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Voltage tester screwdriver it has no power by itself but uses a small neon lamp indicator (connected in series with an appropriate ballast resistor 2.2M-10M to limits current through the device to safe sub-milliamprere level maximum). When the tip is connected to live wire there is a small current that flows to your body to ground and makes the neon glows.
The test light glows brighter as voltage increases. This tester operate across a wide range of voltages starting from 90V up to several hundred volts.
The voltage tester can fail in case you are very well insulated away from ground sources as there is not enough leakage through you to make the neon bulb to light.
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There are some new type Non-Contact-Voltage-Detectors, tester that runs on double a batteries. See some examples: