indicator

• a number or ratio (a value on a scale of measurement) derived from a series of observed facts
• a signal for attracting attention
• a device for showing the operating condition of some system
• (chemistry) a substance that changes color to indicate the presence of some ion or substance
• One who, or that which, shows or points out
• A pressure gauge
• An instrument which draws a diagram showing the varying pressure in the cylinder of an engine or pump at every point of the stroke. It consists of a small cylinder communicating with the engine cylinder and fitted with a piston which the varying pressure drives upward more or less against the resistance of a spring. A lever imparts motion to a pencil which traces the diagram on a card wrapped around a vertical drum which is turned back and forth by a string connected with the piston rod of the engine
• A telltale connected with a hoisting machine, to show, at the surface, the position of the cage in the shaft of a mine, etc
• The part of an instrument by which an effect is indicated, as an index or pointer
• Any bird of the genus Indicator and allied genera
• That which indicates the condition of acidity, alkalinity, or the deficiency, excess, or sufficiency of a standard reagent, by causing an appearance, disappearance, or change of color, as in titration or volumetric analysis

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