ELECTRIC POWER

energy generated through the conversion of other forms of energy, such as mechanical, thermal, or chemical energy. Electric energy is unrivaled for many uses, as for lighting, computer operation, motive power, and entertainment applications. For other uses it is competitive, as for many industrial heating applications, cooking, space heating, and railway traction. For statistical data on production and consumption of electrical energy, see Britannica World Data in the current Britannica Book of the Year. Electric power is the product of current and voltage. A given value of power can be produced by endless combinations of current and voltage values. If the current is direct, electrons progress always in the same direction through the device receiving power. If the current is alternating, electrons move back and forth in the device and in the wires connected to it. For many applications either type of current is suitable, but alternating current (ac) is customarily used because of the greater efficiency with which it can be generated and distributed. A direct current (dc) is required for certain industrial applications, such as electroplating and electrometallurgical processes. The wide-scale production and distribution of electric power was made possible by the development of the electric generator, a device that operates on the basis of the induction principle formulated in 1831 by the English scientist Michael Faraday and the American scientist Joseph Henry independently of one another. The first public power station employing an electric generator began operation in London in January 1882. A second such station opened later that same year in New York City. Both used dc systems, which proved inefficient for long-distance power transmission. By the early 1890s the first practical ac generator was built at the Lauffen power station in Germany, and service to Frankfurt am Main was initiated in 1891. There are two primary sources for driving generatorshydro and thermal. Hydroelectric power is derived from generators turned by falling water. Most other electric energy is obtained from generators driven by steam produced either by a nuclear reactor (q.v.) or by burning fossil fuelsnamely, coal, oil, and natural gas. Until the 1930s hydroelectric-power plants equipped with water-turbine generating units produced the largest percentage of electric energy because they were less expensive to operate than thermal-power plants using steam-turbine units. Since that time, however, major technological advances have reduced the cost of thermal-power generation to the extent that it has become more prevalent than hydroelectric-power production, which, by 1990, constituted only 18 percent of global electricity output. Thermal plants using nuclear energy or gas turbines to run steam-electric units are among these technological innovations. Alternative electric energy sources include magnetohydrodynamic (MHD) generators, nuclear fusion reactors, solar batteries, wind turbines, and geothermal-power stations. Electric energy generated at a central power station is transmitted to bulk delivery points, or substations, from which it is distributed to consumers. Transmission is accomplished by an extensive network of high-voltage power lines, including overhead wires, underground and submarine cables, and microwave systems. Voltages higher than those produced by power plant generators are required when transferring alternating current over long distances, in order to reduce power losses that result from the resistance of transmission lines. Step-up transformers are employed at the generating station to increase the transmission voltage. At the substations other transformers step down the voltage to levels suitable for distribution systems.