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Astronomical Unit - A unit of distance used to express distance in the solar system. It is defined as the geometrical mean distance of the Earth from the Sun and may be taken as 1.496± 0.001 x 108 km (about 93 million miles).
Astronomy - The investigation of the universe outside the universe the Earth, comprising traditional astronomy (the study of distances, distribution and motion of celestial bodies and systems).
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Asynchronous Motor: Induction Motor - An a.c. motor in which currents in the primary winding induce currents in the secondary winding (usually the rotor) which interact with the flux set up by the primary winding to produce rotation.
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Atmosphere -
(1) The gaseous environment of any solid or liquid.
(2) The gaseous envelope surrounding the Earth and retained in the Earth's gravitational field. The various layers, starting from sea level, are known as the Troposphere (up to about 14 km), Stratosphere (up to about 55 km), Mesophere (up to about 80 km) and Ionosphere (up to over150 km), which are separately defined.
(3) A unit of pressure, viz. the standard atmosphere.
Atmosphere, Standard - A unit of pressure referring to sea level and a temperature of 0ºC, and defined as the pressure exerted by a column of mercury 760 mm high, having a density of 13.5951 g/cm3,the local value of the gravitational acceleration being taken as 980.665 crn/sec2. This pressure is equal to 1013250 dyne/cm2, i.e. 1013.250 mbar or 101325 N/m3precisely.
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Atmospheric Pollution - The presence of any undesirable substance in the open air.
Atmospheric Pressure - The pressure per unit area exerted by the atmosphere on an infinitesimal element of area taken at a given point. The meteoriological unit is the millibar (103 dyne/cm3 or l00 N/m3 which is more convenient than the bar (106 dyne/cm3 or 105 N/cm3). One standard atmosphere is, by definition, 1013.25 mbar or 101325 N/m2precisely.
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Atmospheric Tides - Oscillations of the atmosphere which are generated by the gravitational action of the Moon and Sun, or by solar thermal action.
Atomic Absorption Spectroscopy - The study of the absorption spectra from excited atoms.
Atomic Beam: Molecular Beam - A narrowly defined stream of neutral atoms (molecules) moving through a highly evacuated enclosure, the distance between the atoms (molecules) both of the stream and of the surrounding space being so large that collisions or interatomic (intermolecular) forces can be neglected.
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Atomic Energy - The popular name for nuclear energy.
Atomic Heat - Of an element: the product of the atomic weight and the specific heat.
Atomic Mass Unit, Unified - Defined as one twelft of the mass of the 12C nuclide. It replaces the atomic mass unit (a.m.u.) which was related to the 160 nuclide and the atomic weight unit(a.w.u.) which was related to the naturally occurring mixture of oxygen isotopes. It is greater than the a.m.u, by a factor of 1.000318 and than the a.w.u by a factor of 1.000043. The numerical values of many of the fundamental physical constants must therefore be changed when expressed on the unified atomic.
Atomic Number - Of an element: the number of protons in the atomic nucleus, sometimes known as the proton number.
Atomic Physics - The study of the. physical properties of atoms when the atom is regarded as a whole, i.e. as a nucleus associated with which is a number of electrons.
Atomic Scattering Factor - For X-rays or electrons: the ratio of the amplitude of the wave scattered by an atom (equal for X-rays to ZA in the forward direction, where Z is the atomic number and A the amplitude of the incident wave) to that scattered by a single electron under the same conditions.
Atomic Susceptibility - The product of the atomic weight and the magnetic susceptibility per unit mass.
Atomic Units - A system of units employed in theoretical physics such that each of the universal constants e (electronic charge), m (electronic mass) and h (Planck's constant) has the value unity. The numerical values of these constants in any other system of units are then the conversion factors from atomic units to the system in question.
Atomization - The breaking up to a stream of liquid into small particles or droplets to increase the specific surface.
Attenuator - An energy-absorbent material placed in a transmission system to absorb energy flowing in the system.
Audiometer - An. instrument for measuring the acuity of hearing. Measurements may be made with speech signals or with tone signals.
Auger Effect - The emission of an electron (Auger electron), rather than an X-ray photon, from an atom after the removal of an electron from one of the inner shells.
Aurorae. Atmosphere phenomena consisting of the appearance of luminous arcs, rays streamers, etc., of green, red or yellow colour.
Autoradiograph - A photographic record of the radiation from radioactive material in an object (which may be of scientific, technological, medical or bological interest), made by placing the object in close apposition to a photographic film.
Avalanche - (1 ) In metrology: a hurting mass of snow which descends a mountain side, carrying with it rocks or ice (2) Of a burst of ions: all the ions produced from a single primary ion through the process of cumulative ionization.
Average Life: Mean Life - The average lifetime for an atomic or nuclear system in a specified state. For an exponentially decaying system, it is the average time for the number of atoms or nuclei in a specified state to decrease by a factor of e (2.718...). This is 1/1n 2 times the half life and, for a radioactive nuclide it is the reciprocal of the disintegration constant.
Azimuth - .Of a celestial body; the arc of the horizon intercepted between the north or south point of the horizon, according as the observation is made in the northern or southern hemisphere, and the point where the meridian plane passing through the body cuts the horizon.
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