In this way if we move the magnetic needle and follow the alignment of needle head at each position of the needle, we will find that the needle will ultimately comes to the south pole of the magnetic bar following a typical curvature path. This path of travelling a magnetic needle following the alignment of needle itself, from North to South Pole of a bar magnet is referred as magnetic lines of force. There are possibilities of number of such magnetic paths in a magnetic field as shown below.
We can map a magnetic field by using iron filings. Here, we keep a bar magnet below a horizontally placed card board. Now we will sprayed fine iron flings on the card. We will find that, the iron filings set themselves in the form of curve chains between north and south pole of the bar magnet. This curve chains of iron filings indicate the lines of force of the magnetic field of the bar magnet. The lines of force is the line on which if we place a magnetic north pole it will experience the magnetic force in the direction along the tangent drawn on curve (lines of force) on that point. We can also define the line of force as a curve, along which a free to move unit north pole travels from north pole to south pole of a magnet due to influence of magnetic field.