Declinometer

from WordNet (r) 3.0 (2006)

declinometer
    n 1: an instrument for measuring magnetic declination [syn:
         {declinometer}, {transit declinometer}]

from The Collaborative International Dictionary of English v.0.48

Declinometer \Dec`li*nom"e*ter\, n. [Decline + -meter.]
   (Physics)
   An instrument for measuring the declination of the magnetic
   needle.
   [1913 Webster]