The Triplett ET30 Magnetic Field Detector quickly determines the presence of magnetic fields (AC/DC/Permanent). 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. 1-year warranty.