Activity - Of a radioactive substance: the number of nuclear distintegrations occurring in a given quantity of substance per unit time.

Adaptation (Vision) - The change in the sensitivity of the eye after prolonged exposure to. light or dark. Dark adaptation occurs when the subject moves from a bright to a dark environment, and is measured in minutes. Light adaptation occurs when the subject moves in the opposite sense, and is measured in seconds.

Adaptometer - An instrument for measuring the course of adaptation of the eye.

Adiabatic Approximation - An approximation used when the Hamiltonian of a quantum system varies slowly with time.
Adiabatic Change - (1) Any process taking place without gain or loss of heat. (2) A reversible thermodynamical change of a system carried out with no change of entropy, i.e. without any heat entering or leaving the system.

Adiabatic Chart - A chart showing the variations of a pair of thermodynamic variables during an adiabatic change. Also called an isentropic chart.

Adiabatic Demagnetization, Cooling By - Of a paramagnetic salt: the most common method of producing temperaturs below 1 K. It involves the isothermal magnetization of the salt, followed by adiabqtic demagnetization and temperatures as low as about 10-3 K may be obtained in this way.

Admittance - The reciprocal of impedance. The real part is called the condutance and the imaginary part the susceptance.

Advection - Refers. (mainly in meteorology) to the change in any property (e.g. temperature) of a given element of air which is brought about by the motion of the air in a gradient of the property concerned.

Aerial - A conductor or series of conductors, usually in the form of a wire or wires, erected in an elevated position and used for the transmission or reception of radio waves. Also known as antenna.

Aerial Array - A system of two or more spatially arranged. coupled aerials. which is designed to have particular directional radiating or receiving properties.

Aerial Noise - A noise component (analogous to thermal noise) of the output from an aerial or aerial system which is directed at the open sky or at objects which absorb radio waves.

Aerial, Vee - An aerial consisting of a pair of equal straight conductors set at an angle to each other and fed at the junction between them.

Aerial, Yagi - An aerial array containing one radiating element and a number of parasitic ones.

Aerodynamic Heating - The transfer of heat to a solid body immersed within, and moving relative to, a fluid, often air.

Aerodynamic Range - An arrangement for observing the motion of a projectile, aircraft model or missile model in free flight. It is complementary to a wind tunnel.

Aerodynamics - The study of air in motion; by generalization the science of the dynamics of gases.

Aerology - The study of the atmosphere in three dimensions as contrasted with the study of the atmosphere from observations made only at the Earth's surface.

Aeronomy - The study of the physics of upper atmospheric phenomena, including the effects of solar radiation.

Aerophysics - The study of the properties of gases at very high Match numbers.

Aerostatics - The study of gases at rest in equilibrium. An important application is to the equlibrium of balloons and airships.

After-glow - The light emitted by a phosphor after the excitation is removed.

After-image - The image seen, after the retina, or a portion of it. has been fatigued by exposure to intense light or to a continued fixed light stimulus.

Air Coil - A self-supporting coil of wire in which the core is free of any other material. It may be used, for example, as an impedance.

Air-conditioning - The control of the temperature, moisture content and purity of the air by ventilation, heating and cooling and adjustment of humidity.

Aircraft - Any form of flying machine or vehicle supported by air. An aerostat is supported by its own buoyancy; an aerodyne by wings.

Air Engine - A heat engine depending for its operation on the alternate expansion and contraction of a given mass of air.

Airglow - An emission from the upper terrestrial atomosphere which, together with diffused light from the Sun and Stars, contributes to the light of the night sky.

Airship - A power-driven aircraft which is lighter than air.

Air Trajectory - The path followed by an clement of air, as given. for example, by the path followed by a small inert marker.

Albedo - (1) For radiant energy: the coefficient of diffuse reflection i.e. the ratio of the total scattered energy to the incident energy. (2) For light: the ratio of the amount of light reflected in all directions by an element of a diffusely reflecting surface to the amount of light incident on that element.

Alpha Decay - Radioactive decay in which an a-particle is emitted. This lowers the atomic number of the nucleus concerned by two and its mass number by four.

Alpha-Particle - A 4He nucleus emitted during a nuclear transformation. By extension, any 4He nucleus. An α-particle has zero spin.

Alpha-Rays - A stream of α -particles.

Alpha-Ray Spectormetry - (1) The measurement of the distribution of either the kinetic energy or the momentum of a particles from a given radioactive nuclide. (2) The study of a-ray spectra and their implications.

Alphatron - An ionization gauge in which the source of the ionzation is a small α -emitter.

Alternating Current - An electric current which alternately reverses its direction in a circuit in a periodic manner.

Alternating Currnent Bridge - A Wheatstone bridge network using a.c. of audio or radio frequency, for the measurement of resistance, impedance, capacitance or inductance.

Altimeter - An instrument for measuring height above the ground or above sea level.

Altitude - (1) The vertical distance above ground or sea level (2) The angular elevation of a heavenly body the plane of the horizon, measured on the great circle passing perpendicular to that plane through the body in question and the zenith.

Ammeter - An instrument for measuring electric current.

Ampere - The MKSA and SI unit of electric current. It is defined as "the constant current which, if maintained in two straight parallel conductors of infinite length, of neglible circular section, and placed one metre apart in a vacuum, would produce between these conductors a force equal to 2 x to-7 newtons per metre of length.

Ampere, International - The former standard of electric current, defined as the unvarying electric current which, when passed through a specified- solution of nitrate of silver. in water, will deposit silver at the rate of 0.001 1800 g/s. One international ampere is equal to 0.999835 absolute amperes (abamperes). See also: Abampere.

Ampere Law - Relates the magnetic field H to the current which set up the field, by the equation H. dI = I, where the line integral is taken round any, closed path and I is the current enclosed. If the current is flowing in a. conductiong medium I J. ds where J is the current density and ds is a vectorial area element. This reduces to cul H = J ,. which, if there is time variation in charge density (i.e. if the current--and all other field quantities- are not constant) must be modified (as was done originally by Maxwell) to read curl H = J + D, where D is the displacement current density and the sum J + D is known as the total current density. This is the first of Maxwell's equations.