In a vertical card magnetic compass, which damping mechanism reduces oscillations?

• A. Gyroscopic damping
• B. Eddy current damping
• C. Hydrodynamic damping
• D. Mechanical damping

Answer: (B)

Eddy current damping is used because it provides smooth, reliable reduction of oscillations without contact or fluid flow. In a vertical card magnetic compass, a conductive metal element moves within a magnetic field as the card swings. This motion induces eddy currents in the metal. Those currents generate magnetic fields that oppose the card’s motion, turning the oscillatory energy into heat and quickly damping the swing. This non-contact method works well inside the compact instrument housing and remains consistent across operating conditions.

Other damping ideas rely on friction in bearings (mechanical damping) or air resistance inside the housing (hydrodynamic damping), or they depend on a gyroscope for stabilization. In this type of compass, eddy current damping provides the effective, quiet damping needed without those drawbacks.