Electric current

An electric current is charge flowing round a closed circuit. In metal wires it is the free electrons drifting from the battery's negative terminal round to its positive one; long before electrons were known, though, current was agreed to flow from plus to minus, and that conventional direction is still the one drawn. A wire is always full of free electrons, which is why a lamp lights the moment the switch closes even though each electron creeps along slowly. Current is measured in amperes with an ammeter placed in the path of the current, in series. In a circuit with only one path the current is the same everywhere: a lamp doesn't use up current, it turns the charges' energy into light and heat.