCI Ground fault circuit interrupter, which protects humans from electrical shock.

G-glass In the construction of glass-bulb type pH sensors, this glass is used for the membranes of electrodes in processes where the nominal pH value varies around pH 7. Since this type of glass has a wide application range it is also been termed "general- purpose" glass. See E-, L-, and S-glass. ghost point A term used in boiler-water test. 'ing with soap solution. A lather appears to form but will disappear when more soap solution is added. This point represents total calcium hardn'ess and the total hardness of the finalla ther. gib A removable plate that holds other parts or that acts as a bearing or wear surface.

GIF Graphics Interchange,Format [pronounced jiff]; method of saving graphics in electronic memory; used for exchanging data between computers. gigabyte (Gbyte, Gig, GB) 1,073,741,824 bytes or 1,024 megabytes (2 30 bytes). A basic unit for the measurement of mass storage.

Also, used to describe data transfer rates (primarily parallel) as a function of time (Gbps). gigaflops One billion (10 9) floating-point operations per second. . gilbert The CGS unit for magnetomotive force. The SI unit, the ampere (or ampere-turn), is preferred. ;gimbal 1. A cage or frame with two mutually , perpendicular, intersecting axes of rotation.

Free angular movement in two directions is given to any device or mechanism mounted within the frame. 2. A gyro support that gives the spin axis a degree of freedom. gimbal lock A position in a gyro having two degrees of freedom such that the spin axis becomes aligned with an axis of freedom.

This alignment deprives it of a degree of freedom and therefore deprives it of its useful properties. gimbal mount An optical mount that allows the position of a component to be adjusted by rotating it independently around two orthogonal axes. gimlet A small tool for boring holes in wood, leather, and similar materials. It consists of a threaded point, spiral-fluted shank, and cross handle. A tool without the handle and adapted for use in a drill is known as a "gimlet bit." gin A hoisting machine that consists of a windlass, pulleys, and ropes in a tripod frame.

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GIS Graphic Information System; computer system for presenting data in graphic form.

GIW Gain in weight; generally measured in processes that create change in the density of product or the volume of product in some vessel during operations on the product.

GKS Graphical Kernel System; standard interface between application programs and graphics system; ANSI/ISO 2-D international standard. .: ': gland 1. A device for preventing a pressurizedfluid from leaking-out.of a casing at a machine joint, such as at a shaft penetration.

Also known as "gland seal." 2. A movable part that compresses the packing in a stuffing box. See packing follower and lantern ring. glass A hard, brittle, amorphous, inorganic material. Often transparent or translucent, glass is made by fusing silicates (and sometimes borates'and phosphates) with certain basic oxides and then cooling rapidly to prevent crystallization. glass fiber A glass thread that is less than 0.001 in. (0.025 mm) thick. It is used in loose, matted, or woven form to make thermal, acoustical, or electrical insulation. In matted, woven, or filament-wound form it is used to make fiber-reinforced composites. In loose, chopped form it is used to make glass-filled plastics parts. glassine A thin, dense, transparent, super-calendered paper that is made from highly refined sulfite pulp. It is used industrially as insulation between layers of iron-core transformer windings. glassmaker's soap A substance such as Mn02 that is added to glass to eliminate the green color imparted by the presence of iron salts. glass paper 1. An abrasive material that is made by bonding a layer of pulverized glass to a paper backing. 2. Paper made of glass fibers. glass sand The raw material for glassmaking.

It normally consists of high-quartz sand that contains small amounts of the oxides of AI, Ca, Fe, and Mg. glassware Laboratory containers, vessels, graduated cylinders, tubing, and th~.(ike that ~,:: are made from glass. glass wool A relatively loose mass of glass fibers that is used chiefly for insulating, packing, and filtering applications. glassy alloy A metallic material having an amorphous or glassy structure. Also known as "metallic glass." glaze A glossy, highly reflective, glasslike, inorganic, fused coating. See enamel.

glazing / GMR

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glazing 1. T~e process of cutting and fitting glass panes into frames. 2. Smoothing the exposed solder of a wiped pipe joint with a hot iron glazing compound A caulking compound, such as putty, that is used to seal the edges of a pane of glass where it fits into its frame. glitch An undesirable electronic pulse that causes processing errors. glitter A decorative flaked powder that has a particle size large enough so that the individual flakes produce a visible reflection or spar~le. Itjs used in certain decorative paints and m some compounded plastics stock. global 1. Any component that has as its scope the entire system in which it resides. 2. A computer instruction that causes the computer to locate all occurrences of specific data. 3. A value that i1 defined in one program module and used in others. Globals are often referred to as entry points in the module in which they are defined and externals in the other modules that use them. global array A set of data listings that can be referenced by other parts of the software. global common An unnamed data area that is accessible by all programs in the system.

Sometimes referred to as a "blank common." global variable Any variable that is available to all programs in the system. Contrast with

reserved variable. globe body A valve body that is distinguished by a globular-shaped cavity around the port region, wherein the closure member motion is linear and normal to the plane of the port. [ANSI/ISA-75.05.01-2000] globe valve plug guides The means by which the plug is aligned with the seat and held stable throughout its travel. The guide is held rigidly in the body or bonnet. [ANSI/ ISA-75.05.01 - 2000] globe valve 1. A valve with a linear-motion closure member, one or more ports, and a body that is distinguished by a globularshaped cavity around the port region. [ANSI/ !SA-75.05.01-2000] 2. A type of flow-regulatmg valve that consists of a movable disc and a stationary-ring seat in a generally spherical body. In the general design, the fluid enters below the valve seat and leaves from the cavity above the seat. 3. A valve with a linearmoti~n closure member whose seal is perpendICular to the stem motion, that has one or ~ore ports, and that has a body distingUIshed by a globular-shaped cavity around the port region. (See figure.) [ANSI/ISA75.05.01-2000]

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G/I)he Valve globe valve trim The internal parts of a valve that are in flowing contact with the controlled fluid. Examples are the plug, seat ring, cage, stem, and the parts that are used to attach the stem to the plug. The body, bonnet, bottom flange, guide means, and gaskets are not considered to be part of the trim. [ANSI/ISA75.05.01-2000] gl?,ssim~ter "An instrument for measuring the gloss mess of a surface, that is, the ratio of light refl~cted in. a sp.ecific direction to light reflected in all dIrections, usually by means of a photoelectric device. Also known as "glossmeter. " glow discharge A discharge of electrical energy. through a gas, in which the space P?tential near the cathode is substantially hIgher than the ionization potential of the gas.

GLP Good laboratory practice; regulations by ~DA (U.5.) for non-clinical laboratory stud-' Ies. glue 1. In general contexts, a term often used (improperly) to describe an adhesive. 2. In a specific sense, a crude, impure form of commercial gelatine that softens to a gel consistency when wetted with water and dries to form a strong adhesive layer. glued .A mix~d~signal simulation system that combines eXIsting analog and dio'ital simulation software into a hybrid analog or dio-ital simulation system.

0 GM.M ~raphical multimeter; multiple electncal sIgnal tester that can displav waveforms.

G~P Good manufacturing ~ctice; regulatwns by FDA (U.s.) for medical devices.

GMR .Giant r:nagnetoresistive; changes in the electnc~l resl~tance in materials composed of alternating thin layers of metallic elements or magnetic metals separated by nonmao-netic "spacers." Used in memory storage d~vices for much more capacity in disk drives than offered by conventional magnetoresistive methods.

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gnd I graded index fiber gnd Ground; usually chassis ground but can be used for earth ground electrical potential.

Golay cell An infrared detector in which the incident radiation is absorbed in a gas cell, thereby heating the gas. The temperatureinduced expansion of the gas deflects a diaphragm, and a measurement of this deflection indicates the amount of incident radiation. goniometer 1. Generally, any instrument for measuring angles. 2. Specifically, an instrument that is used in crystallography to determine the angles between crystal planes, using X-ray dif{.raction or other means. 3. An instrument that is used to measure refractive index and other optical properties of transparent optical materials. It is also used to measure optical scattering in materials at UV, visible, or IR wavelengths.) go/no-go gauge A composite gauging device that enables an inspector to quickly judge whether specific dimensions or contours fall within specified tolerances. In many instances, the device is so constructed that the part being inspected will fit one part of the gauge easily and will not fit another part if it is within tolerance. The gauge will pass both parts or pass neither if it is not within ~ tolerance. go/no-go test A test in which one or more parameters are determined, but in which 1 only acceptance or rejection of the test object can result, depending on the value(s) mea1 l' sured.

I~ .' GOSIP Government Open Systems I:, Interconnection Profile; U.s. Federal Information Processing Standard 146, which defines a set of open system communication protocols that allow different makes of computers to communicate and users of different applications on these systems to exchange information. gouging Forming a groove in an object by electrically, mechanically, thermomechanically, or manually removing material. The gouging process is typically used to remove shallow defects prior to repair welding. governor A device for automatically regulating the speed or power of a prime mover. In particular, a device that relies on centrifugal force in whirling weights that are opposed by springs or gravity to actuate the controlling element.

GPD Gas plasma displays that emit a glow when excited with small voltage; type of liquid crystal displays (LCD). gph Gallons per hour (volumetric flow rate).

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GPIB General Purpose Interface Bus; IEEE 488 standard; parallel, multiport, 300 Kbs to 1 Mbs, 15 to 28 stations on up to 50 feet. Originally developed by Hewlett-Packard as HPlB for laboratory instrumentation. gpm Gallons per minute (volumetric flow rate). grab-sample point The point in the sample line where the flow of sample fluid can be directed to a portable container. It may be referred to as '''s<UJ.lple point." [ANSI/ISA67.02.01-1999] . . . ,. grab sampling A method of sampling bulk materials for analysis that consists of taking one or more small portions (usually only imprecisely measured) at random from a pile, tank, hopper, railcar, truck, or other â&#x20AC;˘ point of accumulation. graceful degradation A system attribute such that when a piece of equipment fails, the system falls back to a degraded mode of operation rather than failing catastrophically and giving no response to its users. grade 1. To move earth in order to make a land surface of uniform slope. 2. A classification of materials, alloys, ores, units of product, or characteristics according to some attribute or level of quality. 3. To sort and classify according to attributes or quality levels. 4. A classification of oil according to quality. graded index fiber Optical fiber whose core has a nonuniform index of refraction. The core is composed of concentric rings of glass whose refractive indices decrease from the center axis to reduce modal dispersion and thereby increase fiber bandwidth. Such fibers have lower dispersion and broader bandwidth than step-index fibers. lighl source .............

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graded refractive index lens I graphic panel graded refractive index lens A lens in which the refractive index of the glass is not lU1iform. Typically, the index will differ with the distance from the center of the lens. gradient The rate of change of some variable with respect to another, especially a regular uniform or stepwise rate of change. graduation Any of the major or minor index marks on an instrument scale.

GRAFCET Graphe de Commande Etape."

Transition, "step transition function charts"); ¡created in 1979 by French Association for Economical and Applied Cybernetics, GRAFCET Js a popular subset of sequential function chart (SFC), providing a diagrammatic representation of program sequences and supporting alternative sequence selections and parallel sequences. Basic elements are steps and transitiops; originally an enhancement to PLC ladders. grain 1. The appearance or texture of wood, or the woodlike appearance or texture of another material. 2. In paper or matted fibers, the predominant direction most fibers lie in, which corresponds to the directionality imparted during manufacture. 3. In metals and other crystalline substances, an individual crystallite in a polycrystalline mass. 4. In crumbled or pulverized solids, a single particle too large to be called powder. grain boundary The plane of mismatch between adjacent crystallites in a pOlycrystalline mass, as revealed on a polished and etched cross section of the material. grain flow The fibrous appearance on a polished and etched section of a forging. It is caused by the orientation of impurities and inhomogeneities along the direction of the work during the forging process. grain growth An increase in the average grain size in a metal, usually as a result of exposure to high temperature. graininess Visible coarseness in a photographic or radiographic emulsion. It is caused by the clumping together of countless small grains of silver into relatively large masses that are visible to the naked eye or with slight magnification. graining Working a translucent stain while it is still wet in order to simulate the appearance of grain in wood or marble. Tools such as special brushes, combs, and rags used by hand create the desired irregular patterns. grain size 1. For metals, the size of crystallites in a polycrystalline solid. Grain size may be expressed as a diameter, as number of grains per unit area, or as a standard grain-size number that is determined by comparison

with a chart such as those published by ASTM. In most instances, the grain size is given as an average, unless there are substantial proportions that can be given as two distinct sizes. If two or more phases are present, the grain size of the matrix is given.

2. For abrasives, the preferred term is "grit size." grains per cu ft The term for expressing dust loading in terms of weight per unit of gas volume (7,000 grains equals one pound). grains (water) A unit of measure that is commonly used in water analysis to mea5ure impurities in wah~r(17.1 grains = 1 part per million (ppmÂť. gram The CGS unit of mass. It equals 0.001 kilogram, ~hich has been adopted as the SI unit of mass. granular fracture A rough, irregular fracture surface that cal] be either transcrystalline or intercrystalline. Also, it often indicates that fracture took.place in a relatively brittle mode, even though the material in question is inherently ductile. granular structure A nonuniform appearance of molded or compressed material as a result of the presence of particles of varying composition. grapheme The smallest unit of written language. Generally, a single letter for a vowel or consonant; sometimes a combination such as "th" or "ch." graphic Pertaining to representational or pictorial material that is usually legible to humans and is applied to the printed or written form of data such as curves, alphabetic characters, and radar scope displays. graphical display unit An electronics device that can display both text and pictorial representations. graphical user interface (GUI) An operating system or environment that displays program choices and options on the screen as icons (pictures or symbols) and often as windows. Selections are typically made with a mouse or trackball ("point and click").

Invented at Xerox in the 1970s, the graphical user interface was first used on Apple Macintosh and later on Microsoft Windows, Digital Research GEM, and X-Wifl.¢ows. graphic character See graphic." graphic lines Representations of process and signal lines in a pictorial display. graphic panel A master control panel that, pictorially and usually colorfully, traces the relationship of control equipment and the process operation. It permits an operator, at a glance, to check on the operation of a

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graphics object / grid

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far-flung control system by noting dials, valves, scales, and lights. graphics object A visually oriented screenview item, such as a scroll bar, bit map, or icon, that is used in the presentation of some application interface. graphic symbol An easily recognized pictorial representa tion. [ISA-S.S-198S] . graphic symbols Simplified representations of process components and instruments in a pictorial display. graphite flake A form of graphite that is present in gray cast iron. It appears in the microstructure as an elongated, curved inclusion. graphite rosette A form of graphite present in gray cast iron that appears in the microstructure as graphite flakes extending radially outward from a center of crystallization. graphitic carbon Free carbon that is present in the microstructure of steel or cast iron. It is an essential feature of most cast irons but is almost always undesirable in steel. graphitic corrosion Corrosion of gray cast iron. The iron matrix is slowly leached away, leaving a porous structure behind which is largely graphite but that may also be held together by corrosion products. This form of corrosion occurs in relatively mild aqueous solutions and on buried pipe and fittings. graphitic steel An alloy steel in which some of the carbon is present in the form of graphite. graphitization The formation of graphite in iron or steel. It is termed "primary graphitization" if it forms during solidification, and "secondary graphitization" if it forms during subsequent heat treatment or extended service at high temperature. gravimeter A device for measuring the relative force of gravity by detecting small differences in the weight of a constant mass at different points on the earth's surface. Also known as gravity meter. gravimetric A descriptive term that is used to designate an instrument or procedure that utilizes gravitational forces. However, the results or indications of such procedures are not necessarily influenced by the magnitude of the acceleration of gravity. [ISA-RP31.11977] gravitational constant A dimensionless conversion factor in English units that arises from Newton's second law (F = ma) when mass is expressed in pounds-mass (Ibm)' gravit::>meter See densimeter. gravity A weight index of fuels. Liquid petroleum products are expressed either as spe-

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cific, Baume, or API (American Petroleum Institute) gravity. Gravity is the weight index of gaseous fuels expressed as specific gravity related to air under specified conditions.

Gravity is the weight index of solid fuels expressed as specific gravity related to water under specified conditions. gravity convection oven An oven in which there is no mechanical means for circulating air in the oven chamber. gravity metef L A device that uses a V-tube manometer to determiiH~'the specific gravities of solution:;; by direct reading. 2. An electrical device for measuring variations in the gravitational forces through different geological formations. 3. A gravimeter. gray The metric unit for absorbed dose. gray balance In the composition of screen displays and printing, the dot values or densities of cyan, magenta,. and yellow that produce neutral gray. ..:, .. ~~;:. graybody An objecnhat has the same spectral emissivity at every wavelength, or one whose spectral emissivity equals its total emissivity.

Gray code A generic name for a family of binary codes that have the property such that one number can be changed to the next sequential number by changing only one bit in a code for that first number. This type of code is commonly used in rotary shaft encoders to avoid ambiguous readings when moving from one position to the next. See also cyclic code and shaft encoder. gray iron Cast iron that contains free graphite in flake form. So named because a freshly broken bar of the alloy appears gray. grease 1. Rendered, inedible animal fat. 2. A semisolid to solid lubricant that consists of a thickening agent, such as metallic soap, dispersed in a fluid lubricant, such as petroleum oil. grease seal ring See lantern ring. green Unfired, uncured, or unsintered. green strength The mechanical strength of a ceramic or powder metallurgy part after molding or compacting but before firing or sintering. It represents the quality necessary to maintain sharpness of contour ~d physical integrity during handling ancPthe mechanical operations to prepare it for firing or sintering. greenware Unfired ceramic ware. grid 1. A network of lines, typically forming squares, that are used in layout work or in creating charts and graphs. 2. A crisscross network of conductors that is used for shielding or controlling a beam of electrons.

grid circuit I grounding grid circuit An electronic circuit that includes the grid-cathode path of an electron tube in series with other circuit elements. grid control A method of controlling anode current in an electron tube by varying the potential of the grid electrode with respect to the cathode. grid emission The emission of electrons or ions from the grid electrode of an electron tube. grid nephoscope A device for determining the direction of cloud motion. Clouds are sighted through a grid work of bars, and the angular posftion of the grid is adjusted until some feature of the cloud in the field of view appears to move along the majOl:,i:~?,is of the grid. . grinding 1. Removing material from the surface of a workpiece by l.lsing an abrasive wheel or belt. 2. Reducing the particle size of a powder or granular solid. grinding aid A material added to the charge in a rod or ball mill to acceleri1te the grinding process. grinding burn The localized overheating of a workpiece surface as a result of excessive grinding pressures, an inadequate supply of coolant, or both. grinding cracks Shallow cracks in the surface of a ground workpiece. They appear most often in relatively hard materials as a result of excessive grinding friction or high sensitivity in the material. grinding fluid A cutting fluid used in grinding operations, primarily to cool the work piece but also to lubricate the contacting surfaces and carry away grinding debris. grinding medium Any material-including balls, rods, and quartz or chert pebbles-that is used in a grinding mill. grindstone A stone disk that mounted on a revolving axle and used for grinding or tool sharpening. grit A particulate abrasive that consists of angular grains. grit blasting The process of abrasively cleaning metal surfaces by blowing steel grit, sand, or other hard particulate against them to remove soil, rust, and scale. Also known as sandblasting. grommet 1. A metal washer or eyelet, often used to reinforce a hole in cloth or leather. 2.

A rubber or soft plastic eyelet that is inserted in a hole through sheet metal, such as an electronic equipment chassis or enclosure, to prevent a wire from chafing against the side of the hole, damaging its insulation, or shorting out to the chassis. 3. A circular piece of

fibrous packing material used under a bolt head or nut to seal the bolt-hole. grommet nut A blind nut with a round head that is sometimes used with a screw to attach a hinge to a door. groove 1. A long narrow channel or furrow in a solid surface. 2. In a weldment, a straight-sided, angled, or curved gap between joint members used prior to welding to help confine the weld puddle and ensure full joint J.?~J;l~~tration so as to produce a sound weld. . grooved drum A windlass drum whose face has been grooved, usually in a helical fashion, to support a!19 guide the rope or cable wound on it. . grooved tube seat路 A tube seat that has one or more shallow groves into which the tube may be forced by the expander. gross porosity In weld metal or castings, gross porosity is the presence of large or numerous gas holes, pores, or voids that are indicative of substandard quality or poor technique. ground 1. A conducting connection, whether intentional or accidental, between an electrical circuit or electrical equipment and either the earth or some other conducting body that serves in place of the earth. [ANSI/ISA82.02.01-1999 (EIC 1010-1 MOD); ISA12.01.01-1999] 2. A (neutral) reference level for electrical potential that is equivalent to the level of electrical potential of the earth's crust.

3. A secure connection to earth that is used to reference an entire system. Usually the connection takes the form of a rod driven or buried in the sailor a series of rods connected into a grid that is buried in the soil. grounded Referring to the presence or absence of an electrical connection between the "low" side of the transducer element and the portion of the transducer that is intended to be in contact with the test structure. The method for ungrounding should be stated as "internally ungrounded" or "by means of separate stud." [ISA-RP37.2-1982 (R1995)] grounded (earthed) Connected to earth or to some conducting body that serves in place of earth. [ISA-12.01.01-1999] . grounded junction A form ofttermocouple construction in which the measuring junction is electrically connected to its protective sheath so both have the same potential. grounding The act of establishing a conductive connection, whether intentional or accidental, between an electrical circuit or electrical equipment and the earth or some other conducting body that serves in place of

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ground lead / guide

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the earth. [ANSI/ISA-82.02.01-1999 (EIC 1010-1 MOD)] ground lead See work lead. ground loop Circulating current between two or more connections to electrical ground.

This signal can be detected and displayed by electronic instruments. These signals are generally not associated with the variable to be measured and represent noise in t~e measur~ng system. ground loop noise Noise that results when equipment is grounded at points that have different potentials, thereby creating an unintended cutrent path. ground resistance test After electrical grounding and bonding systems are installed, ground resistance is typically tested with a ground resistance tester to demonstrate whether resistance to grotind, in 120 volt systems, is 125 ohms or less. This test is as stipulated by National Electrical Code. How much ground resistance is allowable depends on the specific system requirements and the system interruption restrictions. Five ohms or less is recommended for general equipment and one ohm for instrument grounds. group A classification of combustible materials. [ISA-12.01.01-1999] group addressing In transmission, the use of an address that is common to two or more stations. On a multipoint line, group addressing means that all stations recognize addressing characters but only one station responds. grouping The process of combining two or more computer records into one block of information in order to conserve storage space or disk or tape. Also known as "blocking." group leader The person who holds the highest level of functional supervision but whose responsibilities are oriented solely toward instrumentation and control. [ANSI/ISA67.14.01-2000] group velocity The velocity that corresponds to the rate of change of the average position of a wave packet as it travels through a medium. grouting The placing or injecting of a fluid mixture of cement and water (or of cement, sand, and water) into a grout hole, crevice, seam, or joint for the purpose of forming a seepage barrier, consolidating surrounding earth or rock, repairing concrete structures, or sealing the joint where an equipment base rests on a concrete floor.

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grub screw A headless screw that is slotted at one end to receive a screwdriver.

GSM Global System for Mobile communications; a digital wireless communications technology. guard A shield or cowling that surrounds moving parts so as to prevent workers from being injured or to prevent incidental equipment damage from for~ign objects. guard bit A bit contained in each word or groups of words of mem.ory that indicates to computer hardware or software whether the content of that memory location may be altered by a program.¡See protected location. guard ring An auxiliary, ring-shaped electrode in a counter tube or ionization chamber whose chief functions are to control potential gradients, reduce insulation leakage, or define the active region of the tube. guard vacuum An enclosed evacuated space between a primary vacuum system and the atmosphere. Its primary purpose is to reduce leakage from the seal into the primary system. guest (computer) In process control, the use of a computer that is not the primary or controlling computer and is not used to configure some multiple-part system. A guest computer provides services that are auxiliary to the basic control. Such functions can be collecting historical and archival data, creating a library of application programs, performing background calculations, and so on.

The term was originally used somewhat tongue in cheek by Sam Herb and Scotty Moore in the first edition of Understanding Distributed Process Control in 1981 to describe a system that has a computer that is connected but not required for a process control system's start-up or operation. See host (com0

puter).

G VI Graphical user interface [pronounced: gooey]; operating system or environment that displays program choices and options on the screen as icons (pictures or symbols) and often windows. Selections are typically made with a mouse or trackball (point and click). Invented at Xerox in the 1970s, was first lIsed on Apple Macintosh an.~later on Microsoft Windows, Digital Resea'rch GEM, and X Windows. guide 1. A pulley, idler roll, or channel member that keeps a rope, cable, or belt traveling in a predetermined path. 2. A runway in which a conveyor travels. 3. A stationary machine element-a beam, bushing, rod, or pin, for instance-whose primary function is

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guide bearing I gyro wheel to keep one or more moving elements confined to a specific path of travel. guide bearing A plain bushing that is used to prevent the lateral movement of a machine element while allowing free axial translation, with or without (usually without) simultaneous rotation. Also known as a "guide bushing." guide bushing See bushing.

Guided Acoustical Wave (GAW) A type of touch screen that channels acoustical energy into the full volume of screen material. Compare with sJtrface acoustic wave (SAW). guided bend test A bend test in which the specimen is bent to a predetermined shape in a jig or around a grooved mandrel. guided missile An unmanned airborne vehicle whose flight path ~r trajectory can be altered by some mechanism within or attached to the vehicle in response to either a preprogrammed control sequence or a control sequence that is transmitted to the vehicle while in flight. guided wave A wave whose energy is confined by one or more extended boundary surfaces and whose direction of propagation is effectively parallel to the boundary. guides, closure member The means by which the closure member is aligned with the seat and held stable throughout its travel.

The guide is held rigidly in the body, bonnet, and/orbottom flange. [ANSI/ISA-7S.0S.012000] gutter 1. A drainage trough or trench, usually surrounding a raised surface. 2. A groove around the cavity of a forging or casting die to receive excess flash. guy A wire, rope, or rod that is used to secure a pole, derrick, truss, or temporary structure in an upright position or to hold it securely against the wind. guyed-steel stack A steel stack of insufficient strength to be self-supporting and is thus laterally stayed by guys. gyratory screen A sieving machine that has a series of nested screens whose mesh sizes are progressively smaller from the top to the bottom of the stack. The mechanism shakes the stacked screens in a nearly circular fashion, which causes fines to sift through each screen until an entire sample or batch has been classified. gyro Short for gyroscope. See gt;roscope. gyromagnetic ratio The magnetic moment of a system divided by its angular momentum. gyroscope 1. A transducer that makes use of a self-contained spatial directional reference.

[ISA-37.1-1975 (R1982)] 2. An instrument that

maintains a stable, angular reference direction by virtue of the application of Newton's second law of motion to a mechanism whose chief component is a rapidly spinning heavy mass. gyroscopic couple The turning moment that is generated by a gyroscope in order to oppose any change in the position of its axis of rotation. gyroscopic horizon' A gyroscopic instrument . 路that simulates the position of the natural horizon and indicates the attitude of an aircraft with respect to this horizon. gyro wheel The heavy rotating element of a gyroscope. It consists of a wheel whose rather large mass is distributed uniformly around its rim. In precision gyroscopes, the gyro wheel is specially constructed to have nearly perfect balance.

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