30.19 : Magnetic Damping

Eddy currents can produce significant drag on motion, called magnetic damping. For instance, when a metallic pendulum bob swings between the poles of a strong magnet, significant drag acts on the bob as it enters and leaves the field, quickly damping the motion.

If, however, the bob is a slotted metal plate, the magnet produces a much smaller effect. When a slotted metal plate enters the field, an emf is induced by the change in flux; however, it is less effective because the slots limit the size of the current loops. Moreover, adjacent loops have currents in opposite directions, and their effects cancel. When an insulating material is used, the eddy current is extremely small, so magnetic damping on insulators is negligible. If eddy currents are to be avoided in conductors, they must be slotted or constructed of thin layers of conducting material separated by insulating sheets.

Magnetic damping is used in sensitive laboratory balances. To have maximum sensitivity and accuracy, the balance must be as friction-free as possible. If it is friction-free, it oscillates for a very long time. Magnetic damping is a simple and ideal solution. With magnetic damping, the drag is proportional to the speed and becomes zero at zero velocity. Thus, the oscillations are quickly damped, after which the damping force disappears, allowing the balance to be very sensitive. In most balances, magnetic damping is accomplished with a conducting disc that rotates in a fixed field.