When Einstein was a child, he was much fascinated by compass, and his ability for deep thinking and nuclear understanding distance, magnetic and gravitational forces, enabled him to discover the theory of relativity. As the magnetic forces are acting at some distance, so for the representation of magnetic forces, the magnetic field can be defined. The representation of magnetic field lines in a pictorial way is very useful for visualization of direction and strength of the magnetic field. The magnetic field lines are directed in the direction where the north end of the compass needle is pointing. Traditionally, the magnetic field is also known as the B field.
Description
If small compasses are used for mapping of the magnetic field, around the bar magnet, then they will be pointing in the directions as shown above, such as away from the north pole of the magnet and towards the south pole of this magnet. The use of small compasses for testing of the magnetic field will not cause any sort of disturbance. When there is the placement of small compass in this field then it will align itself in the parallel direction, to the field line at its location, while the north pole is pointing in the direction of B.
The magnetic field is an absolute entity that is used for describing the influence of magnetic forces in a specific region. The magnetic field lines are visual tools that are used for the representation of magnetic fields. They are used for describing the direction of magnetic forces on the north monopole, at any of the given position. As there is no existence of monopoles in nature, so the close connection between electric charges and magnetic monopoles is a useful analogy.
Conventions
A few conventions about filed lines are also adopted. According to the principle the magnetic field lines can be calculated at any place in the space, but their representation in the visual medium is hard. Therefore, density field lines can also be used for an indication of field strength. In the magnetic dipoles, the magnetic field lines, are entering through the south pole and emerging out through the north pole.
Properties of Magnetic Field Lines
The magnetic field lines can never cross. Naturally, there is a bunch of these lines together in the regions where there is the strongest magnetic field. They never start or stop anywhere and they always cause the formation of a closed-loop. In the real world, magnetic field lines can be easily visualized. It is most common when the iron fillings are dropped on a magnetic surface.