Definitions for: Indicator

[n] a device for showing the operating condition of some system
[n] a number or ratio (a value on a scale of measurement) derived from a series of observed facts; can reveal relative changes as a function of time
[n] a signal for attracting attention
[n] (chemistry) a substance that changes color to indicate the presence of some ion or substance; can be used to indicate the completion of a chemical reaction or (in medicine) to test for a particular reaction

Webster (1913) Definition: In"di*ca`tor ([i^]n"d[i^]*k[=a]`t[~e]r), n. [L.: cf.
F. indicateur.]
1. One who, or that which, shows or points out; as, a fare
indicator in a street car.

2. (Mach.) A pressure gauge; a water gauge, as for a steam
boiler; an apparatus or instrument for showing the working
of a machine or moving part; as:
(a) (Steam Engine) 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. See Indicator card (below).
(b) A telltale connected with a hoisting machine, to show,
at the surface, the position of the cage in the shaft
of a mine, etc.

3. (Mech.) The part of an instrument by which an effect is
indicated, as an index or pointer.

4. (Zo["o]l.) Any bird of the genus Indicator and allied
genera. See Honey guide, under Honey.

5. (Chem.) 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

Note: The common indicators are litmus, trop[ae]olin, phenol
phthalein, potassic permanganate, etc.

Indicator card, the figure drawn by an engine indicator, by
means of which the working of the engine can be
investigated and its power calculated. The Illustration
shows one form of indicator card, from a steam engine,
together with scales by which the pressure of the steam
above or below that of the atmosphere, corresponding to
any position of the engine piston in its stroke, can be
measured. Called also indicator diagram.

Indicator telegraph, a telegraph in which the signals are
the deflections of a magnetic needle, as in the
trans-Atlantic system.

Synonyms: index, index number, indicant

See Also: absorption indicator, acid-base indicator, air sleeve, air sock, alpha-naphthol test, annunciator, business index, color, coloring material, colour, colouring material, cursor, device, dial, fact, fuel gauge, fuel indicator, gnomon, indicator lamp, leading indicator, level, Molisch reaction, Molisch test, Molisch's test, oxidation-reduction indicator, pilot lamp, pilot light, point of reference, pointer, price index, price level, reference, reference point, scale, short account, sign, signal, signaling, spirit level, stock index, stock market index, test paper, tidemark, wind sleeve, wind sock

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