diac (semiconductor device). A semiconductor used as a triggering device for a triac. A diac acts in much the same way as two zener diodes installed back to back, with one forward biased and the other reverse biased. No current can flow through the diac until the breakdown voltage of the reverse-biased diode is reached. When this breakdown voltage is reached, the diac immediately turns on and allows the gate current for the triac to flow.
diagonal cutting pliers. A type of wire cutters commonly used by aircraft maintenance technicians. The short, strong jaws are ground so they cut the wire by a chiseling action, rather than by shearing.
diagram. A line drawing used to represent a system or process. A diagram can be used to trace a flow of electrons, fluid, or components in a manufacturing process.
dial (instrument face). The face of an instrument on which calibration numbers are printed. The indication of the instrument is read as the number on the dial to which the pointer is pointing.
dial-indicating torque wrench (precision hand tool). A type of deflecting-beam torque wrench that uses a dial indicator to measure the amount the beam deflects. The dial of the indicator is graduated in foot-pounds, inch-pounds, or meter-kilograms of torque, rather than units of beam deflection.
dial indicator. A precision measuring instrument that uses a compound lever arrangement to amplify the movement of a stylus. The amount the stylus moves, in thousandths of an inch, is read on a circular dial.
diamagnetic material. A material which has a magnetic permeability of less than one, less than that of a vacuum. A diamagnetic material is repelled by a magnet and will try to align itself across the lines of magnetic flux, rather than along them.
diameter (dimension of a circle). The length of a line that touches the circumference of a circle in two places and passes through its center. The diameter of a circle is twice its radius.
diamond. An extremely hard, almost colorless form of pure carbon. Some diamonds reflect light so well they are used as gems, and low-grade diamonds are hard enough to be used as industrial cutting tools.
diamond chisel (metal-cutting tool). A metal-cutting chisel whose cutting edge is in the shape of a diamond. A diamond chisel is used to cut V-shaped grooves in metal.
diamond-point cutting tool (machine tool). A cutting tool used for machining hard metal. The cutting edge of the tool is an industrial diamond.
diamond-point dressing tool (machine tool). A tool used to smooth the surface of an abrasive grinding wheel. An industrial diamond is mounted in the tool, which is clamped in the tool rest in such a way that the diamond can be moved across the face of the abrasive wheel. The diamond wears away any roughness on the wheel and produces a smooth surface.
dibromodifluoromethane (fire extinguishing agent). A colorless, heavy liquid fire extinguishing agent. Dibromodifluoromethane is also known as Halon 1202.
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