The Triplett Model ET30 Magnetic Field Detector quickly determines the presence of a magnetic field including AC, DC and Permanent Magnets. It can test solenoid valves in pneumatic/hydraulic control equipment, relays with coils, electrically controlled solenoid valves in vehicles/machinery/oil burners, and audio speaker coils. It features a built-in self test magnet, LED flashlight and a low battery indicator. Includes (2) AAA batteries.