When metal detector is running, there will be an electromagnetic field generated from the middle sending wiring coil in the detecting area. The symmetric receiving wiring coil on both sides will have induced voltage under the use of alternating electromagnetic from sending wiring coils. The induced voltages from those two receiving wiring coils is equivalent and counteract with each other, two receiving wiring coils will be in equilibrium status.
If there is metal, the electromagnetic field will change and the equilibrium status will be broken. Two receiving wiring coils will send out weak signal,which will be enlarged and treated in the system, finally send out alarm and rejecting signal.