Fig. 9.1 shows a thick, vertical copper rod AB, of negligible resistance, connected into
an electrical circuit. AB passes through a hole in a horizontal card PQRS.
The d.c. power supply produces a current in the circuit, which causes a magnetic field around AB.
Fig. 9.2 shows the view from above of the card PQRS, with the hole through which AB passes.
On Fig. 9.2, draw the pattern of the magnetic field around AB. Include at least four
magnetic field lines.