Airport metal detectors
Metal detectors have been a staple of airport security for decades. These tall, boxy metal detectors are meant to be walked through and, theoretically, they will detect any metal objects that pass through them. However, if this was the case, then there wouldn't be any need for the hand-held metal detectors that travel authority agents and security guards usually have. Knowing how the larger metal detectors work, and their failings, can help take the confusion out of any misunderstanding.
Radio Waves
The way these large, walk-through metal detectors work is fairly simple. One of the walls of the box sends radio waves to the other side, which are then returned. Radio waves will not pass through metal objects, so if the radio waves hit any metal on a person, the waves will be bounce back sooner than the others. The time it takes for the radio waves to travel the full length of the box is set in the machine, so that if any radio waves are bounced back sooner, the alarm in the metal detector is set off.
Adjustments
Most airport metal detectors, though, don't catch all the metal that passes through them. This is because the sensitivity of the metal detector has to be toned down slightly. Otherwise, people with metal fillings in their teeth, metal rivets in their jeans and metal zippers would all set off the alarms. However, this also brings down the effectiveness of the metal detectors. While smaller, metal items may not be detected, the settings are always set high enough that most items considered dangerous will set off the alarm.