Could someone give a basic and concise explanation for it? Its surprising that textbooks talk about this example as how electricity and magnetism were first considered together but not explain why it was the case. Does the needle spin indefinitely? Or does it eventually point towards a specific direction? I'll have a stab at it. A current in a wire sets up a magnetic field around the wire. It is this magnetic field which interferes with the magnetic field set up by the compass thereby disrupting its field causing the compass to spin. This is because the north pole 'want' field lines to be coming out and the south pole 'wants' field lines inwards. The compass will have to move from its original position in order to achieve this configuration again. But once it has achieved this with the new field (assuming a constant current) the compass should stop moving?