A combined motor and generator used to lower the potential difference in
a portion of a circuit, e. g., that part within a building.

A motor-generator is a dynamo whose armature carries two commutators,
with two separate windings, one of fine wire of many turns, the other of
coarse wire of few turns. If the potential of the system is to be
lowered, the main current is passed through the fine winding. This
causes the armature to turn motor-fashion, and a potential difference is
generated by the rotation of the large coils in the field. This
potential difference is comparatively low and by properly proportioning
the windings may be lowered to as great a degree as required.

The same apparatus may be inverted so as to raise potential difference.
It acts for continuous current systems as the induction coil transformer
does for alternating current systems.

Synonym–Continuous Current Transformer.