Magnetic Field Lines
Magnetic field can be represented by magnetic field lines. These lines can indicate the pole of the magnet, and so with the magnetic field strength.
At each point in the field, magnetic field lines are tangent to the magnetic field vector \vec B. Just like in the compass needle, the magnetic field lines point away the north pole of the magnet and towards the south pole of the magnet.
The area where the lines are close to each other indicates an area of stronger magnetic field. The area where the lines are far away from each other represent a weaker magnetic field. And since the direction of the magnetic field at each point is different, the field lines do not intersect each other.
To visualize the above description and characteristics of magnetic field lines and magnetic field, refer to the figures below.
The first figure shows the magnetic field around a single bar magnet represented by the magnetic field lines.
The second figure shows the magnetic field between two bar magnets of the same poles.
The third figure shows the magnetic field between two bar magnets of different poles.
The Earth is also considered as a magnet. Based on a compass magnet, it is more appropriate to say that the compass has “north-seeking” pole and “south-seeking” pole. North-seeking pole points towards the north geographic pole of the Earth while the south-seeking pole points to the south geographic pole of the Earth. The magnetic axis of the earth is said to be not quite parallel to its geographic axis making the compass needle to deviate somewhat from geographic north. The deviation is different in any location and is called as the magnetic declination or magnetic variation. In most points on the earth’s surface, the magnetic field is not horizontal, this angle up and down is called as magnetic inclination. The magnetic field is vertical in the magnetic poles.
Below is a figure showing the magnetic field of the Earth represented by the magnetic field lines.
Another way of representing magnetic field lines is through dots or crosses. If the magnetic field is going out of the page, the magnetic field lines are drawn as dots. These indicate the tips of the arrows coming outward. Magnetic field lines going into the page is represented by crosses. The crosses indicate the feathers of the arrows coming inward. See figure for visualization.