Airport Metal Detectors and Guns

When skyjacking began to be a national pastime and airport security needed strengthening, one of the first steps taken was to perform personal scans by a hand-held metal detector. To some people, this was at least mildly embarrassing.

The succeeding generation of walk-through metal detectors was a distinct improvement, but sometimes provided wildly erratic results. How many times have you seen men forced to go back through the line time and time again after removing coins, keys, pocket knives, cigarette packages and foil gum wrappings from their pockets? I personally witnessed one incident at Fairbanks International Airport where one gentleman was finally reduced to removing his belt with its large buckle, and for good measure, took off his boots as well.

Airport metal detectors give very few false alarms these days, probably owing both to improvements in equipment and experience gained by those who use them. Seldom is it now necessary to remove the change from your pockets, but how does the detector know the difference between a few quarters and a snub-nosed .38?

The answer lies in the rate at which the "signal" generated by a metallic object fades as a person passes between the walls of the detector. When a key--or a gun--is placed in the magnetic field generated by coils in the walls, a small electric current is generated in the receiving loop on one side (the other side contains the transmitting loop). A computer attached to the receiving coil estimates the object's size by calculating the rate at which the current fades. Generally, the smaller the object, the more quickly the current dies down, and the computer will signal only if a "gun-sized" object is detected.

Although they are not infallible, today's airport metal detectors are a distinct improvement over those of a generation ago, which sometimes had ornately bedecked gentlemen passengers doing a modest version of the strip tease.