Back to Table of Contents
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.
Greek symbols may not appear correctly in some browsers. For example
a gamma may appear as γ.
Back to Table of Contents
- An electroacoustic
transducer operating from an electrical system to an acoustical system and
intended to be closely coupled acoustically to the ear.
- See planet, table.
- earth axis
- Any one of a set of mutually perpendicular reference axes established
with the upright axis (the Z-axis) pointing to the center of the earth, used
in describing the position or performance of an aircraft or other body in
- The earth axes may remain fixed or may move with the aircraft or other
- earth current
- A large-scale surge of electric charge within the earth's crust,
associated with a disturbance of the ionosphere.
- Current patterns of quasi-circular form and extending over areas the
size of whole continents have been identified and are known to be closely
related to solar-induced variations in the extreme upper atmosphere.
- The illumination of the dark part of the moon's disk produced by sunlight
reflected onto the moon from the earth's surface and atmosphere. Also called
- Spectroscopic observations reveal that earthlight is relatively richer
in blue light than is direct sunlight; this condition results from the fact
that an appreciable part of the total earth reflection is backward-scattered
light which, in accordance with Rayleigh
law, is relatively rich in the blue and poor in the red.
- earth point
- The point where the forward straight-line projection of a meteor
trajectory intersects the surface of the earth.
- earth radiation = terrestrial
- earth-rate unit (abbr eru)
- A unit of angular drift, as of a gyro, equal to the
rate of angular movement of the earth with respect to the stars, 15° per hour.
- earth satellite
- A body that orbits about the earth; specifically, an artificial satellite
placed in orbit by man.
- earth shine = earthlight.
- earth's rate correction
- A command rate applied to a gyro to compensate
for the apparent precession
of the gyro spin axis with respect to its base caused by the rotation of the
- earth tide
- A periodic movement of the earth's crust caused by the tide-producing
forces of the moon and sun.
- Ebert ion counter
- An ion
counter of the aspiration
condenser type, used for the measurement of the concentration and mobility
ions in the atmosphere.
- The formation of bubbles, with particular reference to water vapor bubbles
in biological fluids caused by reduced ambient pressure; the boiling of body
- Not having the same center; varying from a circle.
- eccentric anomaly (symbol E)
- See anomaly.
- eccentricity (symbol e)
- 1. Of any conic, the ratio
of the length of the radius vector through a point on the conic to the
distance of the point from the directrix.
- 2. Of an ellipse, the
ratio of the distance between the center and focus of an ellipse to its
semimajor axis. Also called numerical eccentricity.
eccentricity e of an ellipse can be computed by the formula
a is the semimajor axis and b is the semiminor axis.
- 3. Of an ellipse, the distance between the center and the focus. Also
called linear eccentricity.
- 1. A wave
that has been reflected or otherwise returned with sufficient magnitude and
delay to be detected as a wave distinct from that directly transmitted.
- 2. In radar, a pulse of reflected radio frequency energy; the appearance
on a radar indicator of
the energy returned from a target. Also called blip.
- echo intensity
- The brightness or brilliance of a radar echo as displayed
on an intensity-modulated
indicator. Echo intensity is, within certain limits, proportional to the
voltage of the target signal or to the square root of its power. Compare echo power.
- echo power
- The electrical strength, or power, of a radar
signal. Echo power is normally measured in watts or dbm (decibels referred
to a milliwatt).
- echo pulse
- A pulse of
radio energy received at the radar after reflection from a target; that is,
signal of a pulse
- echo signal = target
- 1. The reduction in visibility or disappearance of a nonluminous body by
passing into the shadow cast by another nonluminous body.
- 2. The apparent cutting off, wholly or partially, of the light from a
luminous body by a dark body coming between it and the observer.
- 1. The first type of eclipse is exemplified by a lunar eclipse, the
moon passing through the shadow cast by the earth; or by the passage of a satellite
into the shadow cast by its planet; but when the satellite actually passes
directly behind its planet, it may properly be termed an occultation.
- 2. The second type of eclipse is exemplified by a solar eclipse, caused
by the moon passing between the sun and the earth. If the relative positions
and distances are such that at a point on the earth the sun is completely
obscured, the eclipse is total; if the distances are such that, when in
line with the sun, the moon is surrounded by a ring of light, the eclipse is
annular; and when the moon passes to one side of a straight line from
sun to observer and shows a crescent of light, it is a partial eclipse.
- eclipse year
- The interval between two successive conjunctions
of the sun with the same node of the moon's
orbit, averaging 346 days 14 hours 52 minutes 52.42 seconds in 1962, and
increasing at the rate of 0.0276 second annually. See year.
- The apparent annual path of the sun among the stars; the intersection of
the plane of the earth's orbit with the celestial
- The ecliptic is a great
circle of the celestial sphere inclined at an angle of about 23° 27' to
the celestial equator.
- ecliptic longitude = celestial
- ecliptic pole
- On the celestial
sphere, either of the two points 90° from the ecliptic.
- ecliptic system of coordinates
- A set of celestial
coordinates based on the ecliptic as
great circle. See coordinate,
- The points 90° from the ecliptic are the north and south ecliptic
poles. Angular distance north or south of the ecliptic, analogous to latitude,
is celestial latitude. Celestial longitude is measured eastward along the
ecliptic from the vernal equinox through 360°.
- ecological system
- A habitable environment,
either created artificially, as in a manned space vehicle, or occurring
naturally, such as the environment on the surface of the earth, in which man,
animals, or other organisms can live in mutual relationship with one another
and the environment.
- Ideally the environment furnishes the sustenance for life, and the
resulting waste products revert or cycle back into the environment to be used
again for the continuous support of life.
- The study of the environmental relations of organisms. See environment.
- A reservoir in a continuous-flow
oxygen system in which oxygen exhaled by the user is collected for
recirculation in the system.
- 1. = biosphere.
- 2. A volume of space surrounding the Sun, extending from the orbit of
Venus past the orbit of Mars, in which some biologists believe conditions are
favorable for the development and maintenance of life.
- In a fluid, any
circulation drawing its energy from a flow of much
larger scale and brought about by pressure irregularities.
- eddy coefficient = exchange
- eddy stresses = Reynolds
- eddy velocity
- The difference between the mean velocity of fluid flow and the
instantaneous velocity at a point. For example
u' = u - , where u'
is the eddy velocity; u is the instantaneous velocity; and is mean velocity. Also called fluctuation
- Over the same interval which defines the mean velocity, the average
value of the eddy velocity is necessarily zero.
- eddy viscosity
- The turbulent transfer of momentum by eddies giving
rise to an internal fluid friction, in a manner analogous to the action of
molecular viscosity in laminar flow, but taking place on a much larger scale.
- The value of the coefficient of eddy viscosity (an exchange
coefficient) is of the order of 104 square centimeters per
second, or 100,000 times the molecular kinematic viscosity.
- edge effect
- See diffraction,
- In radar, a rectangular display in which targets appear
as blips with
distance indicated by the horizontal coordinate and elevation by
the vertical coordinate. Also called E-scan and E-scope.
- In computer terminology, to arrange, delete, select, or add to
- EDP (abbr) = electronic data processing.
- Of a computer, stored subroutines
and subprograms which are available for use in automatic programming.
- effective aperture = effective
- effective area
- 1. In antenna design, the ratio of the received
power available at the terminals of an antenna to the
power per unit area in the incident wave. For all antennas, effective area
A is related to gain G at a given wavelength by the equation:
A / G = 2 / 4Also called effective aperture. See
- The effective area of an ideal antenna is equal to its physical area
S. In practice, A/S for microwave antennas is always less than
one, a representative value for paraboloids being 0.6.
- 2. Same as scattering cross section.
- effective atmosphere
- 1. That part of the atmosphere
which effectively influences a particular process or motion, its outer limits
varying according to the terms of the process or motion considered.
- For example, an earth satellite orbiting at 250 miles altitude remains
within the ionosphere, but because the air particles are so sparse at this
altitude as to cause no appreciable friction of deflection, the satellite may
be considered to be outside the effective atmosphere. For movement of air
vehicles the effective atmosphere ends at the aeropause
- 2. = optically
- effective earth radius
- See effective
radius of the earth.
- effective exhaust velocity (symbol ce)
- A fictitious exhaust
velocity that would theoretically produce the observed value of jet thrust.
- The effective exhaust velocity ce is determined by the
ce = V + [A(p1 -
p2) g / w]where V is the velocity of the
exhaust gases; A is the nozzle exit area; p1 is static pressure at
the nozzle exit; p2 is ambient pressure; g is the acceleration of
gravity; and w is the weight flow rate of exhaust gases.
- effective multiplication factor (symbol
- The ratio of the neutron
flux in a nuclear reactor to that
supplied by a neutron source.
- effective neutron cycle time
- The lifetime of an average neutron within
from the time it is produced to the time it is fission captured.
- This average takes into account delayed as well as prompt neutrons.
- effective propagation velocity
- The velocity of an electromagnetic
signal which, when multiplied by the transit time for a ray path, gives a
value for actual path length.
- effective radiation = effective
- effective radius of the earth
- A fictitious value for the radius of the earth, used in place of the
geometrical radius to correct for atmospheric refraction when the index
of refraction in the atmosphere changes linearly with height. See modified
index of refraction.
- Under conditions of standard refraction the effective radius of the
earth is 8.5 * 106 meters, or four-thirds the geometrical radius.
If the effective radius is used in ray tracing diagrams, the rays may be drawn
as though they were traveling in straight lines.
- effective Reynolds number
- A fictitious Reynolds
number applied to the flow of air about a body in a wind
tunnel, equal to the free-air Reynolds number at which the effect obtained
is the same as the effect obtained in the wind tunnel.
- effective sound pressure
- The root-mean-square value of the instantaneous pressure of sound waves,
taken over a complete cycle or a period long compared with a cycle, at that
point. The unit is the microbar (dynes per square centimeter).
- effective temperature
- 1. In astrophysics, a measure of the temperature of a star deduced by
means of the Stefan-Boltzmann
law, from the total energy emitted per unit area. Compare brightness
- Effective temperature is always less than actual temperature.
- 2. In physiology, the temperature at which motionless, saturated air would
induce, in a sedentary worker wearing ordinary indoor clothing, the same
sensation of comfort as that induced by the actual conditions of temperature,
humidity, and air movement. Compare sensible
temperature, standard operative temperature, operative
- Effective temperature is used as a guide in air-conditioning practice,
and, on the comfort chart (American Society of Heating and Air Conditioning
Engineers) it appears as a family of curves which serves as one coordinate in
defining comfort zones.
- effective terrestrial radiation
- The amount by which outgoing infrared terrestrial
radiation of the earth's surface exceeds downcoming infrared counter-radiation
from the atmosphere. Also called nocturnal radiation, effective radiation.
- It is to be emphasized that this amount is a positive quantity, of the
order of several tenths of a langley per minute, at all times of day (except
under conditions of low overcast clouds). It typically attains its diurnal
maximum during the midday hours when high soil temperatures create high rates
of outgoing terrestrial radiation. (For this reason the synonym nocturnal
radiation is apt to lead to slight confusion.) However, in daylight hours the
effective terrestrial radiation is generally much smaller than the insolation,
while at night it typically dominates the energy budget of the earth's
- effective wavelength
- The wavelength
corresponding to the effective
- Any device used to maneuver a rocket in
flight, such as an aerodynamic surface, a gimbaled motor, or a jet.
- efficiency (symbol )
- Of a device with respect to a physical quantity which may be stored,
transferred, or transformed by the device, the ratio of the useful output of
the quantity to its total input.
- Unless specifically stated otherwise, the term efficiency means
efficiency with respect to power.
- egads (abbr) = electronic ground
automatic destruct sequencer.
- egads button
- A button used by the range safety officer to initiate destruction of a
rocket vehicle in flight if its course, as plotted during flight, is predicted
to go beyond the destruct line. See egads, impact
- EHF (abbr) = Extremely
High Frequency. See frequency
- eigenmode = normal
mode of vibration.
- See characteristic
- eight ball
- Common name given to a flight attitude
- ejection capsule
- 1. In an aircraft or manned spacecraft, a detachable compartment serving
as a cockpit or cabin, which may be ejected as a unit and parachuted to the
- 2. A satellite, probe, or unmanned spacecraft, a boxlike unit, usually
containing recording instruments or records of observed data, which may be
ejected and returned to earth by a parachute or other deceleration device.
- A device consisting of a nozzle, mixing
tube, and diffuser
utilizing the kinetic energy of a fluid stream to
pump another fluid from a low pressure region by direct mixing and ejecting
- Ekman layer
- The layer of transition between the surface
boundary layer, where shearing stress is constant, and the free
atmosphere, where the atmosphere is treated as an ideal fluid in
equilibrium. Also called spiral layer.
- In Ekman's analysis (see Ekman
spiral), the coefficient of eddy viscosity is assumed constant within this
layer; subsequent calculations have relaxed this assumption.
- Ekman spiral
- As used in meteorology, an idealized mathematical description of the wind
distribution in the planetary
boundary layer of the atmosphere, within which the earth's surface has an
appreciable effect on the air motion. The model is simplified by assuming that
within this layer eddy viscosity and density are constant, the motion is
horizontal and steady, the isobars are straight and parallel, and the geostrophic
wind is constant with height.
- elastic collision
- A collision
between two particles in
which no change occurs in the internal energy of the particles, or in the sum
of their kinetic energies. Commonly referred to as a billiard-ball
- The ability of a body which has been deformed by an applied force to
return to its original shape when the force is removed.
- An elastic substance or fuel used in a solid rocket propellant to prevent
cracking of the propellant grain and to bind
it to the combustion-chamber case.
- elastic wave
- See sound.
- Rubber-like compounds.
- Elastomers are used as pliable components, as in tires, seals, or
- A division of the ionosphere,
usually found at an altitude between 100 and 120 kilometers in the E-region. It
exhibits one or more distinct maximums and sharp gradients of free
electron density. It is most pronounced in the daytime but does not
entirely disappear at night. Also called E1 - layer,
Kennelly-Heaviside layer, Heaviside layer. See sporadic E-layer
(under ionosphere), atmospheric
- There is some evidence to indicate a second layer above the normal
E-layer located at about 150 kilometers, and called the
- Involving the flow of electricity in a conductor. Compare electronic.
- electrical distance
- The distance between two points expressed in terms of the duration of
travel of an electromagnetic
wave in free space between the two points.
- A convenient unit of electrical distance is the light microsecond or
approximately 983 feet (300 meters). In the use of this unit, electrical
distance is numerically equal to transmission time in microseconds.
- electrical element
- See element, sense
- electrical engine
- A rocket
engine in which the propellant
is accelerated by some electrical device. Also called electric propulsion
system, electric rocket.
- Electrical engines can be classified as electrothermal, electrostatic,
or electromagnetic, depending on the nature of the accelerating device.
- electric-current element = electric
- electric dipole
- A pair of equal and opposite charges an infinitesimal distance apart.
- In electromagnetics, the term dipole is often applied to two
equal and opposite oscillating charges an infinitesimal distance apart; in
this sense, it is synonymous with an electric-current element.
- electric discharge
- The flow of electricity through a gas, resulting in the emission of
radiation that is characteristic of the gas and of the intensity of the
current. Also called discharge, gaseous electric discharge, gaseous
discharge. See corona
- electric field
- 1. A region in which a charged particle would experience an electrical
force; the geometric array of the imaginary electric
lines of force that exist in relation to points of opposite charge.
- An electric field is a vector field in which magnitude of the vector is
the electric-field strength and the vector is parallel to the lines of force.
- 2. = electric-field
strength. See atmospheric
- electric-field intensity = electric-field
- electric-field strength
- The electrical force exerted on a unit positive charge at a given point in
space. Electric-field strength is expressed, in the practical system of
electrical units, in terms of volts/centimeter. It is a vector quantity, being
the magnitude of the electric-field vector. Also called electric field,
electric intensity, electric field intensity, electric potential gradient,
- The electric-field strength of the atmosphere is commonly referred to
as the atmospheric electric field.
- electric intensity = electric-field
- electric lines of force
- Imaginary lines defined by the paths traced by unit charges placed in an
electric field. Lines of force are everywhere parallel to the electric
field strength vector. Their principal use is as a convenient means of
picturing the geometry of an electric field. See magnetic
lines of force.
- electric potential
- In electrostatics, the work done in moving unit positive charge from
infinity to the point whose potential is being specified. Sometimes shortened
- electric potential gradient = electric-field
- electric power level = level.
- electric propulsion
- A general term encompassing all the various types of propulsion in which
consists of charged electrical particles which are accelerated by electrical
or magnetic fields, or both; for example, electrostatic propulsion,
electromagnetic propulsion, electrothermal propulsion.
- electroacoustic transducer
- A transducer
for receiving waves from an
electric system and delivering waves to an acoustic system, or vice versa.
- Microphones and earphones are electroacoustic transducer.
- See chemical
- electrochemical transducer
- A transducer
which uses a chemical change to indicate the input parameter.
- 1. A terminal at which electricity passes from one medium into another.
The positive electrode is called anode; the negative electrode is
- 2. In a semiconductor device, an element that
performs one or more of the functions of emitting or collecting electrons or
holes, or of
controlling their movements by an electric field.
- 3. In electron tubes, a conducting element that
performs one or more of the functions of emitting, collecting or controlling,
by an electromagnetic field, the movements of electrons or ions. See anode (electron
tubes) and cathode.
- The science dealing with the forces and energy transformations of electric
currents, and the magnetic
fields associated with them.
- A laterally limited relatively intense electric current located in the ionosphere.
- electrokinetic transducer
- A transducer
that depends for its operation on the dielectric polarization in certain
liquids resulting from viscous shearing stress that accompanies flow through
- Emission of
light caused by an application of electric fields to solids or gases.
- In gas electroluminescence, light is emitted when the kinetic energy of
electron or ions accelerated in an electric field is transferred to the atoms
or molecules of the gas in which the discharge takes place.
- Of or pertaining to magnetism produced by or associated with electricity.
- Magnetoelectric pertains to electricity produced by or associated with
- electromagnetic energy = electromagnetic
- electromagnetic radiation
- Energy propagated through space or through material media in the form of
an advancing disturbance in electric and magnetic fields existing in space or
in the media. The term radiation , alone, is used commonly for this
type of energy, although it actually has a broader meaning. Also called
electromagnetic energy or simply radiation. See electromagnetic
- electromagnetic rockets = plasma
rockets. See electric
- electromagnetic spectrum
- The ordered array of known electromagnetic
radiations, extending from the shortest cosmic rays, through gamma rays,
X-rays, ultraviolet radiation, visible radiation, infrared radiation, and
including microwave and all other wavelengths of radio energy. See absorption
- The division of this continuum of wavelengths (or frequencies) into a
number of named subportions is rather arbitrary and, with one or two
exceptions, the boundaries of the several subportions are only vaguely
defined. Nevertheless, to each of the commonly identified subportions there
correspond characteristic types of physical systems capable of emitting
radiation of those wavelengths. Thus, gamma rays are emitted from the nuclei
of atoms as they undergo any of several types of nuclear rearrangements;
visible light is emitted, for the most part, by atoms whose planetary
electrons are undergoing transitions to lower energy states; infrared
radiations are associated with characteristic molecular vibrations and
rotations; and radio waves, broadly speaking, are emitted by virtue of the
accelerations of free electrons as, for example, the moving electrons in a
radio antenna wire.
- electromagnetic theory
- See electromagnetic
- electromagnetic wave
- A wave produced by oscillation
of an electric charge. See electromagnetic
- 1. Magnetism produced by an electric current.
- 2. The science dealing with the physical relations between electricity and
- electromechanical transducer
- A transducer
for receiving waves from an
electric system and delivery waves to a mechanical system, or vice versa.
- A visible or audible manifestation of atmospheric
electricity. This includes, therefore, not only visible electric
discharges (igneous meteors) but also the sounds produced by them, principally
- An instrument for measuring differences of electric
- A record of the response of a muscle to an electric stimulation.
- The subatomic
particle that possesses the smallest possible negative electric charge
(4.80298 * 10-10 electrostatic units). See physical
- The mass of the electron is approximately equal to 1/1836 that of a
hydrogen atom; its theoretical rest mass
(symbol me) is equal to 9.109 3897(54) * 10-31
kg and its rest energy is equal to 0.510 999 06(15) million
electron-volt. The charge-to-mass ratio for the electron (symbol
e/me) is 1.758796 cc½ / g½.
term electron is usually reserved for the orbital or extranuclear
particle, whereas the term beta particle refers to a nuclear electron.
*The numbers in bold type were taken from the 1987 publication of
Fundamental Physical Constants: 1986 CODATA (Committee on Data for Science
and Technology of the International Council of Scientific Unions) Recommended
Values issued by the U. S. Department of Commerce, National Bureau of
"Digits in parentheses represent one standard deviation
uncertainty in the final digits of the given value, based on a least-squares
analysis with five variables and 17 degrees of freedom."
- electron avalanche
- The process in which a relatively small number of free electrons in
a gas that is subjected to a strong electric field accelerate, ionize gas
atoms by collision, and thus form new free electrons to undergo the same
process in cumulative fashion.
- An avalanche cannot begin until the local electric field strength is
high enough to accelerate a free electron to the minimum
ionizing speed in the space and time interval corresponding to one mean
free path of the electron, for upon collision, the electron usually loses its
forward motion in the direction of the field.
- electron beam
- Specifically, a focused stream of electrons used for neutralization of the
positively charged ion beam in an ion engine.
Also used to melt or weld materials with externally high melting points.
- electron device
- A device in which electricity is conducted principally by electrons
moving through a vacuum, gas, or semiconductor.
- electron gun
- An electrode
structure which produces and may control, focus, deflect, and converge one or
- 1. Involving the flow of electrons in
a vacuum or through semiconductors.
- 2. Of or pertaining to electronics, i.e., to that branch of physics that
treats of the emission, transmission, behavior, and effects of electrons,
especially as applied by means of vacuum tubes, cathode-ray tubes,
photoelectric cells, and the like, together with the associated electrical
- electronic Bohr magneton = Bohr
- electronic data processing
- The use of electronic
devices and systems in the processing of data so as to interpret the data and
put them into usable form.
- electronic missile acquisition (abbr EMA)
- A crossed baseline interferometer
system giving azimuth and elevation
- This system was designed as an acquisition aid for theodolites. The EMA
equipment operates on the Dovap transponder
- That branch of physics that treats of the emission, transmission,
behavior, and effects of electrons.
- electronic sky screen equipment
- See elsse.
- electronic transducer
- A unilateral
transducer that depends for its operation on the generation of a voltage
by the relative motion of the electrodes
in a vacuum tube.
- electronic work function = Helmholtz
- electron tube
- A device in which conduction by electrons
takes place through a vacuum or gaseous medium within a gastight envelope.
- electron-volt (abbr ev)
- A unit of energy equal to
the energy required to move an electron through a potential difference of 1
volt. Often shortened to volt.
- One electron volt equals 1.6020 * 10-9 joule.
- electrostatic memory
- The ability of a substance or device to retain an electrostatic
charge after the charging force is removed.
- electrostatic rocket = ion rocket.
propulsion, ion engine.
- electrostatic storage
- In a computer, storage of
information in the form of electrostatic
- electrostatic-storage tube
- A cathode-ray
tube in which information is stored as positive or negative charges on a
- The phenomenon wherein some dielectric
materials experience an elastic strain when subjected to an electric field,
this strain being independent of polarity of the field.
- electrothermal rocket = electric thermal rocket. See electric
- 1. One of the simple parts of which a complex entity is composed.
- 2. In chemistry, a substance which cannot be broken down by ordinary
chemical means into simpler components.
- 3. In an electron
tube, a constituent part of the tube that contributes directly to the
electrical operation of the tube.
- 4. In a circuit, any
electrical device (such an inductor, resistor, capacitor, generator, line,
electron tube) with terminals at which it may be directly connected to other
- 5. In a semiconductor
device, any integral part of the semiconductor device, any integral part
of the semiconductor device that contributes to its operation.
- 6. = orbital
- elevated pole
- The celestial
pole above the horizon.
- The celestial pole below the horizon is called depressed pole.
- elevation = angle of
- elevation angle = angle of
- ELF (abbr) = extremely
low frequency. See frequency
- A plane curve constituting the locus of all points the sum of whose
distances from two fixed points called focuses or foci is
constant; an elongated circle. See conic
- The orbits of
planets, satellites, planetoids, and comets are ellipses, the primary being at
- A surface whose plane sections (cross sections) are all ellipses or
circles, or the solid enclosed by such a surface. Also called ellipsoid of
- ellipsoid of revolution = ellipsoid.
- It is so named from the fact that it can be formed by revolving an ellipse about
one of its axes.
- Pertaining to an ellipse, or in
the form of an ellipse.
- elliptically polarized sound wave
- A transverse
wave in an elastic medium in which the displacement vector at any point
rotates about the point and has a magnitude which varies as the radius
vector of an ellipse.
- An elliptically polarized wave is equivalent to two superposed plane
polarized waves of simple sinusoidal form in which the displacements lie in
perpendicular planes and are 90° apart in phase.
- elliptical polarization
- The polarization
of a wave
radiated by an electric vector rotating in a plane and simultaneously varying
in amplitude so as to describe an ellipse.
- elliptical system
- A tracking or navigation system where ellipsoids
of position are determined from time or phase summation
relative to two or more fixed stations which are the focuses for the
- ellipticity (symbol e)
- The amount by which a spheroid
differs from a sphere or an ellipse differs
from a circle, calculated by dividing the difference in the length of the axes
by the length of the major axis. Also called compression. See flattening.
- ellipticity ratio
- 1. The ratio of the major axis
to the minor
axis of an ellipse.
- 2. As a measure of elliptical
polarization, the power ratio of the maximum to the minimum electric
vectors of an elliptically polarized antenna.
- The angular distance of a body of the solar
system from the sun; the angle at the earth between lines to the sun and
another celestial body of the solar system. The term is usually used only in
connection with inferior
- The greatest elongation of such a body is its maximum angular distance
from the sun before it starts back toward conjunction. The direction of the
body east or west of the sun is usually specified, as greatest elongation
- elsse (abbr) = Electronic
sky screen equipment.
- An electronic device which indicates the departure of a rocket from a
- EMA (abbr)= electronic
- Large amounts of air in the blood stream which, reaching the heart, cause
it to fail; small amounts are resorbed and cause no symptoms.
- emissance = emittance,
- 1. With respect to electromagnetic
radiation, the process by which a body emits electromagnetic radiation as
a consequence of its temperature only. Compare reflection,
See emittance, emissivity.
- 2. With respect to electric
propulsion and energy conversion, the sending out of charged particles
from a surface causing the generation of these particles; e.g., emission of
ions from an ionizing surface in ion engines.
- emission line
- A minute range of wavelength
(or frequency) in
spectrum within which radiant energy is being emitted by a radiating
substance. See spectral
- emission spectrum
- The array of wavelengths
and relative intensities of electromagnetic
radiation emitted by a given radiator.
- Each radiating substance has a unique, characteristic emission
spectrum, just as every medium of transmission has its individual absorption
- emissive power
- The rate of thermal emission of radiant
energy per unit area of emitting surface. Usually called thermal
- emissivity (symbol )
- A property of a material, measured as the emittance of a specimen of the
material that is thick enough to be completely opaque and has an optically
- emittance (symbol E, )
- 1. The radiant flux per unit area emitted by a body.
- 2. The ratio of the emitted radiant flux per unit area of a sample to that
of a black
body radiator at the same temperature and under the same conditions.
- Spectral emittance refers to emittance measured at a specified
Because of the two common meanings of emittance, it
should be defined when used unless the context allows no misinterpretation.
- In photography, a light-sensitive coating on a film, plate, or paper. See
- emulsion plate
- A plate with a photographic emulsion
specially designed to permit observation of the individual tracks of ionizing
- A thin ceramic
coating, usually of high glass content, applied to a substrate,
generally a metal.
- A satellite of
Saturn orbiting at a mean distance of 238,000 kilometers.
- encoder = analog
to digital converter.
- end-fire array
- A linear antenna
array whose direction of maximum radiation is along the axis of the array.
- Any quantity with dimensions mass * length squared divided time squared.
- energy conversion efficiency
- The efficiency with which a nozzle converts
the energy of the working substance into kinetic
energy, expressed as the ratio of the kinetic energy of the jet leaving
the nozzle to the kinetic energy of a hypothetical ideal jet leaving an ideal
nozzle using the same working substance at the same initial state and under
the same conditions of velocity and expansion.
- energy density
- The sound
energy per unit volume in a sound wave.
The unit is the erg per cubic centimeter.
- energy density spectrum
- The square of the amplitude of
the (complex) Fourier
transform of an aperiodic function. Sometimes called energy
spectrum. See power
- energy equation
- See thermodynamic
energy equation, mechanical energy equation, total
- energy level
- Any one of different values of energy which a
atom, or molecule may
adopt under conditions where the possible values are restricted by quantizing
- During transitions from one energy level to another, quanta of radiant
energy are emitted or absorbed, their frequency depending on the difference
between the energy levels.
- energy management
- In rocketry the monitoring of the expenditure of fuel for flight control and navigation.
- energy spectrum = energy
- A machine or apparatus that converts energy, especially heat energy, into
work. Also called motor.
- engine control
- Any control for
regulating the power and speed of an engine, such as the throttle, mixture
control, manifold-pressure regulator, fuel-pressure control, supercharger
- engine-exhaust trail = exhaust
- engine mount
- A structure used for attaching an engine to a vehicle.
- engine spray
- That part of a pad deluge
that is directed at cooling a rocket's engine or engines during launch.
- English candle = international
- enhanced radiation
- Increased radio wave
radiation from the sun, of several hours or days duration.
- Enhanced radiation is usually accompanied by many bursts.
- A mathematically defined thermodynamic function of state h = u + pv
where h is specific enthalpy; u is specific internal energy;
p is pressure; and v is specific volume. Also called heat
- The change in enthalpy measures the heat imparted to a system during a
reversible isobaric process:
dh = dq where dq is the heat increment per unit mass. For a
dh = cpdTwhere cp is the specific heat
at constant pressure and dT is the temperature increment.
- 1. A measure of the extent to which the energy of a system is unavailable.
A mathematically defined thermodynamic function of state, the increase in
which gives a measure of the energy of a system which has ceased to be
available for work during a certain process:
ds = (du + pdv)/T >= dq/T where s is
specific entropy; u is specific internal energy; p is pressure;
v is specific volume; T is Kelvin temperature; and q is
heat per unit mass. For reversible processes,
ds = dq/T In terms of potential temperature ,
ds = cp (d/)where cp is the
specific heat at constant pressure. See third
law of thermodynamics.
- In an adiabatic process, the entropy increases if the process is
irreversible and remains unchanged if the process is reversible. Thus, since
all natural processes are irreversible, it is said that in an isolated system
the entropy is always increasing as the system tends toward equilibrium, a
statement which may be considered a form of the second law of thermodynamics.
- 2. In communication theory, average
- entry corridor
- Depth of the region between two trajectories
which define the design limits of a vehicle which will enter a planetary atmosphere.
- 1. Of a variable, a curve which bounds the values which the variable can
assume, but does not consider possible simultaneous occurrences or
correlations between different values.
- 2. The bounds within which a certain system can operate as a flight
envelope, especially a graphic representation of these bounds showing
interrelationships of operational parameters.
- An external condition or the sum of such conditions, in which a piece of
equipment, a living organism, or a system operates as in temperature
environment, vibration environment,or space environment.
- Environments are usually specified by a range of values, and may be
either natural or artificial.
- environmental chamber
- A chamber in which humidity, temperature, pressure, fluid contents, noise,
and movement may be controlled so as to simulate different environments.
- environmental lapse rate
- The rate of decrease of temperature T with elevation z in
the atmosphere, - T / z or occasionally T / p,
where p is pressure. See autoconvective
lapse rate, superadiabatic
- The concept may be applied to other atmospheric variables (e.g., lapse
rate of density) if these are specified. The environmental lapse rate is
determined by the distribution of temperature in the vertical at a given time
and place and should be carefully distinguished from the process
lapse rate, which applies to an individual air parcel.
- See E-layer.
- A type of white blood cell or leukocyte which stains a red color with
eosin stain; normally about 2 to 3 percent of white cells in the blood but
tending to decrease during stressful situations and thus usable as an index
- ephemeris (plural, ephemerides)
- A periodical publication tabulating the predicted positions of celestial
bodies at regular intervals, such as daily, and containing other data of
interest to astronomers.
- A publication giving similar information useful to a navigator is
called an almanac.
- ephemeris day
- 86,400 ephemeris
seconds. See ephemeris
- ephemeris second (abbr s)
- This was the fundamental unit of time of the International System of Units
of 1960: 1/31556925.9747 of the tropical year defined by the mean motion of
the sun in longitude at the epoch 1900 January 0 day 12 hours. See ephemeris
for the definition of the latest unit of time in the SI system or look on the
WWW version of the National Institute of
Standards and Technology: Physics Laboratory's International System of Units
- ephemeris time (abbr E.T.)
- The uniform measure of time defined by the laws of dynamics and determined
in principle from the orbital motions of the planets, specifically the orbital
motion of the earth as represented by Newcomb's Tables of the Sun. Compare universal
- Beginning with the volume for 1960 the American Ephemeris and Nautical
Almanac uses ephemeris time as the tabular argument in the fundamental
ephemerides of the sun, moon, and planets.
A gravitational ephemeris
expresses the position of a celestial body as a function of ephemeris time;
and, at any instant, the measure of ephemeris time is the value of the
argument at which the ephemeris position is the same as the actual position at
the instant. The ephemeris time at any instant is obtained from observation by
directly comparing observed position of the sun, moon, and planets with
gravitational ephemerides of their coordinates; observations of the moon are
the most effective and expeditious for this purpose. An accurate
determination, however, requires observations over a more or less extended
period; in practice, it takes the form of determining the time correction T that must be applied to universal time
(U.T.) to obtain ephemeris time:
E.T. = U.T. + TThe
universal time at any instant may be obtained with little delay from
observations of the dirunal motions.
The fundamental epoch from which
ephemeris time is reckoned is the epoch that Newcomb designated as 1900
January 0, Greenwich mean noon, but which actually is 1900 January 0 day 12
hours E.T.; the instant to which this designation is assigned is the instant
near the beginning of the calendar year A.D. 1900 when the geometric mean
longitude of the Sun referred to the mean equinox of data was 279 degrees 41
minutes 48.04 seconds. Ephemeris time is the measure of time in which
Newcomb's Tables of the Sun agree with observation.
The primary unit of
ephemeris time is the tropical year, defined by the mean motion of the sun in
longitude at the epoch 1900 January 0 day 12 hours E.T.; its length in
ephemeris days is determined by the coefficient of T in Newcomb's
expression for the geometric mean longitude of the sun L referred to
the mean equinox of date, given among the elements of the sun.
- A particular instant for which certain data are valid, as the data for
which an astronomical catalogue is computed.
- Eppley pyrheliometer
- A pyrheliometer
of the thermoelectric type. Radiation is allowed to fall on two concentric
silver rings, the outer covered with magnesium oxide and the inner covered
with lampblack. A system of thermocouples
(thermopile) is used to measure the temperature difference between the rings.
Attachments are provided so that measurements of direct and diffuse solar
radiation may be obtained.
This instrument has been adopted by the U.S.
Weather Bureau for station use.
- For instrumentation of NOAA's Surface Radiation Research Branch see
- Equ, Equl
- International Astronomical Union abbreviations for Equuleus. See constellation.
- In astronomy, a small correction
to observed values to remove the effects of systematic errors in an
- equation of state
- An equation relating temperature, pressure, and volume of a system in
- A large number of such equations have been devised to apply equally to
gaseous and liquid phases throughout a wide range of temperatures and
pressures. Of these, the simplest are the perfect
gas law and Van der
- equation of time
- Prior to 1965, the difference between mean time and
time, usually labeled + or - as it is to be applied to mean time to obtain
apparent time. After 1965, the correction to be applied to 12 hours + local
mean time (LMT) to obtain the local hour angle (LHA) of the sun.
- equation of wave motion = wave
- equations of motion
- A set of equations which give information regarding the motion of a body
or of a point in space as a function of time when initial position and initial
velocity are known. See Newton
laws of motion, Eulerian
- The primary
great circle of a sphere or spheroid, such as the earth, perpendicular to
the polar axis;
or a line resembling or approximating such a circle.
- The terrestrial equator is 90° from the earth's geographical poles; the
celestial equator or equinoctial is 90 ° from the celestial poles; the
galactic equator or galactic circle is 90° from the galactic poles. The
astronomical equator is a line connecting points having 0° astronomical
latitude; the geodetic equator connects points having 0° geodetic latitude.
The expression terrestrial equator is sometimes applied to the
astronomical equator. The geodetic equator is shown on charts. A fictitious
equator is a reference line serving as the origin for measurement of
fictitious latitude. A transverse or inverse equator is a meridian the plane
of which is perpendicular to the axis of a transverse projection. An oblique
equator is a great circle the plane of which is perpendicular to the axis of
an oblique projection. A grid equator is a line perpendicular to a prime grid
meridian at the origin. The magnetic equator or aclinic line is that line on
the surface of the earth connecting all points at which the magnetic dip is
zero. The geomagnetic equator is the great circle 90° from the geomagnetic
poles of the earth.
- equatorial bulge
- The excess of the earth's equatorial diameter over the polar diameter.
- equatorial electrojet
- See electrojet.
- equatorial satellite
- A satellite
plane coincides, or almost coincides, with the earth's equatorial plane.
- equatorial system
- A set of celestial
coordinates based on the celestial
equator as the primary great circle; usually declination and hour angle or
sidereal hour angle. Also called equinoctial system of coordinates,
celestial equator system of coordinates.
- equigeopotential surface = geopotential
- equilibrium flow
- Gas flow in
which energy is constant along streamlines
and composition of the gas at any point is not time dependent.
- equilibrium glide
- Gliding flight in which the sum of the vertical components of the
aerodynamic lift and centrifugal
force is equal to the force of gravity.
- equilibrium spheroid
- The shape that the earth would attain if it were entirely covered by a
tideless ocean of constant depth. Compare geoid.
- equilibrium vapor pressure
- The vapor
pressure of a system in which two or more phases of a substance coexist in
equilibrium. See vapor
- In meteorology the reference is to water substance unless otherwise
- equinoctial = celestial
- equinoctial colure
- That great
circle of the celestial
sphere through the celestial poles and equinoxes; the hour circle
of the vernal equinox.
- equinoctial day = sidereal
- equinoctial point
- One of the two points of intersection of the ecliptic and
equator. Also called equinox.
- equinoctial system of coordinates = celestial
equator system of coordinates.
- equinoctial year = tropical
- 1. One of the two points of intersection of the ecliptic and
equator, occupied by the sun when its declination is 0°. Also called
- That point occupied on or about March 21, when the sun's declination
changes from south to north, is called vernal equinox, March equinox,
or first point of Aries; that point occupied on or about September 23,
when the declination changes from north to south, is called autumnal
equinox, September equinox, or first point of Libra.
often used to mean vernal equinox, when referring to the origin of
measurement of right
ascension and celestial
- 2. That instant the sun occupies one of the equinoctial points.
- equivalent-barotropic atmosphere
- See equivalent-barotropic
- equivalent-barotropic model
- A model atmosphere characterized by (a) frictionless and adiabatic flow
(b) hydrostatic and quasigeostrophic
equilibrium, and in which (c) the vertical shear of the horizontal wind is
assumed to be proportional to the horizontal wind itself.
- An equivalent-barotropic atmosphere is, accordingly, an atmosphere in
which the wind does not change direction with height and consequently one in
which the contours and isotherms (on isobaric surfaces, for example) are
everywhere parallel. In such an atmosphere, the vertically averaged motions
are presumably equivalent to those at some intermediate level, the
equivalent-barotropic level. In terms of the motion at this level, assumed to
be an isobaric surface, the
behavior of the equivalent-barotropic model may be described by a single
equation (the vorticity equation) in a single unknown (the height of the
isobaric surface). See barotropic
- equivalent binary digits
- 1. The number of binary places
required to handle the largest quantity which can be handled in some other notation. For
instance 3.323 binary digits are required to convey information equivalent to
that conveyed by one decimal digit.
- 2. The number of places required to express in binary
notation a quantity in some other notation.
- equivalent foot-candle = foot-lambert.
- equivalent pendulum
- A device, usually incorporating accelerometers
which has the same response to acceleration as a pendulum with a specific
period. See Schuler
- equivalent potential temperature
- The potential
temperature corresponding to the adiabatic
e = Ta, e
(1000 / p)0.286where e is the equivalent potential
temperature; Ta, e is the adiabatic equivalent temperature;
and p is the pressure in millibars. This temperature is conservative
with respect to dry-adiabatic and pseudoadiabatic processes.
- equivalent temperature
- 1. Isobaric
equivalent temperature; the temperature that an air parcel would have if
all water vapor were condensed out at constant pressure, the latent heat
released being used to heat the air,
Ti, e = T [1 + (Lw / cp
T)]where Ti, e is the isobaric equivalent
temperature; T is the temperature; w is the mixing ratio;
L is the latent heat; and cp is the specific heat of
air at constant pressure.
- 2. Adiabatic
equivalent temperature; The temperature that an air parcel would have
after undergoing the following (physically unrealizable) process:
dry-adiabatic expansion until saturated; pseudoadiabatic expansion until all
moisture is precipitated out; dry-adiabatic compression to the initial
pressure. This is the equivalent temperature as read from a thermodynamic
chart and is always greater than the isobaric equivalent temperature:
Ta, e = T exp (Lw / cp
T)where Ta, e is the adiabatic equivalent
temperature. Also called pseudoequivalent temperature.
- equivalent width
- In spectrography, a measure of the total absorption
of radiant energy as indicated by an absorption
line or absorption
band. Compare line width.
- The formula for equivalent width W is
A is the fraction of incident radiation which is absorbed at any wavelength,
and 1 and 2 are wavelengths, on opposite sides of
the line or band, where the absorption has dropped to zero.
Thus, in a
plot of A against , the equivalent width
represents the area under the curve, or the width of a fictitious line or band
which absorbs completely throughout its extent but which absorbs the same
total amount of energy as the actual line or band.
- International Astronomical Union abbreviation for Equuleus. See constellation.
- Equuleus (abbr Equ, Equl)
- See constellation.
- 1. Same as radiation,
with respect to emission.
- 2. = terrestrial
- In computer terminology, to expunge, wipe out, or destroy stored
information, usually without destroying the storage media,
as in demagnetizing a magnetic tape.
- A vehicle used to support a rocket for
transportation and for placing the rocket in an upright position within a gantry.
- The region of the ionosphere
in which the E-layer tends to form. See atmospheric
- The E-layer has been observed to be subdivided into two or more
layers, and these are then assigned the designation, E1,
E2, etc. Patchy and intermittent clouds of fairly high ionization,
known as sporadic E-layers, also form in the same general region.
- erf = probability
- The unit of energy or work in the centimeter-gram-second
system; the work performed by a force of 1 dyne acting through a distance
of 1 centimeter.
- An instrument for measuring muscular work.
- International Astronomical Union abbreviation for Eridanus. See constellation.
- International Astronomical Union abbreviation for Eridanus. See constellation.
- Eridanus (abbr Eri, Erid)
- See constellation.
- erosion gage
- An instrument for measuring the effect of dust and micrometeors
on materials exposed to space environment.
- erratic error
- An error caused by an incomplete element in an instrument.
- An example of an erratic error is backlash in a gear train.
- 1. In mathematics, the difference between the true value and a calculated
or observed value.
- A quantity (equal in magnitude to the error) added to a calculated or
observed value to obtain the true value is called a correction.
- 2. In a computer or data-processing system, any incorrect step, process,
- In addition to the mathematically usage, in the computer field the term
is also commonly used to refer to machine malfunctions as machine
errors and to human mistakes as human errors. It is frequently
helpful to distinguish between these as follows: errors result from
approximations used in numerical methods; mistakes result from incorrect
programming, coding, data transcription, manual operation, etc.; malfunction
result from failures in the operation of machine components such as gates,
flip-flops, amplifiers, etc.
- error band
- An error value, usually expressed in percent of full scale, which defines
the maximum allowable error permitted for a specified combination of transducer
- error coefficients
- Partial derivatives showing the variation of a function of several
variables, with one of these variables. These derivatives are used as
coefficients of the variables being changed in a series representation of the
total variation of the function.
- error function = probability
- error signal
- A voltage the magnitude of which is proportional to the difference between
an actual and a desired position.
- In radar use, error signals are obtained from selsyns and
from automatic gain control circuits and are used to control a servo system so
that the resultant motions tend to correct the error.
- The effective
temperature (radiational) of the earth's ozone layer.
- A reddening of the skin due to capillary dilation.
- Several forms of erythema can be caused by undue exposure of the human
body to weather elements. The most common is sunburn.
- E-scan = E-display.
- Of a particle or larger body: to achieve an escape
velocity and a flightpath outward from a primary
body so as neither to fall back to the body nor to orbit it.
- escape rocket
- A small rocket engine attached to the leading end of an escape tower,
which may be used to provide additional thrust to the capsule to obtain
separation of the capsule from the booster vehicle in an emergency.
- escape speed = escape
- escape tower
- A trestle tower place on top of a space capsule, which during liftoff
connects the capsule to the escape rocket.
- The escape tower is of such length as to protect the capsule from the
heat of the escape rocket in case the rocket is used to separate the capsule
from the booster vehicle during ascent. The tower is ultimately separated from
the capsule if ascent is normal.
- escape velocity
- The radial speed
which a particle or larger body must attain in order to escape from the
gravitational field of a planet or star. When friction is neglected, the
escape velocity is
G is the universal gravitational constant (see gravitation);
m is the mass of the planet or star; and r is the radial
distance from the center of the planet or star. Also called escape speed.
- Escape velocity from Earth is 7 miles/sec; from Mars it is 3.2
miles/sec; and from the Sun it is 390 miles/sec. In order for a celestial body
to retain an atmosphere for long periods of time, the mean velocity of the
atmospheric molecules must be considerably below the escape velocity.
- E-scope = E-display.
- E.T. (abbr) = ephemeris
- The doctrine of causes, particularly the causes and reasons for diseases.
- See E-layer note.
- Eulerian angles
- A system of three angles which uniquely define with reference to one coordinate
system (e.g., earth axes), the orientation of a second coordinate system
(e.g., body axes). Any orientation of the second system is obtainable from
that of the first by rotation
through each of the three angles in turn, the sequence of which is important.
- Eulerian coordinates
- Any system of coordinates
in which properties of a fluid are
assigned to points in space at each given time, without attempt to identify
parcels from one time to the next. See equations
of motion. Compare Lagrangian
- Eulerian coordinates are to be distinguished from Lagrangian
coordinates. The particular coordinate system used to identify points in
space (Cartesian, cylindrical, spherical, etc.) is quite independent of
whether the representation is Eulerian or Lagrangian.
- Eulerian correlation
- The correlation
between the properties of a flow at various points in space at a single
instant of time. Sometimes called synoptic correlation. Compare Lagrangian
- Eulerian equations
- Any of the fundamental equations of hydrodynamics expressed in Eulerian
coordinates. These are so commonly used that the designation Eulerian
is often omitted.
- A satellite of
Jupiter orbiting at a mean distance of 671,000 kilometers. Also called
- The physical process by which a liquid or solid is transformed to the
gaseous state; the opposite of condensation. Also called vaporization.
- In meteorology, evaporation usually is restricted in use to the
change of water from liquid to gas, while sublimation is used for the
change from solid to gas as well as from gas to solid.
According to the kinetic
theory of gases, evaporation occurs when liquid molecules escape into the
vapor phase as a result of the chance acquisition of above average,
outward-directed, translational velocities at a time when they happen to lie
within about one mean free path below the effective liquid surface. It is
conventionally stated that evaporation into a gas ceases when the gas reaches
saturation. In reality, net evaporation does cease, but only because the
numbers of molecules escaping from and returning to the liquid are equal, that
is, evaporation is counteracted by condensation.
Energy is lost by an
evaporating liquid; and, when no heat is added externally, the liquid always
cools. The heat thus removed is termed the latent heat of vaporization.
- evaporation coefficient
- The ratio of the actual evaporation
rate to the maximum or Knudsen
rate of evaporation.
- evaporation rate
- 1. The mass of material evaporated per unit from unit surface of a liquid
- 2. The number of molecules of
a given substance evaporated per second per square centimeter from the free
surface of the condensed phase.
- A perturbation
of the moon in its orbit due to the attraction of the sun. This results in an
increase in the eccentricity
of the moon's orbit when the sun passes the moon's line of
apsides and a decrease when perpendicular to it. See lunar
- Evection amounts to 1 degree 15 minutes in the moon's longitude at
- exchange coefficients
- Coefficients of eddy flux (e.g., of momentum, heat, water vapor, etc.) in
flow, defined in analogy to those of the kinetic theory of gases (see eddy). Also called
austausch coefficients, eddy coefficients, interchange coefficients.
- The exchange-coefficient hypothesis states that the mean eddy flux per
unit area of a conservative quantity (suitability expressed) is proportional
to the gradient of the mean value of this quantity, that is,
Mean flux per unit area = -Ce (d/ dN)where Ce is the exchange
coefficient; E is the mean value of the quantity; and N is the direction
normal to the surface. In strict analogy to molecular properties;
Ce would be constant, for turbulent flow Ce turns out to
depend on time and location. See eddy
- 1. An external force, or other input, applied to a system that causes the
system to respond in some way. Also called stimulus.
- 2. The increase in the internal energy of an atomic or molecular system
caused by a collision with another particle of greater energy.
- For atoms in a discharge, excitation usually refers to
increasing the energy level of a bound electron.
- excited atom
- An atom with one or more of its bound electrons in
an increased energy level.
- exclusive OR circuit
- A circuit which
produces an output signal when any
one, but not more than one, input is in its prescribed state. Also called
- exhaust deflecting ring
- A type of jetavator
consisting of a ring so mounted at the end of a nozzle as to
permit it to be rotated into the exhaust stream.
- exhaust stream
- The stream of gaseous, atomic, or radiant particles that emit from the nozzle of a rocket or other
- exhaust trail
- A condensation
trail that forms when the water vapor of an aircraft exhaust is mixed with
and saturates (or slightly supersaturates) the air in the wake of the
aircraft. Exhaust trails are of more common occurrence and of longer duration
trails. Also called engine-exhaust trail.
- exhaust velocity
- The velocity of gaseous or other particles (exhaust stream) that exhaust
through the nozzle or a reaction
engine, relative to the nozzle.
- That field of biology which deals with the effects of extraterrestrial
environments on living organisms and with the search for extraterrestrial
- The outermost, or topmost, portion of the atmosphere.
Its lower boundary is the critical
level of escape, variously estimated at 500 to 1000 kilometers above the
earth's surface. Also called region of escape. See atmospheric
- In the exosphere, the air density is so low that the mean free
path of individual particles depends upon their direction with respect to
the local vertical, being greatest for upward moving particles (see cone of
region). It is only from the exosphere that atmospheric gases can, to any
appreciable extent, escape into outer space.
- Of or pertaining to the exosphere.
- exotic fuel
- Any fuel considered to be unusual, as a boron-base fuel.
- exotic material
- Any structural material which is not presently used in great quantities in
conventional applications. Usually, materials with melting points above 3000°
- expandable space structure
- A structure which can be packaged in a small volume for launch and then
erected to its full size and shape outside the earth's atmosphere.
- expansion wave
- A simple wave or
progressive disturbance in the isentropic
flow of a compressible fluid, such that the pressure and density of a fluid
particle decrease on crossing the wave in the direction of its motion. Also
called rarefaction wave. See compressional
- expiratory reserve
- The volume of air that can be expelled from the lungs after a normal
- An angle equal to 360° minus a given angle. Thus, 150 ° is the explement
of 210° and the two are said to be explementary. See complement,
- explementary angles
- Two angles whose sum is 360°.
- 1. The sudden production of a large quantity of gas, usually hot, from a
much smaller amount of a gas, liquid, or solid.
- 2. Specifically, an explosion, sense 1, produced by combustion of a fuel and an oxidizer.
- The distinction between an explosion, sense 2. and a detonation is that
in an explosion the heat release rate and the number of molecules per unit
volume increase with time more or less uniformly, whereas a detonation is
propagated by an advancing shock front
behind which exothermic reactions take place and thus is (spatially)
- explosion turbine
- A turbine rotated
by gases from an intermittent combustion process taking place in a
- explosive bolt
- A bolt incorporating an explosive which can be detonated on command, thus
destroying the bolt. Explosive bolts are used, for example, in separating a satellite
from a rocket.
- explosive decompression
- A very rapid reduction of air pressure inside a cabin, coming to a new
static condition of balance with the external pressure.
- exponential atmosphere
- 1. = isothermal
- 2. An atmosphere in which the density is given by
= o e-h/Hwhere
is density, o is density at the datum plane, h
is altitude, and H is scale height.
- exposure suit
- A suit designed to protect a person from a harmful natural environment,
such as cold water.
- extended range Dovap (abbr Extradop)
- A baseline
extension of the Dovap system to
provide a coherent
reference to the ground transmitter and all Dovap receivers located beyond
line-of-sight to the ground transmitter.
- The coherent reference is supplied by a cable and is multiplied up to
the proper reference frequency.
- extensive air shower = Auger
- exterior ballistics
- That branch of ballistics
that deals with the motion of projectiles in flight.
- external storage
- In computer terminology, storage media
separate from the machine but capable of retaining information in a form
acceptable to the machine, as floppy disks, removable reels of magnetic tape
or decks of punched cards.
- The attenuation of light; that is, the reduction of illuminance
of a collimated beam of light as the light passes through a medium wherein absorption
- extinction coefficient
- In meteorology, a measure of the space rate of diminution, or extinction,
of any transmitted light; thus, it is the attenuation
applied to visible
radiation. The extinction coefficient is
dI = -I dx or
I = I0 e-x where I is the
illuminance (luminous flux density) at the selected point in space,
I0 is the illuminance at the light source; and x is
the distance from the source.
- When so used, the extinction coefficient equals the sum of the medium's
absorption coefficient and scattering coefficient, each computed as a weighted
average over all wavelengths in the visible spectrum. As long as scattering
effects are primary, as in the lower atmosphere, the value of the extinction
coefficient is a function of the particle size of atmospheric suspensoids. It
varies in order of magnitude from 10 per kilometer with very low visibility to
0.01 per kilometer in very clear air.
- extinction cross section = scattering
- Extradop (abbr) = extended
- Outside our galaxy, which is the Milky Way.
- extraordinary ray
- The refracted component of a beam of radiation
split by having passed through a doubly refracting substance. The other
component is called the ordinary ray. See magnetic
- extraterrestrial life
- Life forms evolved and existing outside the terrestrial biosphere.
- extraterrestrial radiation
- In general, solar
radiation received just outside the earth's atmosphere.
- extremely high frequency (abbr EHF)
- See frequency
- extremely low frequency (abbr ELF)
- See frequency
- extreme value
- In statistics, the upper or lower bound of the random variable which is
not expected to be exceeded by a specified percentage of the population within
a given confidence
- eyeballs in, eyeballs out, eyeballs down, eyeballs up, eyeballs
left, eyeballs right. See physiological