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A homopolar generator is a type of electrical generator that produces direct current (DC) using an electrically conductive disc that rotates within a static magnetic field. The disc is at a right angle to the magnetic field, so its rotation creates an electrical potential difference between the center of the disc and its rim. Brush contacts connect the edge of the disk to the shaft that rotates the disk. The polarity of the electrical current depends on the direction of rotation with respect to the magnetic field. It is referred to as "homopolar" or "unipolar" because only one pole of the magnet is used.
There are actually two modes of operation in which a homopolar generator can produce power. The stationary magnet and rotating disc is the most common method, but if both the magnet and the disc are spinning, power is still generated. This is possible because of the Lorentz force, which is the force the electrons caused by the electromagnetic field.
Michael Faraday invented the homopolar generator in 1831, leading to one of its alternate names, the Faraday disc. Early versions of this device were not particularly efficient sources of electrical power, but they demonstrated the principles now used by commutated dynamos to produce direct current. The inefficiency of early homopolar generators was mainly due to the counterflow of current; the magnet induces the primary flow of current on the part of the disc that is directly below the magnet, but this current circulates towards areas of the disc that are away from the magnetic field. This counterflow heats the disc rather than producing electrical current.
Modern homopolar generators partially resolve this problem by arranging a series of magnets around the perimeter of the disc. This allows the magnetic field to remain steady from the center to the edge of the disc. The degree of counterflow is greatly reduced, which increases the efficiency of the generator.
Commonly used for demonstration purposes, a small homopolar generator produces only a few volts, while larger generators, such as those used in scientific research, can produce a few hundred volts. Some electrical generation systems use multiple homopolar generators to produce thousands of volts, but in general they do not have many practical industrial uses. A homopolar generator can be made to have a very low resistance, so it can produce large amounts of current, occasionally in excess of 1 million amperes.