Testers are one of the simplest solutions, used to test current and voltage continuity in any circuit. Indispensible for any electrician, the testers are very simple and interesting devices. This article will explore the internal structure of one such commonly used circuit tester.The user is required to be on the floor while using the tester so that he can serve as ground when current flows. In order to test the live wire, the user’s body must be in contact with the metal cap on the top of the screwdriver when the tester is put in contact with the wire. This completes a genuine electrical circuit and the bulb will light up once the tester senses voltage.