Magnetism

A compass needle is just a tiny magnet, free to spin. Earth itself acts like a giant bar magnet buried in its core, with a field that reaches out through air, rock, and vacuum. That field exerts a torque on the needle, twisting it until it lines up with the field direction, which is roughly north-south. Nothing has to touch the needle because magnetic fields act at a distance, the same way gravity does.

Every magnet has field lines that loop out of its north pole, bow out through the surrounding space, and dive back in at the south pole, continuing on through the inside of the magnet to close the loop. These lines show the direction a free north pole would move at any point. Where lines bunch up tightly, the field is strong; where they spread out, it is weak.

A changing magnetic field pushing through a loop of wire induces a current in that wire, a two-way link between magnetism and electricity. Slide a magnet into a coil and the flux through the coil is changing, so a current flows and a meter registers it. Hold the magnet still inside the coil and the flux stops changing, so the current drops to zero even though the magnet is still there. This is the same link that connects to generators and motors elsewhere in physical science: motion and changing fields make electricity, and electric current makes its own magnetic field in turn.