Magnetization and Demagnetization

The information in this article was inspired, in part, by "Ordinary Level Physics: A.F Abbott 3rd edition H.E.B." (ISBN 978-0435670054) pages 351 and 352 and the teaching of Mr Black (1970s Kent School, Hostert, BFPO 40).

Magnetising and demagnetising

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.

Magnetisation /demagnetisation by hammering

Hammering a rod will either allow it to be come slightly magnetic if laid along a magnetic field (i.e. North -South) or demagnetise it if laid across the field lines (East-West). Do not try to improve an existing magnet by hammering,: hammering could easily reduce the field strength below that already present.

Magnetisation by stroking

This is, historically, the oldest form of consistently creating magnet. This produces magnets that are not as strong as the electrical methods. There are two methods which have traditionally been given the names “single touch” and “divided touch”.

In the “single touch” method, a magnet is drawn over the rod so as to go completely over the length of the rod. The magnetic domains are then pulled into alignment with as the magnet passes. A useful method of realising the polarity of the induced magnetism is to consider the bar as a compass and which way it would point to the magnet as it finally leaves the bar. Best results are obtained after about twenty passes. The magnet taken in a big loop away from the bar in between passes.

Single and divided touch North will be at the LHS of the bar

In the “divided touch” method two magnets are used at the same time in what may be thought of as a mirroring action. This method produces a stronger magnet than the single touch method.
Beware of polarity: If this method was to be done using two similar poles facing the bar it is possible to create a bar magnet with two like poles at either end! These are termed “consequent poles”.

Magnetisation /demagnetisation by cooling

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.

Electrical methods

Modern methods of magnetisation and demagnetisation tend to use electrical methods as it is easily manufactured and controlled. A current passing through a coil will produce a magnetic field. The strength of the field is proportional to the current.

schematic showing conventions, many, many coils are required.

The polarity of the field is easily seen by examining the path of the conventional current in the coil. If looking at the end of the coil the current is going clockwise it will produce the “south seeking” pole. A capital “S” has the ends following the clockwise rotation. Similarly the other end will be anticlockwise. This produces the “north seeking” pole. A (albeit rounded) capital “N” has the ends following an anticlockwise rotation. These coils can be bought or made.(e.g. CLEAPSS laboratory manual section 12.22.4) or even by modifying some “coil gun” circuits available on the internet- please take care and be aware of potentially very, very high “back EMF” voltages (look for a protecting diode). A high current (capable of being handled by the coil) for a short duration is desirable. It may be necessary to cover the end to stop it acting as a coil gun!.

Demagnetising electrically

This involves taking the bar through decreasing “hysteresis” cycles. An alternating current is used to create a field that swamps the existing field in a magnet. This is gradually reduced. The “B-H” curve shows that each hysteresis cycle reduces the remaining magnetism. This eventually becomes negligible. Alternatively the rod can be drawn out and away from a constant amplitude alternating field.

• Using a “120:120” coil to demagnetise nails.

A 12V, 36W bulb is used (to limit current) in series with a 120:120 demountable transformer coil using a 0-12V (5A or so) a.c. Power supply. The whole of the coil (240 turns), should be used. Nails place in the coil will be felt to vibrate as the voltage is brought up to 12V. The nails can be moved so that all of their length passes through the centre of the coil at this voltage. The voltage is then slowly reduced to zero. The nails should now only have minimal magnetization.

Internal Links

Back to Strength of an Electromagnet
Back to Magnets
Magnetic Hardness
Realigning a compass
Physics Equipment and Apparatus

--D.B.Ferguson 16:07, 7 March 2010 (UTC)

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