From the Blog
Metal Detectors – How They Work
Today there are metal detectors which are small enough to be hand held and you will often find them being used at security points at airports and train stations across the world where they will be able to locate a metal object which has been detected by a walk through system.
There are some which can be transported in order to scan for metal objects in the ground such as nails, loose change or landmines. Any such hand held metal detector operates very much on the same principals as a walk through metal detector as they emit time varying electromagnetic fields and will then listen for waves coming back from conducting objects. There are even some models which will further analyze the returned waves to distinguish which common metal it is.
Certainly for sometime now hand held metal detectors have been used for searching for landmines (unfortunately a many landmines are now made from plastic) and so with new technologies such as neutron activation and ground penetration systems which are being developed in order that searches can be carried out for such landmines that are not made from metal.
Most hand held metal detectors work by use of an electromagnetic field which detects the presence of any metallic objects that are hidden. There are two ways in which metal detectors use electromagnetism either active or passive and below is a short description of each type.
1. Active – This method illuminates some detection space say an opening through which you have to walk (walk through metal detector) or in the space directly in front of you with the use of a hand held version. Energy which is reflected from or passes through the detection space is affected by the presence of any conductive material and the detector detects metal by measuring these effects.
2. Passive – This method does not illuminate the detection space rather it takes advantage of the fact that every unshielded detection space is being permeated by the Earth's natural magnetic field and any Ferromagnetic object (metals such as iron which are capable of being magnetized) that is moving through this detection space will cause a temporary but detectable change in this natural field. Unfortunately, metal objects made from aluminum which is conducting but not Ferromagnetic can not be detected by a passive metal detector.
Metal detectors continue to evolve over time and now you know how they work.