Electric current is the flow of electric charge. The electric charge that flows is carried by, for example, mobile electrons in a conductor, ions in an electrolyte or both in a plasma. The SI unit of electric current intensity is the ampere.

Electric current is measured using an ammeter. A solid conductive metal contains mobile, or free, electrons. These electrons are bound to the metal lattice but not to any individual atom.

Even with no external electric field applied, these electrons move about randomly due to thermal energy but, on average, there is zero net current within the metal. Given a plane through which the wire passes, the number of electrons moving from one side to the other in any period of time is on average equal to the number passing in the opposite direction. As George Gamow put in his science popularizing book, One, Two, Three...Infinity (1947), "The metallic substances differ from all other materials by the fact that the outer shells of their atoms are bound rather loosely, and often let one of their electrons go free.

Thus the interior of a metal is filled up with a large number of unattached electrons that travel aimlessly around like a crowd of displaced persons. When a metal wire is subjected to electric force applied on its opposite ends, these free electrons rush in the direction of the force, thus forming what we call an electric current."

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