The Collaborative International Dictionary of English v.0.48:

Magnetometer \Mag`net*om"e*ter\, n. [Magneto- + -meter: cf. F.
   magn['e]tom[`e]tre.] (Physics)
   An instrument for measuring the intensity of magnetic forces;
   also, less frequently, an instrument for determining any of
   the terrestrial magnetic elements, as the dip and
   declination.
   [1913 Webster]

WordNet (r) 3.0 (2006):

magnetometer
    n 1: a meter to compare strengths of magnetic fields [syn:
         magnetometer, gaussmeter]