I

Source edition 1965. Please read the Introduction to find out about this dictionary and our plans for it. Caution, many entries have not been updated since the 1965 edition.
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Iapetus

A satellite of Saturn orbiting at a mean distance of 3,562,000 kilometers.

ICAO (abbr) = International Civil Aviation Organization. Usually pronounced as a word.

ICAO Standard Atmosphere

See standard atmosphere.

ice frost

A thickness of ice that gathers on the outside of a rocket over surfaces supercooled as by liquid oxygen inside the vehicle.

This ice frost is quickly shaken loose and falls to the ground once the rocket begins its ascent.

ice point

The temperature at which a mixture of air-saturated pure water and pure ice may exist in equilibrium at a pressure of one standard atmosphere.

By decision of the Tenth General Conference on Weights and Measures, Paris, October 1954 the ice point was established as 273.15 degrees K.

ICW (abbr) = interrupted continuous wave.

ISCU (abbr) = International Council of Scientific Unions. Usually pronounced as a word.

ideal exhaust velocity

The exhaust velocity of an ideal rocket.

ideal fluid

1. = perfect fluid.

2. = inviscid fluid.

ideal gas

A gas which conforms to Boyle law and has zero heat of free expansion (or also obeys Charles law). Also called perfect gas.

ideal gas laws = gas laws.

ideal nozzle

The nozzle of an ideal rocket, or a nozzle designed according to the ideal gas laws.

ideal rocket

A theoretical rocket postulated for parameters that are corrected in practice.

An ideal rocket assumes a homogeneous and invariant propellant, observance of the perfect gas laws, no friction, no heat transfer across the rocket wall, an axially directed velocity of all exhaust gases, a uniform gas velocity across every section normal to the nozzle axis, and chemical equilibrium established in the combustion chamber and maintained in the nozzle.

ideal transducer

For connecting a specified source to a specified load, a hypothetical passive transducer that transfers the maximum possible power from the source to the load.

In linear electric circuits and analogous cases, this is equivalent to a transducer which (1) dissipates no energy and (2) when connected to the specified source and load presents to each its conjugate.

ideal velocity

The velocity acquired by an ideal rocket in field free space, under the influence of no external forces except the thrust force.

I-display

In radar, a display in which a target appears as a complete circle when the radar antenna is correctly pointed at it and in which the radius of the circle is proportional to target distance. When not correctly pointing at the target, the circle reduces to a segment of a circle, the segment length being inversely proportional to the magnitude of the pointing error and its angular position being reciprocal to the direction of pointing error. Also called I-scan, I-scope, I-indicator.

IGC-1959 (abbr) = International Geophysical Cooperation, 1959.

igneous meteor

In U.S. weather observing practice, a visible electrical discharge in the atmosphere. Compare electrometeor.

Lightning is the most common and important type, but types of corona discharge are also included.

igniter

A device used to begin combustion, such as a spark plug in the combustion chamber of a jet engine, or a squib used to ignite the fuel in a rocket.

ignition delay

The time lapse occurring between the instance of an igniting action of a fuel and the onset of a specified burning reaction. Also called ignition lag.

ignition lag = ignition delay.

ignore

In computer terminology, a code group or character which indicates that the associated information is to be disregarded.

Igor (abbr)

Intercept ground optical recorder. A long-focal-length telescopic camera used to observe attitude and other details of a rocket in flight.

IGY (abbr) = International Geophysical Year.

I-indicator = I-display.

illuminance

The total luminous flux received on a unit area of a given real or imaginary surface, expressed in such units as the footcandle, lux, or phot. Illuminance is analogous to irradiance, but is to be distinguished from the latter in that illuminance refers only to light and contains the luminous efficiency weighting factor necessitated by the nonlinear wavelength-response of the human eye. Compare luminous intensity.

The only difference between illuminance and illumination is that the latter always refers to light incident upon a material surface. A distinction should be drawn, as well, between illuminance and luminance. The latter is a measure of the light coming from a surface; thus, for a surface which is not self-luminance, luminance is entirely dependent upon the illuminance upon that surface and its reflection properties.

illumination

See illumination, note.

illumonometer = photometer.

ILS (abbr) = instrument landing system

impact

1. A single collision of one mass in motion with a second mass which may be either in motion or at rest.

2. Specifically, the action or event of an object, such as a rocket, striking the surface of a planet or natural satellite, or striking another object; the time of this event, as in from launch to impact.

3. To strike a surface or an object.

4. Of a rocket or fallaway section: To collide with a surface or object, as in the rocket impacted 10 minutes after launch.

impact acceleration

The acceleration generated by very sudden starts or stops of a vehicle.

The term is usually applied in the context of physiological acceleration.

impact area

The area in which a rocket strikes the surface of the earth or other celestial body.

Used specifically in reference to the impact area of a rocket range.

impact line

An imaginary line on the outside of a destruct line and running parallel to it, which marks the outer limits of impact for a missile destroyed under destruct procedures.

impact microphone

An instrument that picks up the vibration of an object impinging upon another, used especially on space probes to record the impact of small meteoroids.

impact predictor

A device which takes information from a trajectory measuring system and continuously computes the point (in real time) at which the rocket will strike the earth; based on the assumption that the rocket power is shut off at that instant and the remaining trajectory is ballistic in nature.

impact pressure

1. That pressure of a moving fluid brought to rest which is in excess of the pressure the fluid has when it does not flow, i.e., total pressure less static pressure.

Impact pressure is equal to dynamic pressure in incompressible flow, but in compressible flow impact pressure includes the pressure change owing to the compressibility effect.

2. A measured quantity obtained by placing an open-ended tube, known as an impact tube or pitot tube, in a gas stream and noting the pressure in the tube on a suitable manometer.

Since the pressure is exerted at a stagnation point, the impact pressure is sometimes referred to as the stagnation pressure or total pressure.

impact strength or impact energy

The amount of energy required to fracture a material.

The type of specimen and the testing conditions affect the values and therefore should be specified.

impact tube

See impact pressure, sense 2.

impeller

1. A device that imparts motion to a fluid; specifically, in a centrifugal compressor, a rotary disk which, faced on one or both sides with radial vanes, accelerates the incoming fluid outward into a diffuser. Also called impeller wheel.

2. That part of a centrifugal compressor comprising this disk and its housing.

impeller blade = impeller vane.

impeller vane

Any one of the vanes on the impeller of a centrifugal compressor, serving to take in air and accelerate it radially outward. Also called impeller blade. Compare compressor blade.

impeller wheel

The vaned rotary disk in a centrifugal compressor. Usually called the impeller.

impingement rate

The rate per square centimeter per second at which molecules strike a plane surface in a gas at rest. Also called rate of incidence.

impinging-stream injector

In a liquid-propellant rocket engine, a device that injects the fuel and oxidizer into the combustion chamber in such a manner that the streams of fluid intersect one another.

implosion

The rapid inward collapsing of the walls of a vacuum system or device as the result of failure of the walls to sustain the ambient pressure.

impulse

1. The product of a force and the time during which the force is applied; more specifically, the impulse is where the force F is time dependent and equal to zero before time t₁ and after time t₂.

2. symbol I_t = total impulse. Compare specific impulse.

impulse noise

Noise generated in discrete energy bursts, not of random nature, which has a characteristic wave shape of its own.

impulse turbine

A type of turbine having rotor blades shaped so that the wheel is turned from the impact of the fluid against the blades, with no pressure drop occurring across the blades.

inactive leg

An electrical element within a transducer which does not change its electrical characteristics as a function of the applied stimulus.

Specifically applied to elements which are employed to complete a Wheatstone bridge in certain transducers.

incandescence

Emission of light due to high temperature of the emitting material. Any other emission of light is called luminescence.

inch (abbr in.)

Exactly 2.540 centimeters.

Prior to July 1, 1959, the inch was 2.54005 centimeters although the conversion factor 2.540 has actually been in use in industry in the United States since 1933.

incidence

1. Partial coincidence, as a circle and a tangent line.

2. The impingement of a ray on a surface. See angle of incidence.

incident ray

A ray impinging on a surface.

inclination

1. = magnetic dip.

2. ( symbol i ). The angle between the plane of an orbit and a reference plane.

The equator is the reference plane for geocentric orbits and the ecliptic is the reference plane for heliocentric orbits.

included angle

In aerodynamics, the angle between free stream velocity and the longitudinal axis of the vehicle.

incoming solar radiation

Full term for insolation.

incourse guidance = midcourse guidance.

increment

A change in the value of a variable. A negative increment is also called decrement.

Ind, Indi

International Astronomical Union abbreviations for Indus. See constellation.

independent variable

Any of those variables of a problem, chosen according to convenience, which may arbitrarily be specified, and which then determine the other or dependent variables of the problem.

The independent variables are often called the coordinates, particularly in problems involving motion in space. Dependent and independent variables can be interchanges, e.g., height and pressure.

index of absorption = absorptive index.

index of refraction (symbol n)

1. A measure of the amount of refraction (a property of a dielectric substance). It is the ratio of the wavelength or phase velocity of an electromagnetic wave in a vacuum to that in the substance. Also called refractive index, absolute index of refraction, absolute refractive index, refractivity. See modified index of refraction, N-unit, potential index of refraction.

It can be a function of wavelength, temperature, and pressure. If the substance is nonabsorbing and nonmagnetic at any wavelength, then n² is equal to the dielectric constant at that wavelength. The complex index of refraction is obtained when the attenuation of the wave power radian, called the absorptive index k, is paired with the index of refraction. It is written n* = n (1 - ik) When the wave passes from one medium n₁ to another n₂, the angle of incidence φ and the angle of refraction φ', both measured with respect to the normal to the interface, are related by
sin φ / sin φ' = n₁* / n₂* = constant
which becomes, for a nonabsorbing medium, the ratios of the (noncomplex) indices of refraction. In the particular case that medium 2 is a vacuum, this ratio is the index of refraction of medium 1. This is known as Snell law, named after Willebrord Snell who discovered it about 1621.

2. A measure of the amount of refraction experienced by a ray as it passes through a refractive interface, i.e., a surface separating two media of different densities. It is the ratio of the absolute indices of refraction of the two media (see sense 1 above). Also called refractive index, relative index of refraction.

Indi

International Astronomical Union abbreviation for Indus. See constellation.

indicator

A device which makes information available but in which there is no provision for storage of such information, as a radar indicator.

indirect wave

Any radio wave which arrives by an indirect path, having undergone an abrupt change of direction by refraction or reflection. See sky wave.

indochinite

See tektite.

induced magnetism

Magnetism acquired by a piece of magnetic material while it is in a magnetic field. See permanent magnetism.

Indus (abbr Ind, Indi)

See constellation.

inelastic collision

A collision between two particles in which changes occur both in the internal energy of one or both of the particles and in the sums, before and after collision, of their kinetic energies.

inert atmosphere

A gaseous medium that because of its lack of chemical reaction is used to enclose tests or equipment..

inert gas

Any one of six gases, helium, neon, argon, krypton, xenon, and radon, all of whose shells of planetary electrons contain stable numbers of electrons so that the atoms are almost completely chemically inactive. Also called rare gas.

All these gases are found in the earth's atmosphere but, with the exception of argon, are found only in very small amounts. Fluorine compounds of the rare gases have only recently been discovered.

inertia

Resistance to acceleration.

inertia force = inertial force.

inertial axes

Axes that are not undergoing acceleration or rotation.

inertial coordinate system

A system in which the (vector) momentum of a particle is conserved in the absence of external forces. Thus, only in an inertial system can Newton laws of motion be appropriately applied.

When relative coordinate systems are used, moving with respect to the inertial system, apparent forces arise in Newton laws, such as the coriolis force.

inertial force

A force is a given coordinate system arising from the inertia of a parcel moving with respect to another coordinate system. The inertial force is proportional and directionally opposite to the accelerating force. Also called inertia force.

For example, the coriolis acceleration on a parcel moving with respect to a coordinate system fixed in space becomes an inertial force, the coriolis force, in a coordinate system rotating with the earth.

inertial guidance

Guidance by means of the measurement and integration of acceleration from within the craft.

inertial navigation

Dead reckoning performed automatically by a device which gives a continuous indication of position by integration of acceleration since leaving a starting point.

inertial orbit

The type of orbit described by all celestial obides, in conformance with Kepler laws of celestial motion.

This applies to all satellites and spacecraft providing they are not under any type of propulsive power.

inertial space

A stationary frame of reference, or set of coordinates, for calculating trajectories.

inertial velocity

Velocity with respect to a fixed system of coordinates.

inferior conjunction

The conjunction of an inferior planet and the sun when the planet is between the earth and the sun.

inferior mirage

A spurious image of an object formed below the true position of that object by abnormal refraction conditions along the line of sight; one of the most common of all types of mirage and the opposite of a superior mirage.

The requirement for the appearance of an inferior mirage is a very large lapse rate of temperature in the layer of air containing the line of sight from observer to object. Compare sinking, stooping.

inferior planets

The planets with orbits smaller than that of the earth: Mercury and Venus.

inferior transit = lower transit.

infinity

1. A point, line, or region, beyond measurable limits.

A source of light is regarded as at infinity if it is at such a great distance that rays from it can be considered parallel. See parallax.

2. Any quantity larger than the largest quantity which can be stored in a register of a specific computer.

inflection

Reversal of direction of curvature.

A point at which reversal takes place is called point of inflection or inflection point.

inflection point

See inflection.

in-flight start

An engine ignition sequence after take-off and during flight. Compare air start, ground start.

This term includes starts both within and above the sensible atmosphere.

information

Any facts or data which can be used, transferred, or communicated.

information content

In a message or a signal from a source, the negative of the logarithm of the probability that this particular message or symbol will be emitted from the source.

The choice of logarithmic base determines the unit of information content. See bit and hartley. The probability of a given message or symbol being emitted may depend on one or more preceding messages or symbols. The quantity has been called self-information.

information gate

In telemetry, a device which, when triggered, allows information pulses to pass.

infrahuman

A live animal other than man used as a substitute for a human in life-science experiments.

infrared (abbr IR)

1. = infrared radiation.

2. Pertaining to infrared radiation, as an infrared absorber.

infrared radiation (abbr IR)

Electromagnetic radiation lying in the wavelength interval from about 75 microns to an indefinite upper boundary sometimes arbitrarily set at 1000 microns (0.01 centimeter). Also called long-wave radiation.

At the lower limit of this interval, the infrared radiation spectrum is bounded by visible radiation, whereas on its upper limit it is bounded by microwave radiation of the type important in radar technology. See electromagnetic spectrum. Whereas visible radiation is generated primarily by intra-atomic processes, infrared radiation is generated almost wholly by larger scale intramolecular processes, chiefly molecular rotations and internal vibrations of many types. Electrically symmetric molecules, such as the nitrogen and oxygen molecules which comprise most of the earth's atmosphere, are not capable of absorbing or emitting infrared radiation, but several of the triatomic gases, such as water vapor, carbon dioxide, and ozone, are infrared active and play important roles in the propagation of infrared radiation in the atmosphere. Since a black body at terrestrial temperature radiates with maximum intensity in the infrared spectrum (near 10 microns), there exist a complex system of infrared radiation currents within the earth's atmosphere.

infrasonic frequency

A frequency below the audiofrequency range.

The word infrasonic may be used as a modifier to indicate a device or system intended to operate at an infrasonic frequency. The term subsonic was once used in acoustics synonymously with infrasonic; such usage is now discourages.

infrasonic sound

Sound whose frequency is below the lower pitch limit, below about 15 cycles per second.

inherited error

The error in initial values used in a computation; especially the error introduced from the previous steps in a step-by-step integration.

inhibitor

Anything that inhibits; specifically, a substance bonded, taped, or dip dried onto a solid propellant to restrict the burning surface and to give direction to the burning process. See restricted propellant.

inhibitor gate

In telemetry, a device which, when triggered, prevents information pulses from passing.

initial mass

The mass of a rocket vehicle at launch.

initial-value problem

A dynamical problem whose solution determines the state of a system at all times subsequent to a given time at which the state of the system is specified by given initial conditions. Also called transient problem. See boundary value problem.

The initial-value problem is contrasted with the steady-state problem, in which the state of the system remains unchanged in time.

initial velocity

The velocity of anything at the beginning of a specific phase of its motion.

injection

1. The introduction of fuel, fuel and air, fuel and oxidizer, water, or other substance into an engine induction system or combustion chamber.

2. The time following launching when nongravitational forces (thrust, lift, and drag) become negligible in their effort on the trajectory of a rocket or spacecraft.

3. The process of putting a spacecraft up to escape velocity.

injector

A device that propels fuel or propellant into a combustion chamber under pressure other than atmospheric. See impinging-stream injector.

inlet

An entrance or orifice for the admission of fluid.

Frequently used in compounds, such as inlet air, inlet air temperature, inlet casing, inlet duct, inlet guide van, inlet port, inlet valve, etc.

inlet pressure

In connection with performance data on pumps, when not otherwise specified, the total static pressure measured in a standard testing chamber by a vacuum gage located near the inlet port. Also called intake pressure, fine pressure, head pressure.

inner liner

Specifically, a tube mounted coaxially inside the outer cover or shell of a combustion chamber. Also called a flame tube or a combustion-chamber liner.

inner planets

The four planets nearest the sun: Mercury, Venus, Earth, and Mars. See planet, table.

inner product = scalar product.

in phase

The condition of two or more cyclic motions which are at the same part of their cycles at the same instant. Also called in step.

Two or more cyclic motions which are not at the same part of their cycles at the same instant are said to be out of phase or out of step.

input

1. The path through which information is applied to any device.

2. The means for supplying information to a machine. See input equipment.

3. Information or energy entering into a system. Compare output.

4. The quantity to be measured, or otherwise operated upon, which is received by an instrument. Also called input signal.

For a thermometer, temperature is the input.

input axis

In a gyro, an axis normal to the spin axis about which a rotation of the base causes a maximum output as a function of this rotation.

input equipment

Specifically, the hardware through which information is fed into a computer.

input signal = input.

insertion

The process of putting an artificial satellite or spacecraftinto orbit.

insolation

(Contracted from incoming solar radiation) 1. In general, solar radiation received at the earth's surface. See terrestrial radiation, extraterrestrial radiation, direct solar radiation, global radiation, effective terrestrial radiation, diffuse sky radiation, atmospheric radiation. 2. The rate at which direct solar radiation is incident upon a unit horizontal surface at any point on or above the surface of the earth. Compare solar constant.

instability

1. The condition of a body if, when displaced from a state of equilibrium, it continues, or tends to continue, to depart from the original condition. Compare stability.

2. Combustion instability.

instantaneous readout

Transmission of data by a radio transmitter instantaneous with the computation of data to be transmitted. See readout station, real time.

instantaneous sound pressure

The total instantaneous pressure at the point of observation minus the static pressure. Often called excess pressure.

in step = in phase.

instruction

1. Information which tells a computer where to obtain the operands, what operations to perform, what to do with the result, and, sometimes, where to obtain the next instruction.

2. = command.

instruction code

An artificial language for describing or expressing the instructions which can be carried out by a digital computer.

In automatically sequenced computers, the instruction code is used when describing or expressing sequences of instructions, and each instruction word usually contains a part specifying the operation to be performed and one or more addresses which identify a particular location in storage. Sometimes, an address part of an instruction is not intended to specify a location in storage but is used for some other purpose. If more than one address is used, the code is called a multiple-address code. In a typical instruction of a four-address code, the addresses specify the location of two operands, the destination of the result, and the location of the next instruction in the sequence. In a typical three-address code, the fourth address specifying the location of the next instruction is dispensed with and the instructions are taken from storage in a preassigned order. In a typical one-address or single-address code, the address may specify either the location of an operand to be taken from storage, the destination of a previously prepared result, or the location of the next instruction. The arithmetic element usually contains at least two storage locations, one of which is an accumulator. For example, operations requiring two operands may obtain one operand from the main storage and the other from a storage location in the arithmetic element which is specified by the operation part.

instrument

To provide a vehicle or component with instrumentation.

instrumentation

1. The installation and use of electronic, gyroscopic, and other instruments for the purpose of detecting, measuring, recording, telemetering, processing, or analyzing different values or quantities as encountered in the flight of a rocket or spacecraft.

2. The assemblage of such instruments in a rocket, spacecraft, or the like.

3. A special field of engineering concerned with the design, composition, and arrangement of such instruments.

instrument landing system

(abbr ILS) A system which provides, in the aircraft, a display of the lateral, longitudinal, and vertical references necessary for a landing.

intake pressure = inlet pressure.

integer

A whole number; a number that is not a fraction.

integral

1. Of or pertaining to an integer.

2. Serving to form a whole or part of a whole, as an integral tank.

integrally stiffened

Of structures, referring to thin-walled components in which increased section wall stiffeners and wall are formed as a single structural member rather than as two separate pieces.

integral tank

A fuel or oxidizer tank built within the normal contours of an aircraft or rocket vehicle and using the skin of the vehicle as a wall of the tank.

integrated trajectory system

(abbr ITS) A multiple trajectory measuring system composed of several angle-measuring-equipment and distance-measuring-equipment sites whereby in-flight selection of station combination can be made to provide the best geometrical solution to space position at any given time of rocket flight.

integrating accelerometer

A transducer designed to measure, and capable of measuring, velocity by means of a time integration of acceleration.

integrator

1. In digital computers, a device for accomplishing a numeric approximation of the mathematical process of integration.

2. A device whose output is proportional to the integral of an input signal.

intensity

1. In general, the degree or amount, usually expressed by the elemental time rate or spatial distribution of some condition or physical quantity, such as electric field, sound, magnetism, etc.

2. With respect to electromagnetic radiation, a measure of the radiant flux per unit solid angle emanating for some source. Frequently, it is desirable to specify this as radiant intensity in order to distinguish it clearly for luminous intensity. Compare emittance.

Occasionally, intensity, is used as synonymous to flux density. This usage does not coincide with accepted photometric and radiometric usage, but is of long standing in meteorology. See sound intensity.

intensity level

In acoustics, ten times the logarithm to the base 10 of the ratio of the intensity I of the sound measured to the reference intensity I₀. The reference intensity I₀ must be stated.

A generally used reference value, especially for air acoustics, is 10^-16 watt per square centimeter.

intensity-modulated indicator

One of two general classes of radar indicators, in which echoes from targets are presented as spots or areas of light whose intensity or brilliance is normally a function of the power of the echo signal. Compare amplitude-modulated indicator. See radarscope.

intensity modulation

The change of the brilliance (or intensity) of the trace on the screen of a cathode-ray tube in accordance with the strength of the applied signal.

interaction parameter

In plasma physics, a measure of the relative importance of the magnetic field and the fluid motion when the magnetic lines are at least partially frozen into the fluid. It is related to the ratio of the magnetic energy density and the fluid kinetic energy.

If the interaction parameter is small, the fluid motion is hardly affected by the field; if it is large, the motion is largely controlled by the field; if it is the order of unity, the two strongly interact, with the net flow a compromise between them.

intercept = altitude difference.

interchange coefficient = exchange coefficient.

interdigitate

To aline two or more sets of fins or projections on a rocket so that each fin or projection of one set lies in a plane between the planes established by fins or projections of the other set or sets.

interface

1. A common boundary between two parts of a system, whether material or nonmaterial.

2. Specifically, in a rocket vehicle or other mechanical assembly, a common boundary between two components. See mating.

3. Specifically, in fluid dynamics, a surface separating two fluids across which there is a discontinuity of some fluid property such as density or velocity or of some derivative of these properties in a direction normal to the interface.

The equations of motion do not apply at the interface but are replaced by the boundary conditions.

interference

1. Extraneous signals, noises, etc. that hinder proper reception of the desired signal in electronic equipment. See babble, clutter, cosmic noise, crosstalk, jitter, static.

2. The mutual effect of two or more meeting waves or vibrations of any kind. Sometimes called wave interference.

interference guard bands

The two frequency bands additional to and on either side of the authorized frequency band, which may be provided to minimize the possibility of interference between different radio channels.

interference region

That region in space in which interference between wave trains occurs.

In microwave propagation, it refers to the region bounded by the ray path and the surface of the earth which is above the radio horizon. Interference lobes and height-gain patterns are formed in this region by the addition of the direct and the surface-reflected wave. In contrast is the diffraction zone which lies below the radio horizon.

interferometer

An apparatus used to produce and measure interference from two or more coherent wave trains from the same source. See radio interferometer.

Interferometers are used to measure wavelengths, to measure angular width of sources, to determine the angular position of sources (as in satellite tracking), and for many other purposes.

interferometric

Pertaining to or measured by an interferometer.

interior ballistics

That branch of ballistics that deals with the propulsion of projectiles, i.e., the motion and behavior of projectiles in a gun barrel, the temperatures and pressures developed inside a gun barrel or rocket, etc. Sometimes called internal ballistics.

intermediate frequency (abbr IF)

The beat frequency used in heterodyne receivers, usually the difference between the received radiofrequency signal and a locally generated signal.

intermediate ion

An atmospheric ion of size and mobility intermediate between the small ion and the large ion.

The mobility of this class of ions lies generally in the interval from 0.01 to 0.1 centimeter per second per volt per centimeter.

intermediate orbit

An orbit tangent to an actual orbit and having the same coordinates but not the same velocity at the point of tangency.

intermittent pressure breathing

Pressure breathing in which different pressures are used at different points in the respiratory cycle, usually with a high pressure during inspiration and lower pressure during expiration.

intermodulation

The modulation of the components of a complex wave by each other in a nonlinear system.

internal ballistics = interior ballistics.

internal efficiency

The efficiency with which a reaction engine, such as a rocket, converts the available thermal energy of its combustion gases into kinetic energy in the exhaust jet, expressed as a ratio.

internal energy

A mathematically defined thermodynamic function of state, interpretable through statistical mechanics as a measure of the molecular activity of the system. It appears in the first law of thermodynamics as du = dq - dw where du is the increment of specific internal energy, dq the increment of heat, and dw the increment of work done by the system per unit mass. The differential du is a perfect differential. Its integral therefore introduces a constant of integration, the zero-point internal energy , so that care must be taken when absolute values of the internal energy are employed.

international candle

The unit of luminous intensity formerly used as the international standard. On January 1, 1948, it was replaced with the candela, which is equal to 58.9/60 or 0.98 international satellite. Also called English candle, British candle.

International Geophysical Cooperation, 1959 (abbr IGC-1959)

An extension of the International Geophysical Year.

International Geophysical Year (abbr IGY)

By international agreement, a period during which greatly increased observation of worldwide geophysical phenomena is undertaken through the cooperative effort of participating nations. July 1957 to December 1958 was the first such year; however, precedent was set by the International Polar Years of 1882 and 1932.

International Gravity Formula

See acceleration of gravity.

international nautical mile

See nautical mile.

International Polar Year

The years 1882 and 1932, during which participating nations undertook increased observation of geophysical phenomena in polar (mostly arctic) regions. The observations were largely meteorological, but included other such as auroral and magnetic studies. This program was continued and expanded in both geographic and scientific scope as the International Geophysical Year.

international standard atmosphere = ICAO Standard Atmosphere; see standard atmosphere.

International Steam Table calorie

(abbr calIT) A unit of heat equal to 4.1868 joules. See calorie.

International System of Units

(abbr SI) The metric system of units based on the meter, kilogram, second, ampere, kelvin, and candela. Also called MSKA system. Other SI units are hertz, radian, newton, joule, watt, coulomb, volt, ohm, farad, weber, and tesla.

International Year of the Quiet Sun

(abbr IQSY) By international agreement, a period, July 1963 to December 1964, during which intensive observations of the sun and related geophysical phenomena are being made. Compare International Geophysical Year.

interpleural

Between the lungs and the chest wall.

interpreter

In computer terminology, a circuit or device which translates an instruction from pseudocode into an instruction or series of instructions which the computer can understand and obey.

interpreter code = pseudocode.

interrogation

Transmission of a radio signal or combination of signals intended to trigger a transponder or group of transponders.

interrogator

1. A radar set or other electronic device that transmits an interrogation.

2. An interrogator-responsor or the transmitting component of an interrogator-responsor.

interrogator-responsor

A radio transmitter and receiver combined to interrogate a transponder and display the resulting replies. Often shortened to interrogator and sometimes called challenger.

intersection

In Boolean algebra, the operation in which concepts are described by stating that they have all the characteristics of the classes involved. Intersection is expressed as AND.

intersector = AND gate.

interval of convergence

See power series.

intervalometer

Any device that may be set so as to accomplish automatically a series of like actions, such as the taking of photographs, or the closure of electrical circuits, at constant predetermined intervals.

inverse

In cartography, same as transverse.

inverse-square law

A relation between physical quantities of the form: x proportional to 1/y² where y is usually a distance; and x terms are of two kinds, forces and fluxes.

inversion

In meteorology, a departure from the usual decrease or increase with altitude of the value of an atmospheric property; also, the layer through which this departure occurs (the inversion layer ), or the lowest altitude at which the departure is found (the base of the inversion ).

This term almost always means a temperature inversion.

inversion temperature

1. In the atmosphere, the temperature at the base of an inversion.

2. Of a gas, a temperature above which the gas gains heat in expansion. See Joule-Thomson effect, note.

inverter

1. A device for changing direct current to alternating current.

2. In computers, a device or circuit which inverts the polarity of a pulse. Also called NOT circuit.

inviscid

Not viscous, not clinging or sticky; frictionless, as in inviscid flow.

inviscid fluid = perfect fluid.

Io

A satellite of Jupiter orbiting at a mean distance of 421,800 kilometers. Also called Jupiter I.

ion

1. A charged atom or molecularly bound group of atoms; sometimes also a free electron or other charged subatomic particle.

An ion pair consists of a positive ion and a negative ion (usually an electron) having charges of the same magnitude and formed from a neutral atom or molecule by the action of radiation. In spectroscopy, the degree of ionization of an atom is indicated by a Roman numeral following the symbol for the element. An un-ionized atom is indicated by the Roman numeral I, a singly ionized atom, one which has lost one electron, is indicated by II, and so on. Thus Fe IX indicates the spectrum of an iron atom which has lost eight electrons.

2. In atmospheric electricity, any of several types of electrically charge submicroscopic particles normally found in the atmosphere. Atmospheric ions are of two principle types, small ions and large ions, although a class of intermediate ions has occasionally been reported.

The ionization process which forms small ions depends upon two distinct agencies, cosmic rays and radioactive emanations. Each of these consists of very energetic particles which ionize neutral air molecules by knocking out one or more planetary electrons. The resulting free electron and positively charged molecule (or atom) very quickly attach themselves to one or, at most, a small number of neutral air molecules, thereby forming new small ions. In the presence of Aitken nuclei, some the small ions will in turn attach themselves to these nuclei, thereby creating new large ions. The two main classes of ions differ widely in mobility. Only the highly mobile small ions contribute significantly to the electrical conductivity of the air under most conditions. The intermediate ions and large ions are important in certain space charge effects, but are too sluggish to contribute much to conductivity. The processes of formation of ions are offset by certain processes of destruction of the ions (see recombination).

3. In chemistry, atoms or specific groupings of atoms which have gained or lost one or more electrons, as the chloride ion or ammonium ion. Such ions exist in aqueous solutions and in certain crystal structures.

ion column

The trail of ionized gases in the trajectory of a meteoroid entering the upper atmosphere; a part of the composite phenomenon known as a meteor. A type of meteor train. See meteor. Compare gas cap.

ion concentration = ion density.

ion counter

An apparatus which counts the number of unit charges of electricity which are contained in a sampled volume of the atmosphere. See aspiration condenser, Ebert ion counter. Compare ionization chamber.

The design of the ion counter depends upon the mobility of the ions under investigation. The general procedure is to pass a sample of the atmosphere through a charged cylindrical condenser. The type of ions collected will depend upon the capacity of the condenser and the polarizing potential. The change in the potential drop across the condenser is a measure of the ionic charge collected.

ion density

In atmospheric electricity, the number of ions per unit volume of a given sample of air; more particularly, the number of ions of a given type (positive small ion, negative small ion, positive large ion, etc.) per unit volume of air. Also called ion concentration.

Measurement of ion density is used in determining efficiency of ionizers in ion engines.

ion engine

A reaction engine in which ions, accelerated in an electrostatic field, are used as propellant. Also called electrostatic engine. See electric propulsion.

ion gage = hot-cathode ionization gage.

ionic conduction

Any electrical conduction where the current is sustained by the motion of ions (as opposed to electrons) within the conductor. All electrical conduction in the atmosphere is of this type.

ionic mobility = ion mobility.

ionization

The process by which neutral atoms or groups of atoms become electrically charged, either positively or negatively, by the loss or gain of electrons; or the state of a substance whose atoms or groups of atoms have become thus charged.

Ionization is a necessary process to produce propellant ions in ion engines.

ionization by collision

The removal of an orbital electron from an atom or molecule by an impacting particle (often, by the absorption of a photon). The atom or molecule is then left with an excess positive charge, i.e., it is positively ionized.

ionization chamber

An apparatus used to study the production of small ions in the atmosphere by cosmic ray and radioactive bombardment of air molecules.

The chamber is an airtight container usually cylindrical in shape and 25 to 50 liters in volume. An insulated electrode is centrally located in the chamber. In operation a potential is applied between the electrode and the chamber wall. The ions produced in the chamber are collected by the electrode system and measured by an electrometer.

ionization gage

A vacuum gage with a means of ionizing the gas molecules and a means of correlating the number and type of ions produced with the pressure of the gas. Various types of ionization gage are distinguished according to the method of producing the ionization. Some common types are: hot-cathode ionization gage, cold-cathode ionization gage, radioactive ionization gage.

ionization potential

The energy required to ionize an atom or molecule. The energy is usually given in terms of electron volts. See work function, note.

ionizer

A filament, grid, or porous body in an ion engine or other device which strips an electron from the outer shell of a neutral atom to form a positively charged ion.

ionizer efficiency

The ratio of the number of ions emitted from an ionizer to the number of neutral atoms entering the ionizer.

ionizing efficiency

See ion pair, ionizer efficiency.

ionizing event

Any interaction by which one or more ions are produced.

ionizing radiation

Any electromagnetic or particulate radiation capable of producing ions, directly or indirectly, in its passage through matter.

ion mobility

In gaseous electric conduction, the average velocity with which a given ion drifts through a specified gas under the influence of an electric field of unit strength. Mobility's are commonly expressed in units of centimeters per second per volt per centimeter. Also called ionic mobility.

In a vacuum, a single gaseous ion subjected to any nonzero potential gradient would accelerate indefinitely; but in the midst of a gas the ion continually experiences collisions with gas molecules. These encounters tend to break up its trajectory into a series of short intervals of acceleration punctuated by deflections. The net result is that the ion's gross motion resembles drift at a uniform velocity. The mobility depends not only upon the nature of the ion and gas but also upon the density of the gas, for the latter controls the mean free path of the ion.

ionophone

A unilateral transducer in which the sound output results from the interaction between an ionic plasma and the surrounding sound transmitting medium.

ionosphere

The atmospheric shell characterized by a high ion density. Its base is at about 70 or 80 kilometers and it extends to an indefinite height.

The ionosphere is classically subdivided into layers. Each layer, except the D-layer, is supposedly characterized by a more or less regular maximum of electron density. The D-layer exists only in the daytime. It is not strictly a layer at all, since it does not exhibit a peak of electron or ion density, starting at about 70 to 80 kilometers and merging with the bottom of the E-layer. The lowest clearly defined layer is the E-layer, occurring between 100 and 120 kilometers. The F1-layer and F2-layer occur in the general region between 150 and 300 kilometers, the F2-layer being always present and having the higher electron density. The existence of a G-layer has been suggested, but is questionable. The portions of the ionosphere in which these layers tend to form are known as ionosphere in which these layers tend to form are known as ionospheric regions, as in D-region, E-region, F-region, G-region. Sudden increases in ionization are referred to as sporadic, as in sporadic E or sporadic D. The above assumption that the ionosphere is stratified in the vertical into discrete layers is currently under serious question. Some evidence supports a belief that ion clouds are the basic elements of the ionosphere. Other investigations appear to reveal the ionosphere as a generally ionized region characterized by more or less random fluctuations of electron density.

ionospheric layer

See ionosphere.

ionospheric recorder

A radio device for determining the distribution of virtual height with frequency, and the critical frequencies of the various layers of the ionosphere.

A pulse at a certain frequency is transmitted vertically, and the time for its return is recorded on an oscilloscope; another pulse at a different frequency is then transmitted and timed. The process is thus repeated until the entire frequency range of interest, usually from 1 to 20 megacycles, has been explored.

ionospheric region

See ionosphere.

ionospheric storm

Disturbance of the ionosphere, resulting in anomalous variations in its characteristics and effects on radio communication. See sudden ionospheric disturbance.

ionospheric wave = sky wave.

ion pair

The pair of ions, one positively and the other negatively charged, formed by the ionization of an initially neutral gas atom when it collides with a high-energy particle.

It is customary to measure the ionizing efficiency of cosmic rays and radioactive materials in terms of the number of ion pairs they produce each second in 1 cubic centimeter of air. The symbol I is usually employed to designate the formation of one ion pair per cubic centimeter per second. Thus, cosmic rays are said to yield about 21 at sea level, and alpha particles from the radioactive gases contribute about 51 over land areas at the surface.

ion rocket

See ion engine.

IQSY (abbr) = International Year of the Quiet Sun.

IR (abbr) = infrared or infrared radiation.

iraser

(Infrared amplification by stimulated emission of radiation). A device which amplifies in the infrared band. See maser.

irradiance = radiant flux density.

I-scan = I-display.

I-scope = I-display.

isentrope

A line of equal or constant entropy. See Poisson equation.

In meteorology, it may be considered an isopleth of potential temperature, i.e., the same as a dry adiabat.

isentropic

Of equal or constant entropy with respect to either space or time.

isobar

A line of equal or constant pressure, specifically, such a line in a weather map.

isobaric

Of equal or constant pressure, with respect to either space or time.

Because isobar is a common meteorological term, isobaric can be taken to mean of isobars, therefore leading to some ambiguity. This use should be avoided.

isobaric equivalent temperature

See equivalent temperature, sense 1.

isochoric

Of equal or constant volume, usually applied to a thermodynamic process during which the volume of the system remains unchanged. Compare isosteric.

isoclinic line

A line through points on the earth's surface having the same magnetic dip. See aclinic line. Compare isogonic line.

isogonic line

A line through points on the earth's surface having the same magnetic variation. See agonic line. Compare isoclinic line.

isogram = isopleth.

isolation

In vibration studies, a reduction in the capacity of a system to respond to an excitation, attained by the use of a resilient support.

isolator = vibration isolator.

isomer

1. One of two or more nuclides having the same mass number A and atomic number Z, but existing for measurable times in different quantum states with different energies and radioactive properties.

The state of lowest energy is the ground state. Those of higher energies are metastable states. To indicate the metastable isomer, the letter m is added to the mass number in the symbol for the nuclide; thus Br^80mCommonly, the isomer of higher energy decays to one with lower energy by the process of isometric transition.

2. One of two or more molecules having the same atomic composition and molecular weight, but differing in geometrical configuration.

isometric transition

A radioactive transition from one nuclear isomer to another of lower energy.

The deexcitation of the nuclei in the metastable state may occur by gamma emission or by internal conversion followed by emission of X-rays and Auger electrons, or both. It is a type of forbidden transition.

isopleth

On a chart or graph, a line of constant value of a given quantity with respect to either space or time. Also called isogram.

isopycnic

Of equal or constant density with respect to either space or time; equivalent to isosteric.

isostasy

A supposed equality existing in vertical sections of the earth, whereby the weight of any column from the surface of the earth to a constant depth is approximately the same as that of any other column of equal area, the equilibrium being maintained by plastic flow of material from one part of the earth to another.

isosteric

Of equal of constant specific volume with respect to either time or space; equivalent to isopycnic. Compare isochoric.

isotensoid structure

A filamentary structure in which the filaments are uniformly stressed throughout for the design loading condition.

isotherm

A line of equal or constant temperature.

A distinction is made, infrequently, between a line representing equal temperature in space, choroisotherm, and one representing constant temperature in time, chronoisotherm.

isothermal atmosphere

An atmosphere in hydrostatic equilibrium in which the temperature is constant with height and in which, therefore, the pressure decreases exponentially upward. In such an atmosphere the thickness between any two levels is given by Z_B - Z_A = (R_dT_v /g) ln (p_A/p_B) where R_d is the gas constant for dry air; T_v is the virtual temperature (K); g is the acceleration of gravity, and p_A and p_B are the pressures at the heights Z_A and Z_B respectively. In the isothermal atmosphere there is no finite level at which the pressure vanishes. Also called exponential atmosphere. See barotropy.

isothermal equilibrium

The state of an atmosphere at rest, uninfluenced by any external agency, in which the conduction of heat from one part to another has, after a sufficient length of time, produced a uniform temperature throughout its entire mass. Also called conductive equilibrium. See diffusive equilibrium, isothermal atmosphere.

isothermal process

Any thermodynamic change of state of a system that takes place at constant temperature.

isotimic

Pertaining to a quality which has equal value in space at a particular time.

isotope

1. On of several nuclides having the same number of protons in their nuclei, and hence belonging to the same element, but differing in the number of neutrons and therefore in mass number A, or in energy content (isomers). For example, ₆C₆¹², ₆C₇¹³, and ₆C₈¹⁴ are carbon isotopes. Small quantitative differences in chemical properties exist between isotopes.

2. A radionuclide or a preparation of an element with special isotopic composition (allobar) as an article of commerce, so called because of the principal use of such materials as radioactive tracers.

3. In common usage, a synonym for nuclide (not recommended).

isotropic

In general, pertaining to a state in which a quantity or spatial derivatives thereof are independent of direction. Also called isotropous.

isotropic antenna = unipole.

isotropic radiation

Diffuse radiation which has exactly the same intensity in all directions.

This should not be called perfectly diffuse radiation because of the likelihood of confusion with the concept of a perfectly diffuse radiator.

isotropic radiator

An energy source that radiates uniformly in all directions. Compare perfectly diffuse radiator.

isotropic turbulence

Turbulence in which the products and squares of the velocity components and their derivatives are independent of direction, or, more precisely, invariant with respect to rotation and reflection of the coordinate axes in a coordinate system moving with the mean motion of the fluid. Then all the normal stresses are equal and the tangential stresses are zero.

Atmospheric turbulence is generally nonisotropic, although isotropic turbulence is that most easily produced in wind tunnel experiments and forms the basis of much of the theoretical analysis of turbulent flow. A related but less restricted type of turbulence is known as homologous turbulence, in which the fluctuations differ only in scale at every point in the flow.

isotropous = isotropic.