By Tim Dees
Metal detectors are commonplace items in our society, as institutions have a greater fear of the threat of terrorism and workplace violence. They range from the large walk-through metal detectors (WTMDs) used in airports and other security checkpoints to devices small enough to conceal in a hand.
Conventional metal detectors are magnetometers that work on the same principle as those used by hobbyists looking for coins on the beach and troops clearing minefields. They work on one or both of two principles:
• Induction coil systems send an alternating current through a coil, producing an alternating magnetic field. When electrically conductive metal enters the magnetic field, eddy currents are produced, creating a separate magnetic field. Another coil measures this additional magnetic field and reveals the presence of the metal.
• Pulse induction detectors fire a spike of high-voltage current into the area where the metal is suspected. In the absence of metal, the pulse decays at a predictable rate. If conductive metal is present, the pulse will create a small current, and the decay of the pulse will be delayed.
WTMDs
Basic WTMDs are configured like a small open doorframe, permitting only one person through at a time. When metal passes through the magnetic field inside the frame, an alarm sounds. More sophisticated WTMDs use lights arranged in a vertical pattern to indicate the approximate height level of the metal.
WTMDs are adjustable for sensitivity, and getting them to a detection level that makes everyone happy is a continuous chore. Set the level too low, and a gun will go through undetected if it’s shielded by enough body mass. People have successfully smuggled guns through metal detectors by carrying them in their armpit, or between their buttocks or thighs. Coordinate your steps just right, and you might get a gun in an ankle holster through if the opposite ankle shields it as it passes through the frame.
When the level is set too high, the tinfoil in a pack of cigarettes or the underwire in a bra will sound the alarm. Most everyone has some metal in or on them someplace, and it’s counterproductive to skin-search everyone who comes through the checkpoint.
Handheld detectors
Handheld metal detectors help to pinpoint and identify the source of the metal. The most common types run on flashlight batteries and beep when they are near metal. More advanced versions are silent (or can be made silent) and alert the operator via small lights or a vibrating handle. There are even some built into a glove.
Security firms will rent metal detectors, and venues such as concert halls and stadiums may have some they can lend you for an unusual situation. If you do make a permanent installation of a metal detector, see that it is manned at all times. Otherwise, people will have an opportunity to test various methods of smuggling metal past the detector before they try it during your event.
Tim Dees is a retired police officer and the former editor of two major law enforcement websites who writes and consults on technology applications in criminal justice. He can be reached at tim@timdees.com.
