Source : Webster's Revised Unabridged Dictionary (1913)

Electroscope \E*lec"tro*scope\, n. [Electro- + -scope: cf. F.
   ['e]lectroscope.] (Physics)
   An instrument for detecting the presence of electricity, or
   changes in the electric state of bodies, or the species of
   electricity present, as by means of pith balls, and the like.

   {Condensing electroscope} (Physics), a form of electroscope
      in which an increase of sensibility is obtained by the use
      of a condenser.

Source : WordNet®

electroscope
     n : measuring instrument that detects electric charge; two gold
         leaves diverge owing to repulsion of charges with like
         sign