Dichroic Mirror - A mirror formed by the deposition on a transparent substrate of alternating layers of high-and low-index dielectric material of suitable thickness.
Dielectric Leakage - The very small steady current which flows in a dielectric subjected to a steady electric filed.
Dielectric Loss - The energy lost by absorption when dielectric hysteresis occurs.
Dielectric Strength: Electric Strength - The maximum potential gradient which a dielectric or insulator can stand without breaking down, under specified conditions.
Dielectric Testing - The testing of dielectrics to ascertain the following quantities: electric strength, volume and surface resistivity, permittivity and loss angle, resistance to arcing and tracking.
Diesel Cycle - A cycle now modified from Diesel's original conception, involving the compression of air to the ignition temperature, followed by the timed introduction of fuel, expansion, and exhaustion.}
Differentiating Circuit. A circuit which produces an output voltage proportional to the rate of change of the input signal.
Dipole, Electric - A pair of electric charge, equal in magnitude but opposite in sign, and separated by a very small distance. If, in a molecule, the centroids of positive and negative charges present are not coincident the molecule constitutes a permanent dipole. If they are coincident the application of an electric field will cause displacement of the positive and negative charges, giving rise to an induced dipole.
Dimension - Of a physical quantity: one of a number of fundamental quantities, such as length, mass and time, in terms of which the physical quantity in question may be expressed. Thus: if L denotes length, M denotes mass and T denotes time, the dimensions of velocity are LT -1 of acceleration LT -2 of force MLT -2 and so on. Electrical, magnetic or thermal dimensions may also be used where appropriate.
Dimensional Analysis - A method based on the principle of the dimensional homogeneity of physical equations--e.g. the two sides of an equation must have the same dimension--by the use of which it is often possible by a consideration of dimensions only to arrive at the form of an equation connecting physical quantities, or to check the validity of the solution of a physical problem.
Dimorphic - Refers to a substance which can exist in two different crystalline forms. The phenomenon involved is known as dimorphism.
Diode - A two-electrode device (thermionic tube or crystal diode) which has markedly undirectional characteristics.
Dichroism - The property of exhibiting two colours. especially that of exhibiting one colour in transmitted light and another in reflected lig ht.
Dichromatism - (1) An effect shown by some materials in which the colour of transmitted light depends on the thickness. (2) A type of colour blindness in which the eye can distinguish only two colours.
Dielectric - Ideally, a medium in which an electric field, once established, may be maintained without loss of energy. If is an insulator containing no free charges.
Dielectric Absorption - The persistence of electric polarization after removal of excitation by a polarizing field.
Dielectric Ageing - The change, usually deterioration, in the properties of dielectrics with time.
Dielectric Amplifier - A device in which the non-linear properties of ferro-electric material are used to amplify a small signal into one of greater powerDielectric Breakdown - The failure of a dielectric to act as an insulator.
Dielectric Constant; Specific Inductive Capacity: Relative Permittivity - Of a medium: the ratio of the capacitance of a capacitor with the medium between the electrodes to that of a capacitor with a vacuum between the electrodes.
Dielectric Field: Dielectric Field, Internal - The average total field which acts on a molecule or group of molecules within a dielectric.
Dielectric Heater - An electrical heater in which dielectric material is heated by dielectric loss in a comparatively strong a.c. field of high frequency.
Dielectric Hysteresis - The lag in time between the applications of an electric field to a dielectric .and the establishment of polarization: in particular the corresponding phase lag when an alternating field. is applied.
Dipole, Magnetic - A mangnet with finite magnetic moment, in which the separation of the poles may be considered vanishingly small.
Dipole Moment - Of an electric diple: the product of the charge on one element of the dipole, and the distance between the charges. It is a vector quanity. The moment of a magnetic dipole may be defined in an analogous fashion.
Dirac Equation - A quantum-mechanical equation introduced to describe the behaviour of the electron and now believed to be capable of describing all elementary particles with spin 1/2. The equation invokes the concept of states of negative energy from which follows the possibility of the existence of the positron.
Direct Conversion Of Heat To Electricity - The generation of electricity by electro-gasdynamic, magnetohydrodynamic, thermoelectric or thermionic methods or by the use of solar energy.
Direct Current - An electric current which is continuous, undirectional and steady.
Direct Pulse - In a pulsed radar system: the pulse of energy radiated direct from transmitter to receiver as opposed to a returned pulse, which arrives at the receiver only after reflection from some object illuminated by the transmitter.
Discharge - (1) The removal of charge from a capacitor. (2) The passage of electric current through a gas or "vacuum" (3) The rate of flow of a fluid at a particular point. (4) The process by which a storage battery delivers electrical energy.Discharge, Corona - An electrical discharge between two electrodes, one of which is of such a shaps as to cause the electric field at its surface to be much higher than that between the electrodes.
Discharge Detector - A test instrument incorporating a sensitive electronic amplifier, used to detect minute electrical transients in voids in solid or liquid insulators.
Discharge, Glow - An electrical discharge between cold electrodes in low-pressure gas, characterized by its low current-density (about 10-6 to 10-1 A cm²) and occurring at a potential above the ionization potential and below the sparking potential.
Discharge, Hollow Cathode - An electrical discharge in which, by' virtue of the hollow shape of the cathode, much higher currents can be sustained by the same voltage than for plane cathodes.
Discharge, Townsend. An electrical discharge with current density so small that the space charge is negligible, which is not self-sustaining, but requires the presence of an external source to maintain the necessary ionization.
Discharge Tube. A tube containing gas at low pressure through which an electric current may be passed.
Disintegration Energy: Q-Value. For a given nuclear disintegration: the amount of energy released in that disintegration: A negative value signifies that energy is absorbed.
Disintegration, Nuclear. The transformation of anatomic nucleus, possibly a compound one.
Dispersion. (1) A suspension of a solid in a liquid. It may range from a colloidal solution to a concentrated paste. (2) The variation of the refractive index of a medium (or by extension, deviation in general) with wavelength.
Dispersion Force. The force of attraction between molecules possessing no permanent dipole.
Dispersion, Rotatory. The variaton with wavelength of the amount by which the plane of vibration of lineraly polarized light is rotated by optically active materials.
Dispersive Power, Optical. (1) Of a particular medium, for light: the ratio of the difference in the deviation of light of two different wavelengths to the deviation of light whose wavelength is an average Of the two. (2) For optical glass: the dispersive power is given by nF-nc where nD, nF and nc are the refractive indices of np -1 the glass for the D, F and C Fraunhofer lines. The reciprocal of this dispersive power is known as the Abbe number or the constrigence.