what are the two methods by which a demagnetised iron bar can be magnetised?
Answers
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”.
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”.
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.
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.
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!.