Take a bar magnet and place it on the glass of an overhead projector. Switch on the projector and bring the magnet into focus on the screen. The idea is to use about 12 small plotting compasses to act as
magnetic field detectors to find out the magnetic field pattern.
Teacher: Now then, we know that there is a magnetic field around the bar magnet. But how does the magnet actually pull and push magnetic objects in that space or field? In what direction? To find out we can use these small plotting compasses, specially made to find out magnetic field patterns.
Teacher: If you remember, each of these compasses is simply a suspended magnet.
Teacher: If I place this first plotting compass close to the north pole of the bar magnet, notice what happens to the plotting compass needle: it lines up so that the south pole of the compass needle is attracted to the north pole of the magnet.
Teacher: If I place this second compass at the other end of the magnet, the north pole of the compass needle is attracted in towards the south pole of the magnet.
Teacher: How about if I place this compass down the side of the magnet. It points along the side and towards the south pole of the bar magnet.
By placing the plotting compasses in key positions around the bar magnet it is possible to start sketching out the shape of the magnetic field pattern, with magnetic lines of force following continuous loops from the north pole of the magnet round to the south. To make this as plain as possible, choose the new position of the compass by moving it in the direction indicated by the needle to form one continuous curved line. It is a good idea to place the bar magnet on an overhead transparency so that you can start sketching out (with a felt-tip pen) the magnetic field lines, which the pupils can then see on the screen. This is the first time that the pupils will have seen this distinctive magnetic field shape.
Teacher: So we can see the plotting compasses lining up and the lines of force looping round from the north pole of the magnet to the south. We call these lines that I'm drawing lines of force and these show the direction in which the force due to the bar magnet acts on the compass needle.