HALF-WAVE RECTIFIER\A rectifier using only one-half of each cycle to change alternating current to pulsating direct current.

HAZARDOUS LOCATION\An area where ignitable vapors or dust may cause a fire or explosion created by energy emitted from lighting or other electrical equipment.

HIGH-INTENSITY DISCHARGE LAMPS\ A general group of lamps consisting of mercury, metal halide, and high-pressure sodium lamps.

HOLE FLOW\In the valence band, a process of conduction in which electrons move into holes, thereby creating other holes that appear to move toward a negative potential. (The movement of holes is opposite the movement of electrons.)

INDICATING DEVICE\A device that indicates an alarm, supervisory, or trouble condition. Frequently, audible and visual devices, such as lamps and flashing lights, are used as indicating devices.

INITIATING DEVICE\A device used to initiate the sequence of electrical events that results in a detection or fire alarm or supervisory signal.

INTEGRATED CIRCUIT\A circuit in which many elements are fabricated and interconnected by a single process (into a single chip), as opposed to a "nonintegrated" circuit in which the transistors, diodes, resistors, and other components are fabricated separately and then assembled.

IONIZE\To convert totally or partially into ions (charged particles). This principle is used in some smoke detectors.

LIGHT-EMITTING DIODE (LED)\A PN-junction diode that emits visible light when it is forward biased. Depending on the material used to make the diode, the light may be red, green, or amber.

LINE\Strands of natural or synthetic fiber twisted together, sometimes referred to as rope.

LUMEN \The unit of light output or light flux. A 1-candlepower light source in the center of a hollow sphere 2 feet in diameter delivers 1 footcandle of light at every point on the inner surface of the sphere. Since the inside surface of the sphere is 12.57 square feet, the light source produces 12.57 lumens. However, to allow for losses, common practice puts the ratio of lumens to candle power at 10 to 1.

LUMINAIRE \A complete lighting unit consisting of a lamp or lamps together with the parts designed to distribute the light, to position and protect the lamps, and to connect the lamps to the power supply.

MAINTENANCE\Day-to-day, periodic, or scheduled work required to preserve or restore a facility or equipment so that it can be effectively used for its designed purpose. It includes work to prevent damage to or the deterioration of a facility that would otherwise be more costly to restore.

MAINTENANCE FACTOR\A factor used to denote the ratio of the illumination on a given area after a period of time to the initial illumination on the same area.

METAL HALIDE LAMP\A discharge lamp in which light is produced by the radiation from a mixture of a metallic vapor (such as mercury) and the products of the disassociation of halides (such as halides of thallium, indium, or sodium).

MOUNTING HEIGHT (ROADWAY)\The vertical distance between the roadway surface and the center of the light source.

MOUSING\Turns of cordage around the opening of a block hook.

NPN\A type of transistor that is formed by introducing a thin region of P-type material between two regions of N-type material.

OVERHANG\The horizontal distance between a vertical line passing through the luminaire and the curb or edge of the roadway.

PHOTOCELL \A light-controlled variable resistor that has a light-to-dark resistance ratio of 1:1,000. Used in various types of control and timing circuits.

PHOTOMETER \An instrument that measures luminous intensity or brightness, luminous flux, or light distribution.

PNP\A type of transistor that is formed by introducing a thin region of N-type material between two regions of P-type material.

PRINTED CIRCUIT BOARD\A flat insulating surface upon which printed wiring and miniaturized components are connected in a predetermined design and attached to a common base.

RECTIFIER\A device that by its conduction characteristics converts alternating current to a pulsating direct current.

REEVING \Threading or placement of a working line.

REFLECTOR\A device used to change the direction of the light rays and redirect them in the desired direction.

REFRACTOR\A transparent medium that passes light rays but bends or deflects them in the desired direction to obtain proper light distribution.

REVERSE BIAS\A condition in which an external voltage is applied to a PN-junction so that the junction offers a high resistance to current flow.

RUNWAY THRESHOLD\The beginning of the runway usable for landing.

SHEAVE\(Pronounced "shiv") A grooved wheel or pulley used to support a cable or rope.

SILICON-CONTROLLED RECTIFIER (SCR)\A semiconductor device that functions as an electrically controlled switch.

SOLID-STATE DEVICE\An electronic device that operates by the movement of electrons within a solid piece of semiconductor material.

STREETLIGHTING LUMINAIRE\A complete lighting device consisting of the light source, globe, reflector, refractor, housing, and support. The pole, post, or bracket is not considered part of the luminaire.

TRANSISTOR\A semiconductor device with three or more elements.

TRIAC\A three-terminal device that is similar to two SCRs back to back with a common gate and common terminals. Although similar in

construction and operation to the SCR, the triac controls and conducts current flow during both alternations of an alternating current cycle.

UNIJUNCTION TRANSISTOR (UJT)\A three-terminal, solid-state device that resembles a transistor but is stable over a wide range of temperatures and allows a reduction of components when used in place of a transistor. It is used in switching circuits, oscillators, and wave-shaping circuits.

VISUAL APPROACH SLOPE INDICA-TOR SYSTEM (VASIS)\A system of angle-ofapproach lights consisting of two bars of lights on each side of the runway near the threshold. These lights show red and white or a combination of both (pink) to the approaching pilot, depending upon the position of the aircraft with respect to the glide path.

WIRE ROPE\A rope formed of wires wrapped around a central core-a steel cable.

ZENER DIODE\A PN-junction diode designed to operate in the reverse-bias breakdown region.

ZONE\An area or division of a building protected by one fire alarm initiating circuit. Sometimes the area and the circuit are referred to interchangeable as the zone. The fire alarm initiating circuit may be connected to represent a certain group of initiating devices instead of a particular area or division of the building.