The Earth's magnetic field - an activity

What the Activity is for

To explore a scale model of the Earth's magnetic field. Pupils can see how a compass reacts to this field.

What to Prepare

• a small bar magnet, embedded in the middle of a foam Earth, about 15 centimetre in diameter (the magnet is placed down a hole made radially through Hudson's Bay in Canada. The south pole should be pointing towards Hudson's Bay)
• a small gimballed bar magnet, free to spin in all 3 planes (a probe)
• a bar magnet, for comparison with the model Earth

What Happens During this Activity

Pupils explore the magnetic field, which is just like that of the Earth.

Particular points to look out for:

• The probe mostly points towards or away from the Earth, not aligned with the surface.
• The field is very like that of a bar magnet.
• The magnetic north pole and geographical North Pole are not in the same place.
• The magnetic south pole and geographical South Pole are not in the same place.
• There is a band around the Earth where the compass lies parallel to the surface: the magnetic equator.
• The effect of the field on the probe gets much weaker quite quickly as the probe moves further from the model Earth.

You might like to make the point that all this pointing up and down leads to hill-walking compasses having to be made for the Northern or Southern Hemispheres. If you take one from the UK to New Zealand, it does not work, bashing one end of the needle against the lid of the case.