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D'alembert Principle - For a moving system of particles connected together by various forms of constraint: states that the external forces acting on the particles, less the effective forces on the particles set up by the constraints, form a system which is in equilibrium.
Dalton Law Of Partial Pressures - States that the total pressure of a mixture of gases under given conditions is the sum of the partial pressures of the individual gases under the same conditions, the partial pressure of a component gas being defined as the pressure that would be exerted. by that gas if it were present alone.
Damping - Of a mechanical or electrical system: denotes the reduction of motion (usually oscillatory) by dissipation of the energy of motion.
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Daughter Product - Of a given radionuclide: that nuclide which is the immediate product of the decay of the radionuclide in question.
Day - (1) Apparent solar day: the interval between two successive transits of the observed Sun (or true Sun) across the median. It i not constant. See also: Time, equation of (2) Mean solar day: the interval between two successive transits of a fictitious mean Sun (which moves at a constant rate equal to the average rate of the true. Sun) across the meridian. One mean solar day is equal to 24 mean solar hours, each of which equals 60 mean solar minutes, each of which equals 60 mean solar seconds. See also: Time, equation of (3) Sidereal day: the interval between two consecutive transits of the first point of Aries across any selected meridian. Neglecting the small effects of precession, etc., it is the interval between two consecutive transits of the same fixed star. It may be defined for most purposes as the period of rotation of the Earth on its axis, and expressed in mean solar time it is 23h, 56 min, and 4.0906s. Like the solar day, the sidereal day is divided into 24 h (sidereal hours) each of which is in turn divided into 60 min, each minute being subdivided into 60s.De Broglie Waves - The waves assoicated with a moving particle, as first propounded by De Broglie. The De broglie wavelength is equal to h/p where h is Planck's constant and p the momentum of the particle.
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constant. At low temperatures the theory correctly predicts that Cv is proportional to T3.
Debye Unit - A unit of dipole moment equal to 10-18 e.s.u. cm. (Note: l e.s.u. of charge is, equal to 10/c coulomb, where c is the speed of light in cm/s).
Decay Product - Of a given radionuclide: any nuclide that arises from that rdionuclide by radioactive decay. See also Daughter p roduct.
Decibel - A logarithmic unit used for expressing power ratios. It is one-tenth of a bel. The number of decibels corresponding to as power ratio W is 10 log 1010 W, the number of bels being log 1010 W. The use of decibels and bels has been extended to cover such quan tities as voltage and current ratio.
Declination - Of a celestial body: the latitude of the body on the celestial sphere, measured from the celestial equator.
Decomposition Voltage - The minimum voltage which will cause steady electrolysis in a given electrolytic cell.
Dee - Of a cyclotron: one of two electrodes in the form of a hollow semicircular box, which are put together like the two halves of a shallow pill box, and between which a.r.f voltage is maintained in such a way that the circulating particles encounter an accelerating field at every passage.
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Defect Structure - A crystalline structure in which some of the atomic positions are occupied by the "wrong" atoms, or even left unoccupied. In such a structure the compound concerned is usually non-stoichiometric.
Deformation - The change in shape or size of a body which
accompanies stress.
Deformation Band - In a metal: a region in a plastically deformed metal crystal which differs in orientation from the rest of the crystal
.Deformation, Elastic - Deformation which disappears when the stress is removed.
Deformation, Plastic - Deformation which occurs at a level of stress above the elastic limit, and which is therefore permanent.
Deformation Texture - The preferred orientation arising in a polycrystalline metal as a result of a deformation process such as rolling or wire drawing.
Degeneracy - of atomic, electronic, or nuclear energy levels: the existence of an atom, electron or nucleus, as the case may be, in more than one possible state corresponding to a given energy level. Such states are known as degenerate states.
Degree - (1) A unit of angular measure. (2) A unit of temperature or
temperature difference.
Deionization Time - The time taken for the ions formed (for example, in a Geiger-Muller tube) as a result of an ionizing event to disappear by diffusion or recombination.
Delayed Neutrons - Neutrons emitted by nuclei in excited states which have been formed in the process of beta decay.
Delay Line - A linen or network specifically designed to introduce a
desired delay in the transmission of a single.
Delta Connection - One of the two most frequently used ways of connecting a three-phase a.c. circuit, the other being the Y connection. The Δ connection has three coils, arraged in a triangle, each angle of which is connected to one of the three phases. The Y connection has three coils arranged in the form of a Y, each point of which is connected to one of the three phases.
Delta Function - A type of function introduced by Dirac to facilitate the quantum mechanical description of scattering processes.
Delta Rays - Electrons ejected from atoms by ionizing radiation andhaving sufficient energy to cause further ionization.
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Demagnetization Energy - The work done in assembling magnetic poles, in a given configuration, against the field of the poles.
Demagnetizing Field - The field acting on a magnetized body in oppostion to the magnetizing force and interpreted as being due to free magnetic poles induced on the surface of the body.
Densitometer - An instrument used for measuring values of photographic density directly, by photometric comparison.
Density - The mass per unit volume of a substance under standard conditions.
Density Bottle: Specific Gravity Bottle - An accurately calibrated glass bottle used, by comparing the weights of the same volumes of a liquid and a reference liquid, to measure the specific gravity of the first liquid.
Density, Number - The number of particles per unit volume or per unit area.
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Debye Temperature - Debye Characteristic Temperature - A temperature θp, characteristic of each substance, appearing in Debye's theory of specific heats and given by θp = hVmax/k, where. h is Planck's constant, Vmax is the maximum frequency of the thermal vibrations of the atoms of the substance, and k is Boltzman's constant.
Debye Theory Of Specific Heats - A theory in which a solid is considered as an elastic continuum whose maximum vibrational frequency is such that the corresponding number of vibrational modes is equal to the total number of degrees of freedom, 3N in the solid, where N is Avogadro's number. The specific heat at constant is given by Cv, = 3R[D(x) - xD'(X)] , where R is the gas constant and D(x) the Debye function, defined as D (x) x0t 3 dt, x x3 et-1 being equal to θD/T and t to hv/kT, where θD is the Debye characteristic temperature, T is the absolute temperature, h is Planck's constant, v is the frequency, and K is Boltzmann's
Daniell Cell - An electrochemical cell based on the reaction Zn+Cu2+ Zn2 + Cu. See also: Electrochemical cell.
Dark Current - The small current that flows in photo-emissive and photoconductive detectors in the absence of any incident light.
Dark-Field Illumination - Illumination of an object in a microscope in such a way that only light diffracted by the object reaches the eye. It is applicable only to the examination of small particles or fine lines, which appear as bright images against a dark background.
Density, Relative; Specific Gravity. Of a given substance: the ratio of the density of the substance to that a standard substance, usually water, at a specified temperature.
Depleted Uranium - Uranium containing a smaller proportion of 235U than is found in nature. The depletion may arise from the irradiation of natural uranium in a reactor (usually the fuel in a thermal reactor) or from the separtion of 235U in a diffusion plant. Depleted uranium is used for radiation shielding, as a dense metal, in alloy manufacture, in the ceramics industry, etc.
Depletion, Isotopic - The reduction of the concentration of one or more specified isotopes in a material or in one of its constituents.Depolarization - In an electrolyte: the control or limitation of the potential of an electrode, during electrolysis, by the addition of an appropriate substance.
Destruction Operator - In the quantum theory of fields: an operator which, when applied to a state vector фn which describes a system containing n particles, will yield a state vector фn-l, which describes a system having (n-l) particles. See also: Creation operator.
Detailed Balancing Principle. States that: in an equilibrium system, any molecular process and its reverse occur with equal frequency.
Deuterium - The hydrogen isotope 12H, of mass number 2, sometimes represented by the symbol D.
Deuteron - The nucleus of deuterium. It has a spin of 1, a positive magnetic moment, and a positive electric quadrupole moment.
Deutron-Deuteron Reaction - The reaction between two deuterons.
Deviation (Optical) - The angle through which a ray of light is bent by refraction or diffraction.
Dew - The deposit of moisture from the atmosphere on a surface whose temperature is below that of the dew point.
Dewar Vessel; Vacuum Flask - A double-walled vessel used for the storage of hot or cold liquids or solids, in which the space between the walls is evacuated and the walls themselves may be silvered, to reduce heat flow to (and from) the inside.
Dew Point - The temperature at which the water vapour present in the atmosphere is just sufficient for the atmosphere is just sufficient for the atmosphere to the saturated.
Dextrogyric: Dextrorotatory - Refers to an optically active substance that rotates the plane of polarization of a transmitted light beam in a clockwise direction, the observation being made looking through the substance towards the light source.
Dial Gauge - An instrument for making small linear measurements, in which linear motion imparted to a plunger is converted into rotational motion and displayed by a pointer on the face of a dial.
Diagnetism - The property shown by many substance; when they are subjected to a magnetizing force, of becoming magnetized in such a direction as to oppose than force, i.e., exhibiting a negative susecptibility (diamagnetic susceptibility).
Dimamagnetism, Langevin Theory Of - A theory that relates the occurrence of diamagnetism to the Larmor precession of the atomic electron orbits. It was originally derived classically, and later corrected quantum-mechanically by Pauli, leading to the following expression for diamagnetic susceptibility:
X=–Ze2N r2, where X is the susceptibility for an assembly of N
6mc2
atoms per unit volume, Z is the atomic number e, m and c have their "T
usual significance and r2 is the mean square radial distance of the
electrons from their nuclei.
Diaphorimeter - A device for measuring sweating.
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