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Compass, Aperiodic, US Army Air Force Type D-12


Compass, Aperiodic, US Army Air Force Type D-12


The US Army Air Force Type D-12 Direct Reading Aperiodic Compass was used at the navigators station across many US WWII aircraft, including heavy bombers B-17, B-24, B-29, medium bombers B-25 and B-26, and transports C-46, C-47, and C-54.  Made by Bendix, part number 1833-1-A, it has a serial number prefix of AF-42, indicative of a manufacturing year 1942. Later versions had a bakelite cover surrounding the 'legs'. 

The navigators' direct reading compass is known as an aperiodic (without a period) compass, since the needle returns to rest quickly and without appreciable overswing when deflected from its position. Radial arms or vanes attached to the card dampen the swing of the needle. This type of compass is mounted horizontally, and is read through a glass cover on the top. The compass consists primarily of a cylindrical metal bowl filled with compass fluid. In the center of the bowl is a semi-float type card to which are attached the magnetized needles and the radial arms. The card is marked with the four cardinal points, with an arrow indicating North. Inside the bowl, a fixed mark acts as the lubber's line. A graduated rotatable ring (verge), with two clamping screws, is fastened to the upper edge of the compass bowl. Across the inside of the ring are two horizontal, parallel white wires, spaced equally on either side of the N and S markings. In making readings, the ring is turned until the two white wires are parallel to the North-South lines on the card. The airplane heading is read as the marking on the ring which is directly above the lubber's line. A box containing magnets which may be turned to compensate for deviation is located on the bottom of the compass.

This compass still contains clear fluid, the glass is intact, and the markings are sharp. It is is a fine example given its age.

Measures ~7 inches diameter and ~4 inches in height.

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