Direct current (DC) is the unidirectional flow of electric charge. Direct current is produced by sources such as batteries, thermocouples, solar cells, and commutator-type electric machines of the dynamo type. Direct current may flow in a conductor such as a wire but can also flow through semiconductors, insulators, or even through a vacuum, as in electron or ion beams. The electric current constantly flows, distinguishing it from alternating current (AC).

A term formerly used for this type of current was galvanic current. The abbreviations AC and DC are often used to mean simply alternating and direct, as when they modify current or voltage. Direct current may be obtained from an alternating current supply by use of a current-switching arrangement called a rectifier, which contains electronic elements (usually) or electromechanical components (historically) that allow current to flow only in one direction.

Direct current may be alternating current with an inverter or a motor-generator set. The first commercial electric power transmission (developed by Thomas Edison in the late nineteenth century) used direct current. Because of the significant advantages of alternating current over direct current in transforming and transmission, electric power distribution is nearly all alternating current today. In the mid-1950s, the high-voltage direct current transmission was developed and is now an option instead of long-distance high-voltage alternating current systems. This DC option is the only technically feasible option for long-distance undersea cables (e.g., between countries such as NorNed). For applications requiring direct current, such as third rail power systems, alternating current is distributed to a substation, which utilizes a rectifier to convert the power to direct current. See War of Currents.

As the power supply, direct current is used to charge batteries in nearly all electronic systems. Vast quantities of direct-current power are used to produce aluminum and other electrochemical processes. Direct current is used for some railway propulsion, especially in urban areas. High-voltage direct current is used to transmit large amounts of power from remote generation sites or to interconnect alternating-current power grids.

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