The magnetic field
Around every magnet there is a space in which it can have an effect. This space is called the magnetic field.
A magnetic field cannot be sensed by the eye, but it can affect another magnet or a moving charge. Some animals, such as certain birds, have been found to use the Earth's magnetic field to find their way.
One way to find whether there is a magnetic field in a space is with a compass. A compass is a small, thin magnet mounted on a pivot so that it can turn freely. The compass needle is that small magnet; with no other magnet nearby, it lines up along the Earth's north–south direction.
Activity 13.1
You will need: a compass, a piece of glass, a piece of iron, a magnet, a piece of plastic, a piece of brass
Place the compass on the table. Bring each object close to it in turn and watch whether the needle deflects.
The observation
The needle deflects only when the magnet is brought near. So the magnet has set up a magnetic field around it.
For extra knowledge
People have known about natural magnets for thousands of years, but the magnetic compass was made by the Chinese in the eleventh century AD.
When a compass is placed at a point in a magnetic field, its needle shows the direction of the field at that point. The direction can change from point to point, and so can the strength. So a magnetic field is a physical quantity with both magnitude and direction.
Figure to be added
A field has size and direction
A magnetic field is the space around a magnet where it has an effect. A compass shows its direction at each point; outside a magnet the field runs from N to S.