Conventional unipolar dynamo machines faced inherent practical limitations despite their conceptual simplicity. In such generators, electrical current is induced in a rotating disk or cylinder placed within a unipolar magnetic field, flowing radially from the center to the periphery or vice versa. While theoretically capable of producing continuous direct current, these machines required either extremely high rotational speeds or very large conductors to generate usable electromotive force. Both approaches created serious difficulties in maintaining reliable electrical contact at the disk’s periphery.

In his 1889 patent, Nikola Tesla addressed this problem by rethinking how voltage is accumulated and collected in unipolar generators, rather than attempting to force higher speeds or larger dimensions.
Tesla’s solution uses two unipolar conducting disks, each mounted in its own magnetic field. Crucially, the magnetic polarity of one field is opposite to that of the other. When both disks rotate in the same direction, the induced currents flow radially outward in one disk and inward in the other. Instead of collecting current at the disk edges, Tesla connects the disks electrically using a flexible conducting belt or band wrapped around their peripheries.

This belt serves as a conductor rather than a current collector, eliminating the need for brushes at high-speed peripheral surfaces. Electrical terminals are instead taken from the shafts of the disks, where stable contact is far easier to maintain. The total electromotive force of the system becomes the sum of the EMFs generated by both disks, achieved without increasing rotational speed or disk size.