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Displacement Law - (1) For complex spectra: states that the arc
spectrum ofan element is similar to the first spark spectrum (singly ionized atom spectrum) ofthe element one place higher in the PeriodicTable, or to the second spark spectrum (doubly ionized atom spectrum) ofthe element two places higher. The analogy also holds for higher orders. (2) For radioactive decay: one ofa set of rules connecting the type ofdecay with the displacement in the Periodic Table (caused by the change in nuclear charge) ofa daughter element relative to the parent.
Dissociation Energy - Ofan electronic state in a diatomic molecule: the energy difference between the isolated atoms ofthe molecule at infinite separation and the lowest rotational vibration level of the electronic state in question. An accurate measurement of the dissociation energy may be obtained from the convergence limit.
Dissonance - Ofmusical notes: the production ofa sound that is unpleasant to the ear when two or more notes are played together. It is a subjective effect and arises from the existence of beats.
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Distortion - Of an optical system: the variation ofthe magnification with the angular distance from the axis. Thus the image of a straight line may appear curved.
District Heating - The provision ofheat and hot water to a number of buildings in a particular locality by means ofa piped service from a central station, the medium of heat transmission being either steam or hot water.
Disturbed Days: Active Days - Five days selected each month as international magnetic "disturbed" or "active" days, on the basis of many magnetic observations.
Diurnal Aberration - The aberration oflight associated with the rotation ofthe Earth about its axis.
Divergent - In nuclear technology: refers to the condition ofa neutron chain reacting medium for which the effective multiplication factor isgreater than unity, i.e. to the supercritical condition.
Divergent Beam Technique - The use of a divergent beam ofX-rays to produce Kossel lines.
Diverter. A resistor shunted across one part of a circuit. It is used, for example, across the field coils of d.c. series motors to control the speed.
Dividing Engine - An instrument for marking or engraving accurate subdividsion on scales.
Dividing Head - A machine tool attachment for rotating the work through any required angle so as to permit various operations (e.g. machining of faces, drilling of holes) to be performed with an accurate angular relationship.
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Domain - Region inside a ferromagnetic material wherein the atomic magnetic moments all point in the same directions. The net magnetization of various domains has been found to be in random directions in an unmagnetized substance. In a magnetized material they all point in the same direction.
Domain Boundary - The transition layer, which may be as great as 1.2/µm. which separates two domains magnetized in differnt directions and in which the change in spin direction between the two domains is assumed to take place gradually. Also known as a Bloch wall.
Domain, Ferromagnetic - A region of spontaneous magnetization in a single direction. Neighbouring domains have different directions of magnetization.
Donnan Distribution Coefficient - A coefficient expressing the distribution of diffusible electrolytic ions on two sides of a membrane in the presence of a non-diffusible ion present on one side only.
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Donnan Potential - The difference in potential set up across a semipermeable membrane which separates a solution containing a non-diffusible ion from one containing diffusible ions.
Donor - An impurity or imperfection in a semiconductor, which donates electrons to the conduction band, leading to electron conduction band, leading to electron conduction.
Donor Level - An electron energy level associated with a donor atom, commonly measured from the bottom of the conduction band.
Doppler Effect - The phenomenon giving rise to a change in the observed wavelength of a radiation which results from the movement of its source relative tothe observer. An approaching source appears to decrease the wavelength and a receding source to increase it.
Dorn Effect - The occurrence of an electrical potential across the ends of a column of liquid when colloidal particles are driven through the column by mechanical forces, as by the action of gravity.
Dose, Absorbed - Of ionizing radiation: the energy imparted to matter per unit mass at a point of interest. The unit is the rad, i.e. 10-4 J/K (100 erg/g).
Dose Equivalent - Of ionizing radiation: an estimate of the biological effect of an absorbed dose.
Dose Equivalent, Maximum Permissible - Of ionizing radition: the largest dose equivalent recevied within a specified period which is permitted by a regulatory committee on the assumption that there is no appreciable probability that somatic or genetic injury will result.
Dose Median Lethal; LD 50 - Of ionzing radiation: the absorbed dose which will kill, within a specified time, 50% of the individuals of a large population of organisms of a given species. Formerly known as mean lethal dose.
Dosemeter: Dosimeter - A instrument used for measuring or evaluating the absorbed dose, exposure, or similar radiation quantity. It may depend on ionization for its operation or may simply involve the darkening of a photographic emulsion.
Dose Of Ionizing Radiation - A general term denoting the quantity of radiation or energy absorbed by a body, and sometime the quantity to which the body is exposed.
Double Electron Excitation - In spectra: the excitation of two electrons rather than one, giving rise to prominent spectral lines, particularly in the alkaline earths.
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Double Quantum Transitions - Radiative transition between atomic or molecular states, which involve the simultaneous emission or absorption oftwo or more photons oflow frequency.
Double Refraction: Birefringence - The property, exhibited by anisotropic crystals, ofpossessing two refractive indices, i.e. ofallowing light to pass through them with two different velocities.
Doublet, Spectral - A spectral line with a multiplicity oftwo. The frequency separation ofthe two levels, expressed as a wave number, is called the doublet interval,
Double Weighing - A method of weighing used to allow for possibl inequalities in balance areas.
Doubling In Molecular Spectra - The splitting of the rotational levels in the specturm of a diatomic molecule which results from the coupling of the electron orbital and spin angular momenta with the nuclear angular momentum.
Downwash (Aerodynamics) - A vertical component of velocity induced by a lifting wing surface in a direction normal to the direction of flight.
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Draconitic Mont - Another name for Nodical month.
Drag Coefficient - For a fluid flowing past a submerged rigid body: the ratio 2F/Aqv2, where F is the longitudinal force on the body, A is the projected area of the body on a plane normal to the direction of flow, q is the density of the fluid and v its velocity.
Drag, Form - n - The drag induced by the lift.
Draper Effect - In a gaseous system exposed to light: the occurrence of a slight expansion followed by a steady state and then by contraction.
Drift Mobility - Of charge carriers diffusing through a solid under the influence of an electric field: the average drift velocity per unit electric field.
Drift Tube - Part of a klystron or a particle accelerator in which charged particles move without being acted upon by magnetic or electric fields.
Drift Velocity - Of charge carriers moving in a solid, liquid or gas under the influence of an electric field: the average distance travelled per second in the direction of the field. The drift velocity per unit electric field is known as the mobility.
Dropping-Mercury Electrode - An electrode used in polarography which supplies a regular series of mercury drops within a solution,
each with a new clean surface available for polarization.
Drude Equation: Drude Law - A relationship between the specific.
rotation of an optically active substance and the wavelength of the light.
Duality, Wave-Corpuscle - Refers 10 the statement that the wave and corpuscular aspects of any physical phenomenon are not contradictory but complementary.
Ductility - Of a metal: the property of being readily drawn out into a wire. It is associated with a fairly low yield stress and a high rate of workhardening which is maintained for a large plastic deformation.
Duplexer - A transmission system designed to permit a commn aerial to be used for transmission and reception of radio and radar signals. Also known as a T-R box or T-R switch.
Dushman-Langmuir Equation - An empircial attempt to give a theoretical expression for the constant D0 in the equation connecting the diffusion coefficient, D, of a solid with temperturc, T, in the manner D = D0 exp (-E/RT) where E is the activation energy and R the gas constant. The expression given for D0 is D0 = Ed2/Nh, where d is the spacing between the crystal planes normal to the diffusion direction, N is Avogardo's constant, and h is Planck's constant.
Dust Core - A mangnetic core composed of pulverized particles held together by a binder. Because of low eddy current losses the cores are suitable for use at high frequencies.
Dynamic Head; Dynamic Pressure - The impact pressure association with a moving fluid.
Dynamics - The study of the behaviour of objects in motion, in particular objects acted on by forces and having variable velocity.Its foundations are Newtons's laws of motion.
Dynamo. Usually denotes an electromagnetic generator which produces current by the rotation of an electical conductor in a magnetic field. Ideally the machine is reversible and may also be used as a motor.
Dynamo Effect In Moving Water - The production of an e.m.f. in water which moves in the Earth's magnetic field, by virtue of the fact that water is a conductor.
Dynamometer. (1) A device for measuring force, mainly the brake horse power of a motor. (2) An electrical measuring instrument depending for its action on the e.m.f between two or more current-carrying coils.
Dyne - The CGS unit of force. It is that force which gives to 1 g an acceleration of 1 cm/scc2, and is equal to 10-5 N.
Dynode - An electrode in photomultiplier tube designed to emit secondary electrons when bombarded by a beam of primary electrons.
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