A test light is a basic circuit testing tool.
It consists of a mobile probe with a small lamp or light bulb attached, connected via an insulated wire or cable to a small clip that can be clamped to a suitable return point for completing the circuit. When each end of the tester is connected to a circuit carrying a voltage, then the lamp will light up. The metal probe is usually sharp enough to penetrate cable insulation so that the tester can connect directly with the conducting wire inside.
