D. The symbol for angle of deviation; electric flux density, diffusion coefficient.
D. A group 00 Fraunhofer lines in the yellow of the solar spectrum; (D)589.6257nm, (D2)-589.0186nm,
(D3) is due to helium.
D/A. Digital-to-analog.
d. A line in the blue of the solar spec trum, having a waveJength of 437.872 nm due to iron.
Dado. (Arch.) (I) A plain, flat, often ornamented surface at the base of a wall, as of a room, (2) One of the faces of a pedestal. (3) A rabbet. (Carp.) A groove cut out across the edge or face of a board to receive another member.
Dado and rabber. (Woodwk.) A joint formed bya rabbeted edge or end of one piece fitted into a groove or dado in another piece.
Da-guerreo-type. (Photog.) An early kind of photograph produced on a metal plate; named after its French inventor, Daguerre.
Dais. (Arch.) A raised platform at the end of a room, or a portion of the floor raised a step above the rest of the floor.
D' Alembert's principle. On a body in motion, the extermil forces are in equilibrium with the inertial forces. Dalton's atomic theory. Matter consists ultimately of indivisible, discrete particles (atoms), and atoms of the same element are identical; chemical action takes place as a result of attraction between these atoms which combine in simple proportions. It has since been found that atoms of the chemical elements are not the ultimate particles of matter, and the atoms of different mass can have the same chemical properties (isotopes). Nevertheless, this theory of 1808 is fundamental to chemistry.
Dalton's law. The law states that the total pressure exerted by a mixture of gases is equal to the sum of the partial pressures. of the gases of the mixture. This is also called Dalton's law of partial pressures.
Dama-scening. (Furn.) Imitating ancient damascus work, by an inlay of one metal upon another.
Dammer. (Paint.) An oleoresin resembling copal. It is obtained from coniferous trees of Asia, Australia, and South America. Used in varnishes. Soluble in hot alcohol, benzine, chloroform, or ether.
Damp. To check or retard the action of (a vibrating string); to cause a decrease in amplitude of; to slow, inhibit, restrain, moderate.
Damped. Progressive energy loss and decrease in amplitude of oscillations due to friction, viscosity or other means. If the damping is such that the system fails to oscillate, the system is said to be critically damped. For greater or lesser degrees of damping it is said to be overdamped or underdamped respectively.
Damper. A device in stringed key~ board instruments to deaden the vibration of the strings; an attachment to keep the indicator of measuring instruments from oscillating excessively, usually a set of vanes in an air space or fluid or a shortcircuited winding in a Ipagnetic field.
Damping. (Elec.) The introduction of a retarding force into or near an electrical recording instrument to reduce the oscillation of the needle and bring it quickly to rest.
Damping coil. (Elec.) A coil mounted near a galvanometer to produce a damping effect; i.e., to bring the needle quickly to a point of rest after deflection.
Damping control: (Tel.) A control on the receiver which aids in removing the horizontal distortion or bulge which may appear on the left side of a picture.
Dampproofing. (Masonry) The treating of a wall with some impervious material to prevent moisture from oozing through.
Dandelion metal. (Metal.) An alloy used as a heavy-duty bearing metal. One composition consists of 72 per cent lead, 18 per cent antimony.and 10 per cent tin.
Dandy mark. (Bookbndg.) A water mark placed in paper by means of a dandy roll.
Dandy roll. (Papermkg.) A cylindri cal roll used on a papermaking machine to produce a watermark.
Daniel cell. A primary cell, with zinc and copper electrodes, the zinc rod being inserted in sulphuric acid cont.ained with in a porous pot, which is itself immersed in a copper pot containing copper sulphate solution. It gives a constant emf of 1.08 volts.
Darby. A two-handled, flat tool used by plasterers, ~specially for working on both walls and ceilings. It is about 7 in. wide and 42 in. long.
Dardelet thread. (Mech.) This screw thread of French design has a 29deg. angle and a taper contact which makes it self-locking.
Dark. A more or less complete absence of light.
Dark current. Residual current in a photocell, video camera tube etc when not illuminated.
Dark space. The comparatively non luminous portion of an electrical discharge through a gas.
D' Arsonval galvanometer. (Elec.) A very sensitive periodic or dead-beat galvanometer in which the indicating coil is suspended in the field of a powerful horseshoe magnet.
Dash. (Auto.) The partition which separates the engine compartment from the driver's compartme.pt. (Print.) A plain or ornamental line between type matter; also a mark
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