Explain how the plotting compasses show that bar AB is a magnet.

The plotting compasses show that bar AB is a magnet because their needles are deflected from the Earth's magnetic North and align themselves with the magnetic field lines produced by the bar. If bar AB were not a magnet, the compass needles would simply point towards the Earth's magnetic North, as shown in Fig. 4.1. The specific directions of the compass needles in Fig. 4.2 indicate the presence and direction of a magnetic field originating from bar AB.