The Collaborative International Dictionary of English v.0.48:

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.
   [1913 Webster]

   Condensing electroscope (Physics), a form of electroscope
      in which an increase of sensibility is obtained by the use
      of a condenser.
      [1913 Webster]