Take a wooden plank and make a hole as shown in figure 4(a). Place this plank on the table. Now place a retort stand on the plank as shown in figure 4(a). Pass 24 gauge copper wire through hole of the plank and rubber knob of the retort stand in such a way that the wire be arranged in a vertical position and not touch the stand. Connect the two ends of the wire to a battery via switch. Place 6 to 10 compass needles in a circular path around the hole so that its centre coincides with the hole. Use 3 (or 9) volt battery in the circuit. Switch on. Current flows through the wire.
It must be a circular line. So we conclude that magnetic field lines are closed lines. The magnetic field lines due to straight wire carrying the current is shown in figures 4(b) and 4(c). This can be verified by sprinkling iron filings around the wire when current flows in the wire.
If the current flow is vertically upwards (out of the page), the field lines are in anticlockwise direction, as shown in figure 4(b). If current flows into the page, in downward direction, the field lines are in clockwise direction as shown in figure 4(c). How do we determine the direction of field lines? It can be easily determined with right hand thumb rule. If you grab the current carrying wire with your right hand in such way that thumb is in the direction of current, then the curled fingers show the direction of the magnetic field as shown in figure 4(d).