How electric tester works?

There are many types of electric testers in use. One kind is a two contacts tester where there will be two connecting leads and one or more neon lamps to indicated the presence and approximate magnitude of test voltage. Here one contact is placed on the wire to be tested and the other lead is placed on the reference level like ground or other circuit parts.

Another kind is a one contact tester which usually is in the form of a screw driver. The end portion is placed in contact with the wire to be tested and the back of the tester is touched by a body part. A neon lamp inside the transparent tester body will glow if the wire is live. The lamp needs only very little electric current to glow that, it uses the capacitance and resistance of the human body to complete the circuit by making a current flow to the ground.