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Fig. 2 Iron filings near the bar magnet align themselves along the field lines.

This happens because the magnet exerts its influence in the region surrounding it.
Therefore the iron filings experience a force. The force thus exerted makes iron filings to
arrange in a pattern. The lines along which the iron filings align themselves represent
magnetic field lines.
(B) Demonstration using magnetic compass
With this demonstration you can draw the field lines of a bar magnet yourself

Procedure of this activity:-
1. Take a small compass and a bar magnet.
2. Place the magnet on a sheet of white paper fixed on a drawing board, using
some adhesive material.
3. Mark the boundary of the magnet.
4. Place the compass near the north pole of the magnet.
5. Here you will notice that the south pole of the needle points towards the north
pole of the magnet. The north pole of the compass is directed away from the
north pole of the magnet.
6. Mark the position of two ends of the needle.
7. Now move the needle to a new position such that its south pole occupies the
position previously occupied by its north pole.
8. In this way, proceed step by step till you reach the south pole of the magnet as
shown in Fig. 3.

Fig. 3 Drawing a magnetic field line with the help of a compass needle.

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