"Metal Linemen Tester Screwdriver They are designed for electricians. Automatically detects and indicates the standard voltage allowing broad application can be used as a screwdriver. Designed for detection of standard voltage in cables, cords, circuit breakers, lighting fixtures, switches, outlets and wires. Also excellent for detection voltage in security, entertainment, communications, environmental control, and irrigation systems. Tube light of circuit tester pen will emit light when the pen detects alternating current voltage, Tester screwdriver pen has two kinds of applications,one for measuring circuit electrical pen and the other for screwdriver. You can use it to directly test AC voltage as a voltage tester, you also can use it as slot screwdriver, When you use before testing, it is necessary to insert the screwdriver to the bottom, hold the finger against the end of the handle.The tube turns light when there is electricity and operate safely, AC circuit tester light detector is for indoor use only and to avoid strong light Product Features : USAGE METHOD : Before testing, it is necessary to insert the screwdriver to the bottom, hold the finger against the end of the handle.The neon tube turns light when there is electricity and operate safely MULTI FUNCTIONAL : You can use it to directly test AC voltage as a voltage tester pen, you also can use it as a  slot screwdriver APPLICATION : Suitable for use to home, shop, garage, automotive, motorcycles, boats, personal watercraft, snowmobiles  " Read Less