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Eagle Mounting - A compact mounting of a concave diffraction grating based upon the principle of the Rowland circle.
Earth - The third planet in the solar system in order of distance from the Sun. Its diameter is about 12600 Km, its mass about 6 x 1024 kg, its density about 5.6, and its period of rotation about 24 hours. It has one satellite, the Moon. See also: Following entries.
Earth Currents - Natural electric currents circulating in the ground. They are due to the existence of a natural electric field (the Earth current fie/d), which at a giver point, varies continually in magnitude and direction.
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Earth Inductor - An instrument for measuring the magnetic dip by means of dynamo-electric action. It depends on the interaction between a hand-operated rotating coil and the ambient magnetic field of the Earth.
Earth, Mantle Of - That region of the Earth's interior between the crust and the core. Its upper boundary is the Mohorovicic discontinuity.
Earth, Motions Or - (1) Translation: the movement about the Sun. The centre of mass of the Earth and Moon, which is about 4000 km from the centre of the Earth, moves about the Sun in approximately an ellipse, the motion being somewhat disturbed by the attractions of other bodies. (2) Rotation: the movement of the Earth about its own axis. The time taken per rotation shows slight variations, as does the inclination of the axis. The oscillation of the Earth's pole about the mean position, arising from periodic variations in the plane of the Moon's orbit, is known as nutation, and has a period of about 18.6 years. Its effect is to produce small changes in the precession, which is the slow revolution of the Earth's axis about the pole of the ecliptic in a period of about 26,000 years.
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Eclipse, Solar - The interposition of the Moon between the Sun and the Earth so as to blackout the view of the Sun as seen from the Earth. It may be total, when the Moon completely covers the Sun; partial, when the Moon partially covers the Sun; or annular, when the lunar disk is completely within the solar disk.
Economizer - In a steam-engine: a heat exchanger in which the waste heat of the flue gases is used to heat the feed water before it enters the boiler.
Eddington Theory - A theory based on the idea that, in describing a physical system, it is essential to take account of the observer, the nature of the observations, and the processes of measurement, as in relativity and quantum theory.
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Echelon, Echelle And Echelette Gratings - Dispersive diffraction gratings designed to bridge the gap between conventional gratings and dispersive interferometers as regards resolution, dispersion, and free spectral range.
Echo Box. A high-Q resonator used mainly for checking the performance of radar receivers. The cavity is set ringing by a pulse of energy and the receiver performance is checked during the decay of the oscillations.
Echo Sounding - The determination of the range and direction of underwater objects by the observation of pulses of ultrasonic energy reflected from .them. The technique may be used for measuring the depth of water below a ship's hull, for locating shoals of fish, or, in the form of Sonar (or Asdic) for the detection of submarines.
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Elasticity - The property whereby a body, when deformed by an applied load, recovers its previous configuration when the load is removed. According to Hooke's law the strain is proportional to the stress within the elastic limit.
Elastic Limit - The stress above which a body ceases to deform elastically and at which plastic defomation sets in.
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Earth-Air Current - The downward electric current observed over most of the Earth, of about 10-6 A/km2.
Earth, Core or - That region of the Earth's interior extending from the mantle to the centre of the Earth. There is an outer core (believed to be liquid) of about 2000 km in thickness and an inner core (believed to be solid ) of radius about 1200 km, separated by a transition region of about 140 km in thickness.
Eclipse, Lunar - The interposition of the Earth between the Sun and Moon so as to cut off the Sun's light from the Moon's surface. It may be total, when all the Sun's light is cut off; or partial, when only some of the light is cut off. An analogous, state of "affairs may also occur for a planet, leading to a planetary eclipse.
Elastic Modulus - For an elastic material: the ratio of the stress to the resulting strain. According as the stress produces elongation, shear, or compression, we speak of the Young's modulus. shear modulus (or modulus of rigidity), or bulk modulus (or compression modulus) respectively. Similarly, the torsional modulus refers to twisting and the flexural rigidity to be bending.
Earth Potential - The electric potential of the Earth, usually regarded aszero.
Earthquake - A violent movement of the Earth's surface which occurs. when a large amount of energy (up to as much as 1025 erg, i.e. 1018 J) is released within a few seconds from focal region.
Earthquake Intensity Scale - A scale used to express the observed effects of an earthquake. The most common is the modified Mercalli scale, which ranges from I (recorded but not felt) through V (many awakened, some dishes broken) to XII (total damage).
Earth, Rotation Of - The rotation of the Earth about its axis. Its period varies slightly by up to 0.002 s/day and the inclination of the axis is also subject to variations, mainly precession and nutation.
Earth Tides - Periodic elastic deformations in the body of the Earth which are directly analogous to the ocean tides, and, like them, are produced by the gravitational attraction of the Sun and Moon. They cause small but measurable fluctuations in the gravitational acceleration.
Easy Glide - In the plastic deformation of a single crystal: the occurrence of considerable deformation with a small increase in stress, in a region following the elastic region (with increasing stress) and preceding the onset of work hardening.
Ebullioscopy - (1) The measurement of the elevation of the boiling point of a solvent by the addition of a non-volatile solute. (2) The determination of molecular weight by such a measurement.
Eddy - A macroscopic element of fluid possessing rotational motion.
Eddy Conductivity - The apparent thermal conductivity of a fluid in the presence of eddies.
Eddy Current. A local current induced in a conducting medium subjected to a varying magnetic field.
Eddy Current Lag - The large in the achievement of the new equilibrium value of the induction when the magnetic flux is changed. With a.c. the induction (and hence the eddy current) decreases from the surface to the centre of the material. I
Eddy Viscosity - The apparent viscosity of an incompressible fluid when it is undergoing turbulent motion.
Edge Wave - (1) A form of wave generated when an atmospheric disturbance passes over a continental shelf or sloping sea bottom or when an outside wave disturbance is- in resonance with a bay or inlet. (2) Another name for boundary wave.
Effuser - A device to convert pressure into velocity, e.g. that part of a wind tunnel, upstream of the working section, in which the air or gas is accelerated.
Effusion - The molecular flow of gases through small apertures.
Ehrenfest Theorem - States that the motion of a quantum-mechanical wave packet will be identical to that of the classical particle it represents if any potentials acting upon it do not change appreciably over the dimensions of the packet
Eigenfunction - Of a differential equation: a solution satisfying specified boundary conditions and corresponding to a particular eigenvalue. Also known as an eigensolution.
Eigenvalue - Of a differential equation: one of the special values of a given parameter for which a non-trivial solution (an eigenfunction) exists.
Einstein Characteristic Temperature - A temperature θE, characteristic of each substance, appearing in Einstein's theory of specific heats and given by θE = hv/k, where h is Planck's constant, v is the frequency of thermal vibration of the atoms of the substance, and k is Boltzmann's constant.
Einstein Equation For Photoelectric Emission - An equation given the kinetic energy of a photoelectron in terms of the quantum energy of the incident photons. It represents one of the earliest successes of the quantum theory, and may be written as E = hv-W, where E is the kinetic energy in question, h is Planck's constant, v is the frequency of the incident light, and W is the excess of energy needed by the electron to escape.
Einstein Formula In Radiation Theory - States that the transition of a system from a higher to a lower quantum state is accompanied by the emission of radiation of frequency (Em - En)/h, where Em and En are the two energy levels involved and h is Planck's constant.
Einstein Theory Of Specific Heats - A theory in which a solid is regarded as an assembly of bound atoms each of which vibrates independently as a three-dimensional harmonic oscillator, all the atoms vibrating with the same frequency. The specific heat at constant volume Cv is given by Cv = 3 Ry2ey (ey - 1) 2,
where R is the gas constant and y = θE /T, θE being the Einstein characteristic temperature, and T the absolute temperature.
Elastic Constants - Of a crystal: the coefficients of the relations by which the components of the elastic strain are express d as linear functions of the stress components.
lastic Wave - Any form for wave motion by which energy is transmitted through a body by virtue of the elastic properties of that body. In general there are two types of elastic wave: compressional (or longitudinal) and shear (or transverse).
Elastodynamics. The study of the mechanical properties of elastic
waves.
Elastomer - A material, usually synthetic, having elastic properties resembling those of rubber.
Electret. A dielectric body that retains and electric moment after the
externally applied field has been reduced to zero. It is the electrical
analogy of a permanent magnet.
Electrical Attenuator. An arrangement for introducing a known attenuation into a given circuit.
Electrical Battery. Two or more electrical cells connected and used as a single unit, as in an accumulator. When it is connected to a load of substantially constant voltage which is just sufficient to keep the battery in a fully charged condition, it is known as a floating battery.
Electrical Cable. An electrical conductor surrounded by a dielectric and a sheath, or an assembly of conductors so surrounded, used for the transmission of electric power or for communication.
Electrical Circuit, Equivalent. A representation of an actual electrical network, obtained by replacing the network components by
fictitious equivalents which, for the particular problem under
investigation, express the relevant properties of the network to the
desired degree of accuracy.
Electrical Circuit, Three Phase. A combination of three single-phase
circuits in which the three applied voltages successively differ in
phase from each other by one-third of a cycle.
Electrical Cycle. One complete set of instantaneous values of an electrical quantity in an indefinite repetition of events. The time of such a cycle is the time between corresponding points in for example, a voltage or current waveform.
Electrical Engineering. The practical application of the principle of electricity and magnetism. It is concerned mainly with power of communication. |
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