The current that flows steadily in one direction is usually called a direct current. We know that the electrical system in an automobile and an air plane, the telegraph, the telephone and the trolly-bus use the direct current. Direct current is also used to meet some of the industrial requirements. For industry and many other purposes, however, most cities make use of another type of electric current which flows first in one direction and then in another. It was given the name of an alternating current. We know that the alternating current is the very current that makes radio possible.