A beam balance was designed for the long‐term measurement of permanent magnet stability. The balance measures an air‐gap magnetic field change with a precision of one part in 30 000. The balance is supported on crossed flexures and operates on the torque‐balance servo principle. A fixed torque is applied to the balance arm. Servo action causes the permanent magnet assembly to rotate in such a way as to create an opposition torque which is equal to that applied. The magnet speed required to establish torque balance depends on the square of the magnetic field. This speed is measured with great precision by a preset counter. A large number of magnet assemblies are measured.