This circular layout also represented the end of the bipolar motor as an industrial power source. It was possible to place a second set of field coils and pole pieces within the same size of casing, giving a four-pole arrangement. Because of the more efficient provision of field flux around the entire circumference of the armature, this give a motor of almost twice the power, for the same armature current. Armature current, and the associated commutator and brushgear, represented one of the most expensive parts of the motor to manufacture.
