\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
   analysis.
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.