aberration
n. (in optics) a defect in the image formed by an optical device (e.g. a lens). In chromatic aberration the image has coloured fringes as a result of the different extent to which light of different ...
absolute space
Space that exists as a background to events and processes and is not affected by objects or other entities in the universe. The idea underpins Newtonian physics, although many physicists have always ...
absolute time
Time that exists independently of any events or processes in the universe. Like absolute space, absolute time is a basic concept in Newtonian physics.
acceptor
A substance that is added as an impurity to a semiconductor because of its ability to accept electrons from the valence bands, causing p-type conduction by the mobile positive holes left.
acceptor levels
Energy levels of an acceptor atom in a semiconductor, such as aluminium, in silicon. These energy levels are very near the top of the valence band, and therefore cause p-type conduction. See also ...
acoustoelectronic devices
Devices in which electronic signals are converted into acoustic waves. Acoustoelectronic devices are used in constructing delay lines and also in converting digital data from computers for ...
actinometer
Any of various instruments for measuring the intensity of electromagnetic radiation. Modern actinometers use the photoelectric effect; earlier instruments depended either on the fluorescence produced ...
active device
1. An electronic component, such as a transistor, that is capable of amplification.2. An artificial satellite that receives information and retransmits it after amplification.3. A radar device that ...
aerial
The part of a radio or television system from which radio waves are transmitted into the atmosphere or space (transmitting aerial) or by which they are received ( receiving aerial). A directional or ...
after-heat
Heat produced by a nuclear reactor after it has been shut down. The after-heat is generated by radioactive substances formed in the fuel elements.
age of the earth
The time since the earth emerged as a planet of the sun, estimated by dating techniques to be about 4.6 × 109 years. The oldest known rocks on earth are estimated by radioactive dating to be about ...
algebraic sum
The total of a set of quantities paying due regard to sign, e.g. the algebraic sum of 3 and −4 is −1.
alternating current
abbr.: a.c. or AC; an electric current that varies with time in a sinusoidal manner. The abbreviation a.c. may also be applied to a voltage of varying polarity.
alternator
An alternating-current generator consisting of a coil or coils that rotate in the magnetic field produced by one or more permanent magnets or electromagnets. The electromagnets are supplied by an ...
ammeter
An instrument that measures electric current. The main types are the moving-coil ammeter, the moving-iron ammeter, and the thermoammeter. The moving-coil instrument is a moving-coil galvanometer ...
Ampère's law
A law of the formdB = (μo I sin θ dl)/4πr2,where dB is the infinitesimal element of the magnitude of the magnetic flux density (see magnetic field) at a distance r at a point P from the element ...
Ampère's rule
A rule that relates the direction of the electric current passing through a conductor and the magnetic field associated with it. The rule states that if the electric current is moving away from an ...
analytical geometry
A form of geometry in which points are located in a two-dimensional, three-dimensional, or higher dimensional space by means of a system of coordinates. Curves are represented by an equation for a ...
