This current
creates an electromagnetic field. The direction of the current flow is reversed several thousand times every second. After a few second,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