balancing coil

from 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.]