Closed Cycle - Of a heat engine: a cycle of operation in which the same
heat-transfer fluid is used repeatedly.
Closed Shell - Of electrons in an atom or molecule characterizes a system in which all the quantum states of a particular shell are filled.
Close-packed Structure - The structure obtained when spheres of equal radii are packed together so as to occupy a minimum volume. Two such structures. are possible; face-centred cubic and close-packed hexagonal.
Cloud - A visible aggregate of small particles of ice or water suspended in free air, formed by condensation when warm moist air rises into cooler regions. Clouds may be classificd according to form and height.
Cloud Chamber. An intrument for making the -tracks of ionizing particles visible as rows of supersaturated vapour.
Cloud Searchlight - A searchlight apparatus used to measure the height of the cloud base by observation of the patch of light at the cloud base of interest. Radar may be used in the same way.
Clusec - A unit of inleakage into agas system. It is 10 cm3 of gas per
second at l m of mercury pressure, i.e. 1/100 of a lusec.
Clutter - Spurious responses on a radar display caused by reflections from large objects on the ground, rain, storm clouds, etc.
Coanda Effect - The effect whereby a two-dimensional jet of fluid, discharged tangentially along a convex solid surface, may remain attached to the surface for a considerable distance.
Coaxial Line - A radio-frequency transmission line consisting of a central copper conductor, embedded in insulating material or supported by insulating beads, surrounded by a cylindrical sheath which may be either a solid metal tube or a woven braid of thin copper wires. Such a line has no external field.
Cockcroft .Walton Apparatus - A device for producing large d.c. voltages. of up to about 2 million with output currents of several milliamperes and with good voltage stability. It is used for accelerating particles for research in nuclear physics.
Coefficient of Performance (C.O.P.).
Of a refrigerator or heat pump: the radio of the quantity of heat extracted to the amount of work expended.
Coelostat - An instrument for continuously reflecting the same region of the sky into the field of view of a fixed telescope, in spite of the rotation of the Earth.
Coercive Force - The magnetic field necessary to reduce to zero the residual intensity of magnetization arising from hysteresis. The value of the coercive force corresponding to magnetic saturation is the coercivity.
Coherence - The quality of being coherent. Sometimes defined as the quality that expresses the correlation between two stochastic processes.
Coherence Time - As applied to a typical stochastic process such as a wave travelling through space: the time interval over which appreciable phase correlation exists.
Coherent - As applied to emitted or scattered waves: implies the existence of a definite phase relationship between two. or more waves. Where no such relationship exists the waves are said to be incoherent.
Coherent Units - Units based on a set of basic units from which all derived units may be obtained by multiplication or division without the introduction of numerical factors. Examples of coherent units are those of the CGS system and the International System (SI).
Cohesion - The-force between the atoms or molecules of a given material by virtue of which the material resists physical separation.
Cohesion Pressure - Of a liquid t where E is the internal energy and V the volume of the liquid. Also known as the internal pressure.
Cohesive Energy - Of a solid: the work required to dissociate one mole of the substance into its free constituents.
Coincidence Counter - A system of counters which produces an output pulse only when a coincidence occurs, i.e. when two or more particles or photons arrive within the resolving time of the system.
Coincidence Counting - The use of a coincidence counter to distinguish particular types of effect from unrelated effects, such as background effects.
Cold Cathode - A cathode of low work function, which emits electrons at room temperature in, for example, an electron tube, a gas discharge tube, or an X-ray tube.
Cold Emission - (1) Of electrons: the emission of electrons from a cathode by the application of a sufficiently high-voltage gradient. Sometimes called auto-emission or auto-electronic emission. (2) Of light: the emission of visible light at ordinary temperatures, as in chemiluminescence, fluorescence, and phosphorescence.
Cold Storage - The application of refrigeration to preserve perishable goods for prolonged periods by temperature and humidity control in a closed space.
Collimation - The limiting of a beam of radiation to the required dimensions.
Collimator - (1) For .light: an assembly involving slits, apertures, lenses, etc. for the production of a parallel of light. (2) For X-rays, neutrons. etc.: a contrivance, usually in the form of a tube with terminating slots, which restricts the angular divergence of a beam of rediation, thus producing an approximately unidirectional beam for the study of such processes as scattering and diffraction.
Collins Machine - A compact unit for the liquefaction of helium and hydrogen.
Collision - An interaction between free particles (including photons, atoms and nuclei), aggregates of particles, or rigid bodies, in which they come near enough to exert a mutual influence, with exchange of energy, momentum or charge. Actual contact is not necessarily implied.
Collision Diameter - Of molecules in a collision: the distance of closest approach between the centres of two molecules taking part in the e concept is used in the collision theory of chemical reactions.
Collision, Elastic A collision in which the total kinetic energy is unchanged.
Collision Frequency - For a given particle in a specified environment at a given temperature and pressure: the number of collisions undergone per unit time.
Collision, Head-on - A collision in which the relative motion of the colliding particles is along the same line between centres both before and after “impact".
Collision, Inelastic - A collision in which the total kinetic energy changes.
Collision Mean, Free Path - The average distance traversed by a particle between successive collisions.
Collision Probability - For a collision of a particular type between two particles: the ratio of the cross-section for the type of collision in question to the cross-section for all types of collision between the two particles.
Colorimeter - An instrument designed for the measurement of colour specification. See also: Tintometer.
Colorimetry - The measurement of colour specification.
Colour - A sensation normally produced in the eye by light, but which may be evoked by pressure, electric or magnetic fields, drugs or pathological conditions. Colour cannot be measured: what a colorimeter measures is colour specification.
Colour Analyser - A device used to provide information about, or to control, processes that involve colour changes.
Colour Atlas - A series of material colour standards arranged in a systematic way so as to cover as much of colour space as is practicable.
Colour Centres - Crystal defects in an optically transparent crystal which introduce absorption bands and therefore affect the colour of the crystal.
Colour Comparator. An instrument to facilitate comparison of coloured samples.
Colour Content - Degree of saturation of a colour.
Colour Contrast - The subjective impression of contrast arising from the simultaneous stimulation of different parts of the retina by different colours.
Colour Coupler - A substance which, when added to certain types of photographic developer, will form a colour or dye when "coupling" with the oxidized developer.
Colour Emissivity - The ratio of the energy radiated in a given (narrow) wavelength band to that radiated by a black body in the same band at the same temperature.
Colour Filter - 2A layer, film or plate that alters the relative intensities of the component wavelengths of light passing through it.
Colour, Hue Of - The property by which a colour is identified as tending to resemble the appearance of some particular wavelength of light in the visible spectrum. .
Colour Index-The difference between the photographic and visual magnitudes of a star, from which may be. deduced the effective temperature.
Colour Photography - Photographic reproduction in colour, based upon the Helmholtz theory of colour vision. All processes involve the taking of photographs through red, blue and green Colour filters.
Colour Space - A there-dimensional conceptual space used in studying colour phenomena. Any desired attributes of colour may be employed to describe and specify a colour space (e.g. brightness, hue and saturation) and these may be arranged in appropriate coordinate systems.
Colour Specification - A set of parameters which serve to define uniquely a colour stimulus in terms of some colour system. Only three such parameters are needed for any colour system, on account of the trivariance of human colour vision.