The Collaborative International Dictionary of English v.0.48:

Compensator \Com"pen*sa`tor\, n.
   1. One who, or that which, compensates; -- a name applied to
      various mechanical devices.
      [1913 Webster]

   2. (Naut.) An iron plate or magnet placed near the compass on
      iron vessels to neutralize the effect of the ship's
      attraction on the needle.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Autotransformer \Au`to*trans*form"er\, n. [Auto- + transformer.]
   (Elec.)
   A transformer in which part of the primary winding is used as
   a secondary winding, or vice versa; -- called also a
   compensator or balancing coil.
   [Webster 1913 Suppl.]