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
Object
After a few second