A direct-current generator consists of a coil spinning in one direction inside a magnetic field. The induced current is proportional to the angle between the coil and the field; it is maximized when the coil is perpendicular to the magnetic field and zero when it is parallel to it. Two segmented slip rings and brushes (the "commutator") serve to reverse the polarity, since the voltage of the coil is reversed every half-turn.

Increasing the number of coils or the field of the iron core increases the induced electromotive force , which, in turn, increases the current.

The expression to calculate the electromotive force is , where is the magnetic induction, is the length of the wire, is its velocity, and is the angle between the wire and the magnetic field.