A generator is a machine that converts mechanical energy into electrical energy. Originally called a dynamo, it makes use of the principle of electromagnetic induction, which was discovered in 1831 by the English physicist Michael Faraday. He reversed the work of Hans Christian Oersted (1777-1851), who had discovered that an electric current created a magnetic field. By Faraday's reasoning, it should be possible for a magnetic field to generate an electric current. (American inventor Joseph Henry came to the same conclusion independently.) Faraday took a coil of wire, attached it to a galvanometer and then inserted a magnet into the coil. As the magnet moved, its magnetic lines of force were cut by the coil and the galvanometer registered current. The same thing happened when the magnet was removed; electricity was induced to flow in the coil, although the flow was in the opposite direction. In fact, the results were the same whether it was the magnet or the coil that was in motion. When the movement stopped, so did the electrical flow, even if the magnet was still within the coil.
Faraday's next step was to construct a device that would induce a continuous flow, not just bursts. He accomplished this by, once again, reversing the work of another person. Dominique Arago (1786-1853) had shown that a permanent magnet suspended over a copper wheel (a nonmagnetic conductor) was deflected when the wheel turned. This was because the electric current that was generated by the interaction of the permanent magnet and the moving wheel created its own magnetic field which affected the original magnet. Faraday placed a copper wheel (the conductor) between the poles of a permanent magnet and set it spinning. The magnetic field induced an electric current that could be drawn off the spinning wheel with a wire and put to work.
The first electrical generator, possibly the single greatest electrical discovery in history, operated on three principles. First, there must be a conductor in which to induce voltage; second, a magnetic field must be close enough to the conductor for the lines of force to be cut; and third, either the conductor or the magnetic field must be moving. Following Faraday's lead, Hippolyte Pixii invented a hand-driven generator in 1832 in which the magnet revolved and the conductor (coils) remained stationary, more practical than Faraday's first device. André Ampère (1775-1836) suggested using a commutator to transform the power from that generated as alternating current (AC) to direct current (DC), for which there was great interest in the scientific community. In 1846 Ernst Werner von Siemens devised a generator that did not need permanent magnets. His dynamo-electric principle used self-generated electricity to activate electromagnets. The residual magnetism in the electromagnets and their circuitry was enough to start the process. Siemens, along with his brother Charles von Siemens and J. G. Halske (1814-1890), set up a very successful business manufacturing electrical devices.
Originally, the term dynamo was applied equally to generators of AC and DC. Today the term alternator refers to an AC generator, and the dynamo applies to DC only. The dynamo made use of brushes in physical contact with the rotating commutator. There were some disadvantage with DC dynamos--the brushes caused arcing, and there was a limit to how far DC could be carried by power lines. Increasing the voltage to "push" the voltage greater distances made the arcing worse. The alternator, with its reliance on electrical induction, eliminates arcing and, with the use of transformers, produces a voltage that high tension wires can conduct to great distances.
Today, most of the world's electricity is produced by generators, which still operate under Faraday's basic conditions. Portable generators are used in the home during times of electricity blackout, in the field during times of emergencies, and for recreational purposes--anywhere electricity is needed away from the local power grid. Typically powered by gasoline engines, they can be small enough to be carried in one hand but can provide minimal electricity needs for comfortable living in the home. Automobile-based portable generators, which derive their electric power from an automobile's engine, can deliver up to 5,000 watts for as long as two days.
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