Compassa

Meet Compassa, a compass needle who always turns to line up with the field.

A compass needle is a small magnet with a magnetic moment pointing from its south pole to its north pole. In a magnetic field B, it feels a torque τ = m × B that twists it until the moment lines up with the field. The torque is largest when the needle is at right angles to the field and zero once it is aligned. William Gilbert argued in 1600 that Earth itself behaves like a giant magnet.

Earth's surface field is roughly 25 to 65 µT. For a needle with magnetic moment 0.01 A·m², at right angles to a 50 µT field, the torque is τ = mB = 0.01 × 50 × 10⁻⁶ = 5 × 10⁻⁷ N·m, small but enough to turn a light, balanced needle.

Compasses guided sailors for centuries and are still carried as a backup to GPS. Phones use tiny electronic magnetometers in the same role.

A compass points to magnetic north, not geographic north, and the difference, called declination, varies from place to place and drifts over the years. Nearby iron or electric currents also pull it off line.