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Walk-Through Metal Detectors for Personnel
(Chapter 3   Metal Detection, Continued)

Do metal detectors really work?—The basics

Exhibit 2.18Metal detectors work very well—they are considered a mature technology and can accurately detect the presence of most types of firearms and knives. However, metal detectors work very poorly if the user is not aware of their limitations before beginning a weapon detection program and is not prepared for the amount of trained and motivated manpower required to operate these devices successfully.

A metal detection device in school security applications is used primarily to locate undesirable objects that are hidden on a person's body. When a questionable item or material is detected by the device, the detector produces an alarm signal; this signal can be audible, visible (lights), or both. Unfortunately, a metal detector alone cannot distinguish between a gun and a large metal belt buckle. This shortcoming is what makes weapon detection programs impractical for many schools; trained employees are needed to make these determinations.

Metal detectors are usually not effective when used on purses, bookbags, briefcases, or suitcases. There is usually a large number of different objects or materials located in or as part of the composition of these carried items that would cause an alarm.

If you ask the average person what a metal detector does and what property to which it is most sensitive, the answer to the first question would probably be that it is a device that detects only metal. The answer to the second question likely would be that a metal detector is more likely to detect metal objects with heavier mass. Both answers are incorrect.

A metal detector actually detects any conductive material—anything that will conduct an electrical current. The typical pulsed-field portal metal detectors generate electromagnetic pulses that produce very small electrical currents in conductive metal objects within the portal archway which, in turn, generate their own magnetic field. The receiver portion of a portal metal detector can detect this rapidly decaying magnetic field during the time between the transmitted pulses. This type of weapon detection device is "active" in that it generates a magnetic field that actively looks for suspicious materials or objects. A magnetometer, a passive device, was much more in use 20 years ago in the detection of weapons. The magnetometer depends on the Earth's magnetic field-it looks for a distortion caused by the presence of ferromagnetic (attracted to a magnet) material.

Counter to intuition, the mass of a particular object is not significant in metal detection. The size, shape, electrical conductivity and magnetic properties are the important properties.

For example, when a long thin wire is taken through a portal (walk-through) metal detector, and the wire is in any geometry except one in which the two ends (or any two points on the wire) are touching, it will rarely be detected. However, shape this same wire into a closed circle and the metal detector will most likely go off, even though the mass of the wire has not changed.

Delving even deeper into metal detector sensitivity, consider the orientation of an object. Take the same closed-loop wire described in the previous paragraph. Lay this loop on its side so that it is parallel to the ground. In this configuration, the portal metal detector is less likely to see it, but, if the wire loop is upright and parallel to the side panels of the metal detector, the detector will be much more likely to go off in this orientation.

Some people fear the use of a metal detector on themselves because of the possible side effects of being subjected to the magnetic field. This fear is unfounded; metal detectors emit an extremely weak magnetic field, weak enough to be of no concern even to heart patients with pacemaker-type devices. Indeed, the use of an electric hair dryer subjects the user to a much stronger field than would be received by a metal detection device.

Another widely held belief about metal detectors is that they are a straightforward technology, where the equipment does all the work. This is not true at all. The average first-time consumer will undoubtedly expect a metal detector to be much smarter and more helpful than it can possibly be. A metal detector is only as good as the operator overseeing its use.

In many facilities, the misconception exists that someone known by the operator, such as a fellow employee or a security person, should be allowed to circumvent the system. It must be clearly established that in order to ensure the integrity of any routine metal detection program, everyone must be subjected to the program requirements, including students, parents, teachers, custodial and maintenance staff, security personnel (except for sworn police officers who are required to carry a weapon), school administrators, and visitors. To require less would be counterproductive and prejudicial. Signage can be of great help: a sign at the school entrance explaining the importance of the detectors in maintaining a safe and comfortable learning environment provides policy notification. If a more aggressive approach is needed for a particular community, entry signs could spell out a particular school or district policy that requires the screening of all who enter the school, with access denied to those who refuse.

 



Research Report:   The Appropriate and Effective Use of Security Technologies in U.S. Schools