PI metal detectors operate on a totally different principle, although they still depend upon the principle of the changing inductance of a search coil when metal is near it. You can see in Figure 7 that a pulse generator creates pulses. The pulses are amplified and connected to an electronic switch. When this pulse is connected to the coil, a magnetic field occurs around it. When the current through the coil is interrupted, the magnetic field around the coil is also interrupted. Due to self-inductance, we will see a strong pulse of reverse-polarity coming from the coil. To prevent self-oscillation, we add a damper resistor to the circuit. When metal comes near the search coil, the reverse-polarity pulse (occurring when the drive current is interrupted), will take longer to decay than normal. This longer pulse, although relatively small, is still large enough to be detected. To boost it, we take many successive measurements, and feed the reverse-polarity decay signals to an integrator. The integrator’s voltage output is amplified and fed to an indicator. The advantage of this type of metal detector is the simple construction of the search coil and the fact that soil conditions do not affect the detector unless there are minerals present in the ground. The drawback of this method is that it does not distinguish amongst different types of metals. Also, the search coil head must not include any metal parts. Another drawback is the higher power consumption and a rather complicated design.