Chain Reaction - A series of reactions in which one of the agents necessary to the series is itself produced by the reactions so as to cause similar reactions. Examples are: chemical chain reactions and nuclear chain reactions.
Chain Reaction, Neutron. A nuclear chain reaction in which the agent. is a neutron which has been produced in the fission process.
Chandrasekhar Absorption Constants - Constants expressing the absorption of stellar radiation by interstellar clouds. The clouds are pictured, not as separate, but as a continuous Cloud of fluctuating density.
Chandrasekhar Method of Envelopes - A method used in the study of the behaviour and eventual breakdown of certain models of stars of masses, which takes full account of the radiation pressure, neglected in the usual mass-luminosity treatments.
Chapman-jouget State - A point on the Hugoniot curve of pressure versus temperature in a detonating gas at which the detonation velocity equals the sum of the particle velocity and the sound velocity in the burned gas.
Characteristic Curve - Of a discharge: the curve showing the variation in resultant current with applied voltage.
Characteristic Impedance. (l) Of a transmission system: the square root of the ratio of the series impedance to the shunt admittance. (2) In an electrical cable: the impedance which would be measured at the input terminals of an infinitely long line having the same characteristics per unit length as the cable in question.
Characteristic Radiation - X-rays consisting of a series of groups of discrete wavelengths which are characteristic of the emitting element and independent of its state of chemical combination.
Characteristic Wavelength Of An Electromagnetic Wave Front - The characteristic velocity divided by the frequency.
Charge Conjugation - The operation of changing the signs of all electric charges and electromagnetic fields in a system.
Charge Density, Electric - The charge per unit volume. It is sometimes used to mean the surface charge density, which is the charge per unit area of a conductor, for example, where the charge must reside on the surface.
Charge, Electric - A quantity of electricity, resident, for example, on a body, or transmitted through a medium. The unit of charge in the International System (SI) is the coulomb. The "natural" unit is the charge on the electron, which is 1.60 x 10-19 C. See also: entries under Units.
Charles Law: Charles=Gay-Lussac Law - For a perfect gas: states that the volume of a given mass of any gas at constant pressure rises with temperature by a constant fraction of the volume at O°C, Le. the coefficient of expansion is the same for all gases. The fraction is about 1/273 or 0.003 66 per'C and is a fair approximation for many gases.
Chattock Gauge - A sensitive pressure gauge for measuring very small pressure differences.
Chemical Potential - A thermodynamic variable bearing a relation to the flow of matter similar to that which temperature bears to the flow of heat, i.e. metter flows in a chemical reaction from  a region of higher chemical potential to one of lower.
Chemical Recorder - A device which records signals on impregnated paper through the chemical action produced by the flow of current in the chemical with which the paper is impregnated.
Chemiluminescence - The emission of light during a chemical reaction at an intensity which cannot be attributed to temperature radiation. It arises from the fluorescence of electronically excited molecules or atoms.
Chemistry - The study of the chemical properties. of elements and compounds. and of the causes, courses and consequences of the reactions in which they take part.
Child-Langmuir Equation - Relates the current density, I, which can be drawn under space-charge-limited conditions between infinite parallel plane electrodes to the potential, V, applied between the electrodes. It may be written as I = AV3/2, where A is a constant, known as the perveance.
Chladni Figures - The figures, demonstrating the vibrations of a glass or metal plate, obtained when sand, sprinkled on the plate, collects along the nodal lines.
Choke - An arrangement (e.g. an inductance coil) whereby energy at high frequency is prevented from leaving a system at some point while energy at much lower frequencies may be extracted or fed in as required.
Choke Coupling - An arrangement for the connection of two sections of a transmission system which provides a minimum loss of high ­frequency energy without the necessity for conductive contact between the sections.
Chopper - (1) For neutrons: a mechanical shutter, opaque to neutrons apart from slits or openings which pass through it, rotating at a high speed in a well-collimated beam of neutrons so as to produce bursts of neutrons. (2) For light: a component for periodically interrupting the light in an instrument so that a.c. amplifiers, etc., can be used in the measuring apparatus.
Chromatic Aberration - An aberration in an optical system arising from the variation of refractive index and focal length with wavelength.
Chromatic Colour - A colour which is not a grey.
Chromaticity - The colour quality of light, definable by its chromaticity coordinates. 
Chromaticity Chart: Chromaticity Diagram - A plane diagram formed by plotting one of the three chromaticity coordinates against either of the others.
Chromatic Parallax - In an optical instrument: the parallax between a line image and a graticule when the wavelength of the incident light is varied.
Chromatic Sensation - One which gives a sensation of colour, i.e. not a grey.
Chromo-optometer - An optometer in which the chromatic aberration. of the eye is used to determine its refraction.
Chromosphere - That Layer of the Sun's atmosphere that can be seen for a few seconds as a red coloured crescent at certain periods of a solar eclipse. It occupies the region from about 0 to 8000 km above the level represented by the sharp limb of the visible Sun.
Chronograph - An instrument for recording the time at which a given event takes place. It may involve pen recording on a rotating drum or photographic recording of an oscillograph.
Chronometer - A balance wheel type of clock fitted with a very accurate escapement.
Chronon. A assumed quantum of time.
Circle Diagram - An Argand diagram used in electrical engineering to represent, for example the magnitude and phase of a vector impedance.
Circuit -  The closed path along which electric currents can flow. See also: entries under Electrical circuit.
Circuit Breaker - A switch designed to interrupt a circuit automatically when an excessive current flows.
Circular Electric Wave - A wave in which the lines of electric force are circles. It can be generated by a circular filament carrying a current which is uniform along the filament.
Circular Magnetic Wave - A wave in which the lines of magnetic force are circles, e.g. in the magnetic field in a plane at right angles to a radiating dipole.
Circulating Electromagnetic Wave - A wave in which the equiphase surfaces are semi-infinite planes originating from an. axis-the axis of circulation.
Clark Rule - A modification of the Morse rule relating r, the equilibrium internuclear distance of a diatomic molecule, and ω, the equilibrium vibrational frequency. It states that ωr3 n = k - K where n is the group number, assigned according to the number of shared electrons, k is a constant characteristic of the period. and K a correction for singly ionized molecules of that period (zero for neutral molecules).
Classical Physics - That part of physics in which matter or energy is considered to be a continuum e.g. classical mechanics as opposed to quantum mechanics, or classical thermodynamics as opposed to quantum statistics.
Claude-Heylandt Liquefaction Process - A modification of the Claude process in which some of the precooled air is diverted from the compressor to a nozzle, where adiabatic expansion produces liquefaction, the unliquefied air being recirculated.
Claude Liquefaction Process - A method of liquefaction in which compressed air which has been cooed by its adiabatic expansion in a cylinder is used to precool the incoming air before it, in turn, is expanded.