An electric pen, commonly known as an electric pen, is a commonly used electrical tool. It can be used to determine the zero and live wires in household lighting circuits, as well as to determine whether there is leakage in household appliances. The test pen mainly consists of a contact used as a screwdriver, a neon tube, a step-down resistor, and a pen cap (for grounding). The front end of the testing pen is a metal probe used to make contact with the tested equipment. The back end is also a metal object, which may be a metal hook or a metal sheet, used for contact with the human body. The middle insulating tube contains neon lamps, resistors, and pressure springs that can emit light. Regardless of the type and structure of the test pen, their working principle is the same. Test pens can be divided into three categories based on the measured voltage: high-voltage test pens can be used to detect projects with a voltage above 10kV; The low-voltage testing pen is suitable for detecting live facilities with a voltage range below 500V; When the testing voltage range is between 6~24V, people often use weak current test pens. We learned Ohm's law in high school. I=U/R. The principle of the testing pen is based on this