Metal detectors
The principle of a metal detector is quite simple. Inside the machine are wire coils. A transmitter sends an electronic current through the coils.
The current flow current creates an electromagnetic field. The direction of the current flow is reversed several thousand times every second.
After a few seconds,this current or pulse slows down and disperses. Any metallic object like keys,coins,a knife or even a cell phone which happens
to enter this electromagnetic field will influence the speed of the current. If the pulse hits a metallic object,it will take a few milliseconds longer
for the magnetic pulse to go away. This delay sets off an alarm.