Which metals make an alarm go off depends on the technology used by the machine. Alarms frequently employ a very low-frequency or induction balance system. In a VLF system, a transmitter sends a signal through a coil and a receiver that picks up frequency changes when metal passes by the transmitter. Large, thick objects that contain highly conductive metals like gold, silver and copper tend to set off VLF alarms. Some typical items that set off an alarm include keys, coins and cellphones.
Magnetic metals, such as iron nickel and cobalt, do not cause phase shifts but can still set off an alarm. For instance, prosthetic arms and other surgical implants usually contain a high percentage of iron and often set off metal detectors, according to Beth Israel Deaconess Medical Center.
The attributes of a metallic object, especially its thickness, can determine whether or not it sets off an alarm. In general, the thicker a metallic item, the better the chance it sets off an alarm. Depth can also dampen the effect of metal on an alarm. Items buried within the body, such as a pacemaker, increase an object's resistance to electricity.
If you have a pacemaker or other surgical implant, alert security before going through a metal detector. A doctor can issue you a pacemaker identification card. If you carry metallic objects, most security checkpoints allow you to bring them into the facility if you agree to run the objects through an x-ray machine. Metal detectors can employ a single type of detection system or multiple ones, so you never know exactly what will set off an alarm.
