The Collaborative International Dictionary of English v.0.48:

Astatic \A*stat"ic\, a. [Pref. a- not + static.] (Magnetism)
   Having little or no tendency to take a fixed or definite
   position or direction: thus, a suspended magnetic needle,
   when rendered astatic, loses its polarity, or tendency to
   point in a given direction.
   [1913 Webster]

   Astatic pair (Magnetism), a pair of magnetic needles so
      mounted as to be nearly or quite astatic, as in some
      galvanometers.
      [1913 Webster]

WordNet (r) 3.0 (2006):

astatic
    adj 1: not static or stable