All public access to the airport to go through the terminal, where everyone must walk through metal detector, and all items must be checked by X-ray machine.
　　Almost all airport metal detectors are based on pulse induction (PI) principles. A typical pulse induction system uses a bow tilting at the side of the coil as transmitter and receiver. The technology can be sent through the coil current is strong and short pulses. Each pulse generates a brief magnetic field. When the pulse ends, the magnetic field polarity reversal and the rapid decay of the magnetic field, resulting in a sharp electrical spike. The spike will last several microseconds (millionths of a second), then leads to another in the conduction current in the coil. The subsequent current is called the reflected pulse, it only lasts about 30 microseconds. Then send another pulse, and this process is repeated. Typical, based on pulse induction metal detector sends about 100 pulses per second, but different manufacturers and different types of products the number of pulses per second, there may be very different, ranging from about 25 pulses per second to more than 1,000 between pulses.
　　If you have metal objects walk through metal detector, the pulse will be generated within the object a reverse magnetic field. When the pulse magnetic field decay and cause the reflected pulse, the metal objects in the reflected pulse magnetic field will extend the time required to completely disappear. The process works something like echoes: If you are only a few in a room with hard surface cries, you may only hear a very brief echo, or completely not hear the echo. But if you have a lot of hard surfaces in the room shouting, echo duration will be longer. In a pulse induction metal detector, metal objects from the target they produce the magnetic field will "echo" superimposed on the reflected pulse, which makes the reflected pulse duration when compared with no metal objects a little longer.
　　Metal detector in a sampling circuit for monitoring the reflected pulse duration. By its comparison with the expected duration, the circuit can determine whether the reflected pulse decay time extended by another magnetic field. If the reflected pulse decay time, decay time longer than the normal number of microseconds is possible that the metal objects interfere with the decay process.
　　Sampling circuit to monitor a weak signal is sent to a device called the integrator. Integrator reads the signals from the sampling circuit, it will zoom in and converted to DC. DC voltage is connected to an audio circuit, and in this circuit is converted to sound using the sound of metal detectors indicate that the target object. If you find the target items, screeners who will require passengers to remove all metal objects and then again through the detector. If the metal detector to indicate the presence of metal, so screeners will use a sensor technology based on the same hand-held pulse detector to find the specific reasons.
　　The market of many new walk through metal detector is multi-region. This means that they have multiple transmit and receive coils, each coil are at different heights. This is essentially equivalent to a single device containing multiple metal detectors.