Electric current is the result of the relative motion of net electric charge. In metals, the charges in motion are electrons. The magnitude of an electric current depends upon the quantity of charge that passes a chosen reference point during a specified time interval. Electric current is measured in amperes, with one ampere equal to a charge-flow of one coulomb per second.

A current as small as a picoampere (one-trillionth of an ampere) can be significant. Likewise, artificial currents in the millions of amperes can be created for special purposes. Currents between a few milliamperes to a few amperes are common in radio and television circuits. An automobile starter motor may require several hundred amperes.

The total charge transferred by an unvarying electrical current equals the product of current in amperes and the time in seconds that the current flows. If one ampere flows for one...