• Take a battery (12 V), a variable resistance (or a rheostat), an ammeter (0–5 A), a plug key, and a long straight thick copper wire.
• Insert the thick wire through the centre, normal to the plane of a rectangular cardboard. Take care that the cardboard is fixed and does not slide up or down.
• Connect the copper wire vertically between the points X and Y, as shown in Fig. (a), in series with the battery, a plug and key.
• Sprinkle some iron filings uniformly on the cardboard. (You may use a salt sprinkler for this purpose.)
• Keep the variable of the rheostat at a fixed position and note the current through the ammeter.
• Close the key so that a current flows through the wire. Ensure that the copper wire placed between the points X and Y remains vertically straight.
• Gently tap the cardboard a few times. Observe the pattern of the iron filings. You would find that the iron filings align themselves showing a pattern of concentric circles around the copper wire (Fig. (b)). Concentric circles represent the magnetic field lines.
• The direction of the magnetic field can be obtained by using a compass. If the current direction is reversed, the direction of the magnetic field is reversed. Fig. (c and d) shows the pattern of magnetic field lines surrounding wire. When the current flows out of the paper [Fig. (a)] the field lines point anticlockwise. When the current flows into the paper [Fig. (e)] the field lines point in a clockwise direction around the wire. The right-hand thumb rule helps to remember this.
Right Hand Thumb Rule
If a current carrying conductor is imagined to be held in your right hand such that the thumb points along the direction of current, then the direction of the wrapped fingers will give the direction of magnetic field lines. Accordingly if current in wire is vertically upward, the magnetic field lines are anticlockwise, while if current in wire is vertically downward, the magnetic field lines are clockwise.
