Testers which are used for testing voltage at some plug (more like a screw driver with led & external metal tail), when you insert it in a plug & touch external metal point LED glows giving indication about voltage present. If you don't touch metal it won't glow even if it is in plug having voltage. I want to know how led glow when I touch the metal.How does the circuit gets completed. Even if sit on plastic or wooden chair, it still works same way? I don't understand how circuit gets completed.