variometer

from WordNet (r) 3.0 (2006)

variometer
    n 1: a measuring instrument for measuring variations in a
         magnetic field

from The Collaborative International Dictionary of English v.0.48

Variometer \Va`ri*om"e*ter\, n. [L. varius various + -meter.]
   (Elec.)
   An instrument for comparing magnetic forces, esp. in the
   earth's magnetic field.
   [Webster 1913 Suppl.]