A coil of insulated wire wound around a laminated steel pole shoe in an electrical motor or generator. Each motor or generator has two or more field coils that may be connected either in parallel or in series with the armature.

Direct current flowing through the field coils of a generator produces magnetic flux which induces a voltage in the conductors of the armature as they pass in front of the pole shoes. The amount of voltage induced in the armature is determined by the strength of the magnetic field. A voltage regulator is normally used to control the amount of current flowing in the field coils, and thus the strength of the magnetic field.

In a motor, torque is produced by the reaction between the magnetic flux produced by current flowing in the field coils and the flux produced by current flowing through the armature coils.