# Giant model of a magnet

##### Demonstration

A demonstration to link theory and experiment.

#### Apparatus and materials

Plotting compasses, as many as possible

Bar magnet

#### Health & Safety

Some magnetising and de-manetising coils are powered by mains a.c. Only use them if they have been inspected and tested for electrical safety.

#### Procedure

a Arrange as many compasses as possible in a regular crowd to fill a rectangle on the bench top, as a model of a bar magnet.

b To 'magnetize' the model, draw a large magnet once over the compass needles from end to end of the crowd.

c To 'demagnetize', wave the large magnet arbitrarily over the model, taking the magnet slowly farther and farther away while waving it.

#### Teaching notes

1 This model illustrates an early, simple theory of a magnet (but does not do justice in detail to the modern domain theory). It is so important that students should see it clearly at close range, not at a distance on a remote high lecture bench. Students should be encouraged to take turns in small groups to try it. Before students try the model, warn them not to bring the large 'magnetizing' magnet very near the compass needles.

2 If the compass needles are mounted in transparent cells, a crowd of them can be shown on an overhead projector. A projection model can be made cheaply by pushing points of sewing needles through a thin sheet of Perspex, as pivots for compass needles from cheap plotting compasses. Install a transparent lid over the array to avoid losses in storage.

This has the advantage of providing a large picture for a demonstration, and the disadvantage of being more 'special' than a crowd of ordinary compasses.

3 If compass needles acquire reverse magnetization, demagnetise them using an alternating current. Then remagnetize them by placing them in a coil in which a direct current is turned on momentarily. Not all will be magnetised the 'right' way at the first try. Remove the ones that are, and repeat with the rest.

This experiment was safety-checked in December 2004