With only basic materials such as a permanent magnet, electrical wires and a battery, a teacher can demonstrate different ways to magnetize an iron nail. He can turn a nail into an electromagnet, or permanently magnetize it by rubbing it with another magnet. Simple experiments such as these can teach basic principles of magnetism in science class and generate discussions about natural phenomena such as the earth’s magnetic field.
TL;DR (Too Long; Didn't Read)
You can magnetize a nail by touching a magnet to it, by prolonged contact with a magnet, or by building an electromagnet from it.
Prolonged Contact With a Magnet
The fastest way to magnetize a nail involves creating prolonged contact with a permanent magnet of sufficient strength. You can purchase permanent magnets in hardware and hobby supply stores and even in junkyards. Often made of hard steel, permanent magnets retain their magnetism once they’ve been magnetized. If held in contact with one end of a nail, the nail will begin to exhibit magnetism and will be able to pick up small iron objects such as paper clips and iron filings. Though it will lose its magnetism once it breaks contact with the magnet, very long exposure, such as months of contact, will permanently magnetize the nail.
Rubbing With a Magnet
Rubbing the nail with the permanent magnet causes a stronger, more lasting magnetism in the nail. For this to work, only one pole of the magnet must stroke the nail from one end to the other in a single direction. The magnet must be lifted completely off the nail after each stroke before beginning the next one. The nail’s magnetism increases with each stroke. It usually takes around 20 to 30 strokes before the nail becomes sufficiently magnetized. Stroking with one pole of a permanent magnet works because it aligns the atoms in the nail to "line up" in the same polar direction, giving the nail a north and a south magnetic pole.
Batteries and Wire
The third method of magnetizing a nail uses electromagnetism and requires a length of insulated copper wire, some pliers and a battery or series of batteries taped together. Expose about an inch of copper wire from either end of the wire, and wrap the middle portion of the wire tightly about the nail. More wraps of wire will give you a stronger magnet. Attach each exposed end of the copper wire to opposite battery terminals to complete the electromagnet. Current flowing through the wire in a loop around the nail creates a magnetic field. The electromagnet becomes stronger with each additional coil twisting around the nail. Increasing the battery voltage has the same effect. Note that the current may make the magnet warm or hot after a few minutes; take care so the magnet doesn't become so hot it burns.
To make the class understand that the magnetism created was only temporary, the teacher can then demonstrate ways of demagnetizing the nail. To demagnetize a nail magnetized by prolonged contact or rubbing, banging the nail on a hard surface or dropping it to the floor shakes up the aligned atoms with sharp impact. For the electromagnet, simply detaching one end of the copper wire from its terminal kills the magnetic field.