Oscillation is a combination of all of the other errors, and it results in the compass card swinging back and forth around the heading being flown. When setting the gyroscopic heading indicator to agree with the magnetic compass, use the average indication between the swings.

Vertical Card Magnetic Compasses
The floating-magnet type of compass not only has all the errors just described, but lends itself to confused reading. It is easy to begin a turn in the wrong direction because its card appears backward. East is on the west side. The vertical card magnetic compass eliminates some of the errors and confusion. The dial of this compass is graduated with letters representing the cardinal directions, numbers every 30°, and marks every 5°. The dial is rotated by a set of gears from the shaft-mounted magnet, and the nose of the symbolic airplane on the instrument glass represents the lubber line for reading the heading of the aircraft from the dial. Eddy currents induced into an aluminum-damping cup damp oscillation of the magnet. [Figure 3-21: A vertical card magnetic compass]
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Eddy currents: Current induced in a metal cup or disc when lines of flux cross it from a moving magnet.
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