static, term formerly use to describe electrical noise in radio reception, especially noise that originates outside a transmitter and receiver, e.g., in the atmosphere or in man-made devices. In general, a frequency modulation (FM) radio receiver is less susceptible to such noise than one using amplitude modulation (AM). Many receivers are equipped with devices, such as automatic limiting and silencing circuits, to lessen the effect of such noise on reception.

pitot static system, device for measuring the rate at which a fluid flows. Among the principal applications of the device are an airspeed indicator for aircraft and a distance and speed indicator for ships. The device contains a short tube with one open end that faces directly toward the stream of air or other fluid. When no fluid is moving into the opening, a minimum pressure, called the static pressure, is exerted against it. When a stream is flowing into it, the pressure rises by an amount that depends on the velocity of the stream. Behind this tube is another tube with a number of small vents at right angles to the direction of the first tube. When a stream raises the pressure against the opening of the first tube, the pressure against the vents is still equal to the static pressure. A suitable gauge compares these pressures, using the static pressure as a reference, and gives a reading in units of velocity. For an aircraft this reading is called "indicated airspeed"; "true airspeed" requires a correction, made automatically by some airspeed indicators, for the local density of the air. Because of winds, neither reading accurately measures the speed in relation to a point on the surface of the earth. That value is known as groundspeed. Although airspeed is normally given in knots or miles per hour, for supersonic aircraft it may be given in Mach numbers, which give the ratio of the aircraft's speed to the speed of sound. For navigational purposes Mach numbers are converted to true airspeed.

Condition in the course of a reversible chemical reaction in which no net change in the amounts of reactants and products occurs: Products are reverting to reactants at the same rate as reactants are forming products. For practical purposes, the reaction under those conditions is completed. Expressed in terms of the law of mass action, the reaction rate to form products is equal to the reaction rate to re-form reactants. The ratio of the reaction rate constants (i.e., of the amounts of reactants and products, each raised to the proper power), defines the equilibrium constant. Changing the conditions of temperature or pressure changes the reaction's equilibrium; a high temperature or pressure may be used to “push” a reaction that at ordinary conditions makes little product. See also H.-L. Le Châtelier.

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Condition in which the net force acting on a particle is zero. A body in equilibrium experiences no acceleration and, unless disturbed by an outside force, will remain in equilibrium indefinitely. A stable equilibrium is one in which small, externally induced displacements from that state produce forces that tend to oppose the displacement and return the body to equilibrium. An unstable equilibrium is one in which the least departures produce forces tending to increase the displacement. A brick lying on the floor is in stable equilibrium, while a ball bearing balanced on a knife-edge is in unstable equilibrium.

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Undesired sound that is intrinsically objectionable or that interferes with other sounds being listened to. In electronics and information theory, noise refers to those random, unpredictable, and undesirable signals, or changes in signals, that mask the desired information content. In radio, this noise is called static; in television, it is called snow. White noise is a complex signal or sound covering the entire range of component frequencies, or tones, all of which possess equal intensity.

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