How does a metal detector work?

Have you ever hunted for buried treasure? 

You could just start digging anywhere and hope that you would find something. 

Or you could use a metal detector to find out exactly where metal objects are buried under the ground. 

Magnetism makes a metal detector work. Do you know how?

The kind of metal detector that treasure hunters use is made up of a flat coil of wire fixed to one end of a short pole. At the other end is a handle, and a box containing a battery and electronic circuits.
As the treasure hunter moves the coil above the ground, the battery produces an electric current that sends out a magnetic field from the coil.
This field makes any buried metal object, such as a coin, send back its own magnetic field. When the metal detector receives the coin's field, a light flashes on the handle of the pole.

The magnetic field around the moving coil creates electric currents in any metal object, such as a coin, that is buried in the ground.
These currents create a magnetic field around the buried object.
The metal detector's electronic circuits detect the magnetic field made by the currents.
A light on the handle of the pole flashes to tell the treasure hunter that the detector has found a metal object.

Can you think of any other uses for a metal detector? 

There are metal detectors in vending machines to test whether the coins put in the slot are real or fake.
There are metal detectors at airports so that security guards can make sure no one takes dangerous weapons, such as guns, onto airplanes.
Metal detectors under the road control some sets of traffic lights, where a major road crosses a minor road. The lights will change to 'Go' for the minor road only when a car comes along. Magnetism makes all these different kinds of metal detectors work.