The operating principle of electromagnetic electrical generator was discovered in the years of 1831–1832 by Michael Faraday. The principle, later called Faraday’s law, is that an electromotive force is generated in an electrical conductor which encircles a varying magnetic flux.
He also built the first electromagnetic generator, called the Faraday disk, a type of homo polar generator, using a copper disc rotating between the poles of a horseshoe magnet. It produced a small DC voltage.
This design was inefficient, due to self-cancelling counter flows of current in regions that were not under the influence of the magnetic field. While current was induced directly underneath the magnet, the current would circulate backwards in regions that were outside the influence of the magnetic field. This counter flow limited the power output to the pickup wires, and induced waste heating of the copper disc. Later homo polar generators would solve this problem by using an array of magnets arranged around the disc perimeter to maintain a steady field effect in one current-flow direction.
Another disadvantage was that the output voltage was very low, due to the single current path through the magnetic flux. Experimenters found that using multiple turns of wire in a coil could produce higher, more useful voltages. Since the output voltage is proportional to the number of turns, electrical generator could be easily designed to produce any desired voltage by varying the number of turns. Wire winding became a basic feature of all subsequent generator designs.
Independently of Faraday, the Hungarian Anyos Jedlik started experimenting in 1827 with the electromagnetic rotating devices which he called electromagnetic self-rotors. In the prototype of the single-pole electric starter (finished between 1852 and 1854) both the stationary and the revolving parts were electromagnetic. He also may have formulated the concept of the dynamo in 1861 (before Siemens and Wheatstone) but didn’t patent it as he thought he wasn’t the first to realize this.
The Faraday disk was the first electric generator. The horseshoe-shaped magnet (A) created a magnetic field through the disk (D). When the disk was turned, this induced an electric current radially outward from the center toward the rim. The current flowed out through the sliding spring contact m, through the external circuit, and back into the center of the disk through the axle.