The Collaborative International Dictionary of English v.0.48:

Hygroscope \Hy"gro*scope\, n. [Gr. ? wet + -scope: cf. F.
   hygroscope.] (Physics)
   An instrument which shows whether there is more or less
   moisture in the atmosphere, without indicating its amount.
   [1913 Webster]

WordNet (r) 3.0 (2006):

hygroscope
    n 1: hygrometer that shows variations in the relative humidity
         of the atmosphere