Magnetisation
Take a piece of steel rod. It is not a magnet at first.
Coil a wire around it and connect the wire to a battery. The electric current will turn the rod into a magnet.
When the coil of wire is removed, the steel rod starts to lose its magnetism. But some magnetism remains. So the rod has become a magnet.
Turning the rod into a magnet is called magnetisation. The rod is said to be magnetised.
To completely remove the magnetism, one way is to use alternating current. This is an electric current that keeps changing direction. It is supplied using an a.c. generator.
Put the magnetised rod into the coil of wire again, and connect the coil to an a.c. generator. Then pull the rod out of the coil slowly.
The magnetism of the rod is removed. We say it is demagnetised.
The alternating current keeps changing the directions of the north and south poles of the rod. As we pull out the rod, the strength of this magnetism gets weaker and weaker. When the rod is completely removed, the magnetism becomes zero.