The Collaborative International Dictionary of English v.0.48:

Magnetograph \Mag*net"o*graph\, n. [Magneto- + -graph.]
   (Physics)
   An automatic instrument for registering, by photography or
   otherwise, the states and variations of any of the
   terrestrial magnetic elements.
   [1913 Webster]

WordNet (r) 3.0 (2006):

magnetograph
    n 1: a scientific instrument that registers magnetic variations
         (especially variations of the earth's magnetic field)