This very useful device quickly indicates rotation direction. It can also be used to detect the magnetic field produced by AC. The red and white disc spins on its axle when the tester is placed close to a sufficiently strong AC field. When placed close to a pump that is turning the disc will also rotate. No plumber or heating contractor should be without this very handy keyring tool. Quickly indicates rotation direction Detects AC fields Detects solenoid valves