At an airport, a person is made to walk through the doorway of a metal detector, for security reasons. If she `//` he is carrying anything made of metal, the metal detector emits a sound. On what principal does this detector work?

Explanation

At airport entrance , there is kept a metal detector of door way type. Frame of this door way, consists of a coil having large no. of tightly wounded turns, there is connected with a capacitor. This LC circuit is in resonance with ac source connected with it. Suppose the coil has inductance, `L_(0) = ( mu_(0) N^(2) A)/( l)`
Now, when any person carrying some metallic object, walks through this door way, inductance of coil, walks through this door way, inductance of coil changes from `L_(0)` to L where `L = ( mu N^(2) A)/( l ) `. Here L` gt gt gt gt L_(0)`. Because of this, there is a sudden change in the impedance of a circuit and hence sudden change in the current passing through the LC circuit. This sudden change in current is immediatly noted and electronic circuit kept in the metal detector sounds "Beep... Beep..". This alarm gives indirect proof that the person walking through the door way is carrying some metallic object.