The Collaborative International Dictionary of English v.0.48:

Electrometer \E`lec*trom"e*ter\, n. [Electro- + -meter: cf. F.
   ['e]lectrom[`e]tre.] (Physics)
   An instrument for measuring the quantity or intensity of
   electricity; also, sometimes, and less properly, applied to
   an instrument which indicates the presence of electricity
   (usually called an electroscope).
   [1913 Webster]

   Balance electrometer. See under Balance. Electro-metric

WordNet (r) 3.0 (2006):

electrometer
    n 1: meter to measure electrostatic voltage differences; draws
         no current from the source