Movement of electric charge carriers is known as electric current. In a wire, electric current is a flow of electrons that have been dislodged from atoms and is a measure of the quantity of electrical charge passing any point of the wire per unit time. Current in gases and liquids generally consists of a flow of positive ions in one direction together with a flow of negative ions in the opposite direction. Conventionally, the direction of electric current is that of the flow of the positive ions. In Alternating Current (AC) the motion of the charges is periodically reversed while in Direct Current (DC) it is not. Direct current is produced by batteries, fuel cells, rectifiers, and generators with commutators. Direct current was supplanted by Alternating Current (AC) for common commercial power in the late 1880s because it was then uneconomical to transform it to the high voltages needed for long-distance transmission. Techniques developed in the 1960s overcame this obstacle, and direct current is now transmitted over very long distances, though it must ordinarily be converted to alternating current for final distribution. For some uses, such as electroplating, direct current is essential. The Ampere is an unit of electric current in the meter-kilogram-second system or the SI system. It is the steady current that when flowing in straight parallel wires of infinite length and negligible cross section, separated by a distance of one meter in free space, produces a force between the wires of 0.0000002 newtons per meter of wire. It was previously defined as the current when one Coulomb of charge flowed in one second. Now instead one Coulomb is said to be the charge when one Ampere flows for one second.