This is a so called Aperiodic compass.The RAF used high quality compasses of the aperiodic type. They settled onto a true course after a turn without any overcompensation.The reason they were so reliable was due to their sophisticated features; a strong magnetic moment, small inertia and heavy damping.

Compasses were prefixed either P for Pilot usage or O for the Observer.

The magnet system of this compass features two strong bar magnets close to the central pivot point and eight indicator arms (The spider arms) that move within the damping effect of the alcohol filled bowl. That's the reason why this magnet system moves slowly and does not overshoot.

This Azimuth Compass was designed to allow navigators to obtain an accurate bearing of a geographical feature. The compass would be mounted on a fixed pole and aligned with the aircraft. By rotating a bezel on the top of the compass body, the sighting aperture would be brought into line with the feature and its bearing could then be read off the compass card through the glass prism mounted above.

A small lamp housing next to the prism allowed use at night.