Both alternating current and direct are measured in ampers. But how is the ampere defined for an alternating current ?

Explanation

In direct current 1 ampere = 1 coulomb/sec. However, in alternating current, direction of current is reversed every half cycle. Therefore, a.c. ampere is defined in terms of some property, which is independent of direction of current. As is known, a.c. ampere is defined in terms of Joule's heating which is proportinal to `I^(2)`.