The imaginary lines which represent the direction of the magnetic field are known as magnetic lines of force. Outside the magnet, their direction is from the North Pole to the South Pole and inside the magnet, these are from south to the North Pole. Their density decreases when they move from an area of higher permeability to an area of lower permeability.
Properties of magnetic lines of force:
(i) Magnetic lines of forces are closed continuous curves, extending through the body of the magnet. They flow from the south pole to the north pole within a material and north pole to south pole in the air.
(ii) The direction of the line of force is from the North Pole to the south pole outside the magnet while it is from south pole to north pole inside the magnet. In a single bar magnet, as shown to the right, they attempt to form closed loops from pole to pole.
(iii) The tangent to the magnetic line of force at any point gives the direction of the magnetic field at that point, (i.e) it gives the direction of magnetic induction (B) at that point.
(iv) They never intersect with each other. These lines do not intersect, because if they do so then it would mean two values of the magnetic field at a single point, which is not possible.
(v) The crowd where the magnetic field is strong and thin out where the field is weak. 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 weak.
(vi) They all have the same strength. The strength of the magnetic lines is the same throughout and is proportional to how close are the lines. Their density decreases (they spread out) when they move from an area of higher permeability to an area of lower permeability. Their density decreases with increasing distance from the poles.
(vii) Magnetic lines of forces are closed continuous curves, that emerge out from the north pole and enter at the south pole. Inside the magnet, these lines are directed from south to north pole.
(viii) The direction of the magnetic field at any point is along the tangent to the magnetic line of force passing through that point. The magnetic lines of force between the two unlike poles contract longitudinally. The expansion of lines of magnetic field accrues in lateral position and contraction in longitudinal position.