These mains tester screwdrivers contain a neon indicator in the clear plastic handle which lights up when the point of the screwdriver is touched on voltages between 220250V & your thumb is forming an earth contact with the metal stud set into the top of the handle. The mains tester has a plastic insulating sleeve on the screwdriver blade & comes in two sizes 140mm overall length 55mm blade length and 190mm overall length 100mm blade length. The 140mm tester is fitted with a metal pocket clip.