To describe the phenomena related to magnets, lines are used to depict the force existing in the area surrounding the magnet. These lines are called the magnetic lines of force. These lines do not exist actually, but are imaginary lines that are used to illustrate and describe the pattern of the magnetic field. As shown in the figure below, the magnetic lines of force are assumed to originate from the north pole of a magnet, then pass through the surrounding space and then arrive at the South Pole. Then these lines travel inside the magnet from the South Pole to the North Pole and hence complete the loop.
Lines of force are the lines in any such field the tangent of which at any point gives the field direction at that point and its density gives the magnitude of the field. Hence, magnetic lines of force are basically the lines of force which represent the direction of the magnetic field. The imaginary path traced by an isolated (imaginary) unit north pole may also be defined as a line of force. Magnetic lines of force are closed curves. Outside the magnet their direction is from north pole to south pole and inside the magnet these are from south to north pole.
They don’t have any origin or end and do not interact because if they do so then it would mean two value of magnetic field at a single point, which is not possible. At the poles of the magnet the magnetic field is stronger because the lines of force there are crowded together and away from the poles the magnetic field is week. i.e. magnetic field intensity depends on the number of lines of force. The number of magnetic lines of force passing through unit normal area is defined as magnetic induction whereas the number of lines of force passing through any area is known as magnetic flux. The lines of force can emerge out of the north pole of magnet at any angle and these can merge into the South Pole at any angle.
The direction of magnetic line of force is the direction of force on a North Pole, so the magnetic lines of force always begin on the North Pole of a magnet and end on the South Pole of the magnet. When a small magnetic compass is placed along a lie of force, it sets itself along the line tangential to it. Hence, the line drawn from the South Pole of the compass to its North pole shows the direction of the magnetic field.