In today’s world, we regularly encounter walk-through metal detectors. They are a common element of the security protocol put in place to ensure public safety at airports, sporting matches, concerts and other potentially high risk events, but How Do Metal Detectors Detect Metal??

The basic explanation of metal detection relies on a rule of physics that states that wherever there is a changing electric field, there is also a changing magnetic field as they are two sides of the same coin (electro-magnetism) see, Maxwell’s equations for a deeper explanation.

Your basic metal detector exploits this with two coils, a transmitter and receiver coil. Firstly the transmitter coil has an electric current running through it which creates a magnetic field according to the law above. This magnetic field interacts with metals that it passes over and incites an electric current in the metal, causing, you guessed it again, a small magnetic field to form.

The receiver coils job is to pick up on that magnetic field and, using the same principle demonstrated above (in reverse) create a current through a circuit, which on a basic metal detector contains a speaker! Congratulations you’ve found a bit of metal.

Now, gate metal detectors are slightly more complicated than that as you can imagine.

Pulse Induction Technology.

Metal detectors rely on Pulse Induction technology to detect metal. Short magnetic fields are generated by sending powerful bursts or pulses of electric current through a metal coil. Depending on the model, metal detectors can send out between 25 and 1000 pulses per second. These pulses create short magnetic fields, which reverse polarity and rapidly collapse resulting in sharp spikes of energy lasting only a few microseconds. These spikes trigger a subsequent pulse (lasting about 30 microseconds). When metal moves through these magnetic fields, a reflected magnetic field is created. The reflected magnetic field reacts with a receiver coil to trigger an audible alert. Essentially, the magnetic field of a metal object slows down the disappearance of the reflected pulse. The length of the reflected pulses are monitored and compared against the expected length. If a reflected pulse is longer than expected, even by a few microseconds, it is typically indicative of the presence of metal.

Multi-Zone Metal Detectors

Most modern walk-though detectors have a number of transmit and receiver coils, each at a different height. These are referred to as multi-zone metal detectors.  Multi-zone metal detectors are more effective for security purposes because they are essentially comprised of several individual metal detectors positioned to more thoroughly screen a subject.

If you are interested in learning more about metal detectors, or wish to discuss your security needs, please contact Todd Research by calling +44 1480 832202 or sending an email to [email protected].