Where on the Earth's surface would a magnetic compass experience the strongest directional force from the Earth's magnetic field?

• A. At the magnetic equator
• B. Near the magnetic poles
• C. Near the prime meridian
• D. Over the International Date Line

Answer: (A)

The turning force on a compass needle comes from the torque produced by the Earth's magnetic field, and the needle will respond most strongly when there is a strong horizontal magnetic component to push it to align with the field lines. Since the needle lies in the horizontal plane and rotates about a vertical axis, only the horizontal part of the field matters for producing that torque.

At the magnetic equator, the field is entirely horizontal and is as strong as the horizontal component gets, so the compass experiences the largest turning force to align with the field. Near the magnetic poles, the field is vertical, so the horizontal component is near zero and the torque on the needle is minimal. The prime meridian and the International Date Line aren’t special in this magnetic sense, so they don’t correspond to a stronger directional force.

So the strongest directional force on a magnetic compass occurs at the magnetic equator.