This simple circuit diagram are used to sense electromagnetic radiation, it can even detect hidden wrings. The electric field will induce a small voltage at 1mH inductor (the sensor) and this induced voltage will be amplified by the opamp. This circuit used a headphone that connected to the opamp output and will give an audio indicator of the electric field. To set the gain amplifier, set the R4 POT value.
For example of the applications, the electric field around a main transformer can be heard as a 50Hz human frequencies. You can even hear the telephone conversation by keeping the sensor near a telephone line. Use a 9V DC volts for this circuit, its recommended to use radial type inductor for L1 and use electrolytic type in all capacitors with rate at least 15V.
source : www.circuitstoday.com