Self-demagnetisation takes place when the opposite poles of a magnet are joined. What is the reason behind this?

A magnet is an object that has a magnetic field. The word magnet comes from the Greek "magnítis líthos" (μαγνήτης λίθος), which means "magnesian stone". Magnesia is an area in Greece (Now Manisa, Turkey ) where deposits of magnetite have been discovered since antiquity.

How to demagnetize materials

 

Permanent magnets can be demagnetized in the following ways:

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Heat. Heating a magnet past its Curie point will destroy the long range ordering.

Contact. Stroking one magnet with another in random fashion will demagnetize the magnet being stroked, in some cases; some materials have a very high coercive field and cannot be demagnetized with other permanent magnets.

Hammering and/or Jarring. Such activity will destroy the long range ordering within the magnet.

Being placed in a solenoid which has an alternating current being passed through it. The alternating current will disrupt the long range ordering, in much the same way that direct current can cause ordering.

In an electromagnet, ceasing the flow of current will eliminate the magnetic field. However, a slight field may remain in the core material as a result of hysteresis.You can demagnetize a magnet in several different ways. One would be to hit both ends of the magnet using a hammer. This causes the ordering in the magnet to be destroyed. Another is to rub one magnet with another one. This should demagnetize one of the magnets. You can find more information here: http://www.ehow.com/how_5071154_demagnet...