Connects to Magnetic Pick-up (above) for audio translation of AC magnetic field frequency and intensity. Makes finding sources of AC magnetic fields very easy. Listen carefully to hear the hum of 50 or 60 Hz, plus clicks, pops, and shrill tones of harmonics, surges, and “dirty” power. Even has an output where you can connect earphone or external speaker. Input: 1/8in stereo. Requires 9V battery.