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Methods of Magnetisation and Demagnetisation

Check us out at Re-magnetising a magnet is often necessary if the magnet has been mistreated. Occasionally magnets are required to be made from pins etc. in order to make compasses. Also there are often requests to make a tool (e.g. screwdriver) magnetic so that it is complies with a desired function (e.g. difficult to retrieve screws are not lost). Sometimes a tool may have inadvertently become magnetic with unwanted consequences. There are a few methods of effecting the magnetisation of an object. However, it is important to make sure that the object is of the right stuff. Trying to make a permanent magnet from a rod not capable of retaining the magnetism will just waste time. So, what materials hold magnetism? Obviously any "old magnets" will be useful, certain steels and some iron based rods such as nails and the steel shafts of screwdrivers can also hold magnetism. Magnetically soft iron will magnetise but will loose the magnetism very quickly. This makes it ideal for electromagnets. Some stainless steels have very poor retention of field so should not be used. Tip:To test suitability as a permanent magnet:- bring the material you wish to magnetise in close proximity to a strong magnet. Then use the magnetised item to pick up drawing pins, paper-clips or iron filings. If, after removing the magnet for a few minuets, there is little adhering to the item, it will not be useful as a permanent magnet. Gentle tapping, for example with a pencil, should still leave some magnetised items still in contact. This method can create a magnetised bar without any apparent magnet being present (i.e. just using the earth's field). A stronger field may, of course , be used by placing the cooling bar between two magnets or in an electrically created field. Care should taken that the heated bar is thermally isolated from the field magnets, so as not to destroy their properties. The bar is heated to above a temperature (technically called the curie point) which varies from metal to metal, however most steels will be hotter at red hot. At this point the bar is no longer ferromagnetic but paramagnetic. As the bar cools it becomes ferromagnetic again and the domains are aligned with the external field. It may be of interest to try heating an old, weak magnet (all the paint will be burned off!) to red hot using a pair of tongs in a Bunsen flame and then placing it on a piece of heat mat with a rare-earth magnet underneath. Demagnetisation can be achieved by allowing the bar to cool in an East-West orientation shielded from magnetic influences.
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