The metal detector operates by means of electromagnetic induction. When an item of metal passes through a magnetic field, or vice versa, the field will generate electric eddy currents that circulate in the metal. These currents then create their own magnetic field. The metal detector device is able to detect the field that is emitted.
The detecting coils within the unit do the transmitting and the receiving. These coils overlap one another to induce a current in each other. Under normal circumstances the two currents would cancel out. However, the metal objects magnetic field causes distortions in the balance and therefore a low current emerges in the receiving coil.
The changes in the balance of the magnetic field are amplified and either fed as audio tones to a pair of headphones or to a meter mounted in the detector handle.
Top of the range metal detectors usually feature a discriminator, this allows the detector to ignore certain types of metal and concentrate on the more precious ones.
