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.]WordNet (r) 3.0 (2006):
variometer n 1: a measuring instrument for measuring variations in a magnetic field