Direct current is the unidirectional flow of electric charge. Direct current is produced by sources such as batteries, solar cells, commutator-type electric machines of the dynamo type. It can also flow in a conductor such as a wire, but can also flow through semiconductors, insulators, or through a vacuum. The DC current flows in a constant direction appose to alternating current (AC). Historically, direction current was called galvanic current.
Direct current may be found from an alternating current supply by use of a current-switching arrangement. This arrangement is called a rectifier. A rectifier contains electronic elements or electromechanical elements that allows current to flow only in one direction. DC may also be obtained from AC with an inverter or a motor-generator set.
Direct current is used in nearly all electronic systems as the power supply. Very large quantities of direct current power are used in the production of aluminum and other electrochemical processes. DC is also used in some railway propulsion. High-voltage direct current is used to transmit large amounts of power from remote generation sites or to interconnect alternating current power grids.
Although DC stands for direction current, it also can refer to constant polarity. Under this definition, DC voltage can vary in time. This can be seen in the raw output of a rectifier or the fluctuating voice signal on a telephone line. Some forms of direct current have almost no variation in voltage, although, it may still have variations in output power and current.