And Chinese found in nature in ancient Greece there is a natural to be magnetized rocks as magnets, a thin plate, they can absorb magic and always points to the same direction after random oscillation, we Chinese people are just using the characteristic of the magnet invented the compass, used to tell direction on the sea, is one of the four great inventions of ancient China.

Below is a picture of a strong magnet

A strong magnet

Magnets attract not only materials containing iron, but also materials containing cobalt and nickel, and we can use this property of the magnet to make a detector that, if it encounters materials containing iron, makes a beeping sound.

The detector is ready. How to use it?
The driving force of the vehicle detector actually comes from the nature of the magnet.
The magnets on the controller and the iron attract each other, connecting the wire to the iron sheet, so that the circuit is switched on and the buzzer goes off.

Note: when using the detector, it is necessary to make the magnet attract the iron metal to contact with the wire, so that the circuit can be connected. If only the magnet attracts the iron metal, without contact with the wire, the buzzer will not sound, because the ferrite magnet cannot conduct electricity.