Metal Detectors

METAL DETECTORS have long been mainstays of building security and concealed weapon or contraband detection in correctional institutions. The tragic events of September 11, 2001 focused more attention on the role of metal detectors, specifically in airport settings. Since assuming control of air travel security, the Transportation Safety Administration has established more stringent uniform performance standards for all metal detectors used in U.S. airports (Fiorino).

The TSA has also integrated explosive detection equipment into the passenger and cargo screening procedures, but metal detection and X-Ray equipment remain the primary means relied upon by law enforcement authorities to address the increased threat of terrorism at the most likely targets.

In other countries where terrorism is even more of an ongoing concern than it is in the United States, metal detectors have become a routine part of everyday life, such as in Israel where walk through metal detection equipment has been installed at entrances to malls, shopping areas and even restaurants (Dershowitz, p.56). There are three types of metal detectors in general use today: walk-through, hand-held, and extended arm.

Walk-through detectors are usually installed at building entry points or anywhere demarcating a transition from an unsecured area to one of greater security. Hand-held units are effective for use in lower volume areas, but it is common to find them used in conjunction with walk-through detectors. Typically, individuals that provoke a signal from the walk-through equipment can be more particularly screened by the hand-held device. Most metal detectors utilize an electromagnetic field generated by passing an electric current through a wire coil.

In the case of walk-through metal detectors, the electromagnetic field is projected in the form of a wall between the borders formed by the equipment; hand-held devices project a circular field surrounding the length of the device. In the case of walk through detectors, subjects (and any potential targets) pass completely through the electromagnetic field, whereas hand-held devices are employed by passing them close enough to potential targets to come within the known dimensions of the magnetic field surrounding the device.

Extended arm detectors function exactly the same as hand-held devices, except that their design allows them to be more easily deployed to bring their electromagnetic field closer to the ground for their specific use in detecting buried weapons or contraband (USDJ, p.21). Generally, metal detectors are capable of detecting objects that are sufficiently electrically conductive to cause a detectable change in the electromagnetic field generated by the source in the detector. Metal objects conduct electricity by transmitting the charge via the electrons of the atoms the object comprises.

Electrically conductive materials are detectable directly, as well as indirectly, by virtue of a secondary magnetic field generated by the target object itself, induced by the eddy current caused by exposure to the (primary) magnetic field of the detector source (USDJ, p.26). Electrical conductivity is measured in Siemens units per meter (S/m). The electrical conductivity of air, bakelite (an industrial plastic), diamond, distilled water and glass are all negligible.

Sea water registers approximately 4 S/m, by virtue of the electrical conductivity of salt ions within the liquid rather than electrons as in the case of metals (USDJ, p.24). Copper (rated at 57,000,000 S/m) is very highly conductive, which, quite naturally, is why it is preferred for use in wiring. Other metals such as aluminum, brass, lead, stainless steel and cast iron range from 35,000,000 S/m to 1,000,000 S/m, respectively, and graphite rates at 100,000 S/m (USDJ, p.25).

Non-ferrous metals are detectable by virtue of the inductance change they produce within the coil in the source of the primary magnetic field generated by the detecting device. Essentially, the introduction of the target object into the primary magnetic field alters the inductance of the coil in the same manner as would a change or additional turn in the coil itself. Ferrous metals are detectable both in this manner as well as by virtue of the magnetic permeability of the target object (Sheets, p.23).

Magnetic permeability refers to the property of some materials to become permanently or temporarily) magnetized by polar alignment of their inherent microscopic component magnetic domains through exposure to a magnetic field. Permeability is relative, and varies from non-permeable or diamagnetic materials such as copper, lead, silver and water, to highly permeable ferromagnetic materials such as cobalt, iron, nickel and steel. Paramagnetic materials such as air and aluminum are far less permeable than ferromagnetic substances and slightly more permeable (or more readily magnetized) than diamagnetic materials (USDJ, p.27-28).

Electrical conductivity and magnetic permeability are the main parameters measured by and detectable to metal detectors, but many other factors also contribute to the specific susceptibility of a given object to detection by subjection to an induction field. The spatial orientation of the target object in relation to the magnetic field is the most significant additional factor, because detectability varies profoundly depending on whether its surface or its edge is perpendicular to the magnetic field USDJ, p.30).

Likewise, the shape of the target object also contributes to its susceptibility to detection, and specifically, the length of its perimeter edges rather than merely its overall surface area, by virtue of a property known as the current path length of the interaction between magnetic field and object, which varies in direct proportion to total perimeter of its edges (USDJ, p.32). The human body itself is also somewhat conductive, which is known as body capacitance.

Generally, metallic objects are sufficiently more electrically conductive (and magnetically permeable) than biological tissues, so proper calibration and operation of walk-through detectors is usually sufficient to distinguish between them. Hand-held devices offer the additional flexibility of probing specific areas of the body possibly used in attempts to shield objects from detection (Sheets, p.27). Metal detectors are susceptible to a source of error introduced by false positives, particularly in the case of extended arm equipment employed in proximity to walls or floors containing conductive or ferromagnetic construction materials.

Finally, the opposite problem can result where the equipment is adjusted to recognize metals more likely to.