To test the presence of 'Alternating current' (AC), electric line tester is used. In AC, phase line gives both
positive and negative components for example, an electric bulb glows when current is passed through the bulb from 'phase' to 'neutral'. The neutral is at lower potential. Electricity flows from higher potential to lower potential as heat flows from higher temperature to lower temperature and water flows from higher level to lower level.

Electric line tester, when we keep the pointed end of the tester in contact with the wire through which electric current is passing and touch the metal cap at the other end, a small amount of current passes through the neon bulb, high resistance in the tester and through our body to the earth (which is at zero potential). Here the body completes the electric circuit which makes the neon bulb to glow. If the bulb glows, it means the current is passing through the wire and the device under test is functioning otherwise, the current is not passing through the device or line under test is faulty. High resistance in the tester allows only small amount of current passes through our body.