Test light

A test light, test lamp, voltage tester, or mains tester is a simple piece of electronic test equipment used to determine the presence or absence of an electric voltage (usually alternating current (AC) ) in a piece of equipment under test.

The test light is an electric lamp connected with one or two insulated wire leads. Often, it takes the form of a screwdriver with the lamp connected between the tip of the screwdriver and a single lead that projects out the back of the screwdriver. By connecting the flying lead to an earth (ground) reference and touching the screwdriver tip to various points in the circuit, the presence or absence of voltage at each point can be determined, allowing simple faults to be detected and traced to their root cause. For higher voltages, a statiscope consisting of a neon glow tube mounted on a long insulating handle can be used to detect AC voltages of 2000 volts or more.

For low voltage work (for example, in automobiles), the lamp used is usually a small, low-voltage incandescent light bulb. These lamps usually are designed to operate on approximately 12 V; application of an automotive test lamp on mains voltage will destroy the lamp and may cause a short-circuit fault in the tester.

For line voltage (mains) work, the lamp is usually a small neon lamp connected in series with an appropriate ballast resistor. These lamps often can operate across a wide range of voltages from 90V up to several hundred volts. In some cases, several separate lamps are used with resistive voltage dividers arranged to allow additional lamps to strike as the applied voltage rises higher. The lamps are mounted in order from lowest voltage to highest, this minimal bar graph providing a crude indication of voltage.

...
Wikipedia