The M2 Artillery Compass is a special-purpose instrument designed for high accuracy. It combines a surveyors compass with a clinometer, level and a plumb built into one instrument. It was in use during World War II, based on an earlier Compass, M1 that was not graduated in mils. The M2 has readings for 0-359 degrees and 0-6400 mils.
The unmounted M2 compass provides an alternate instrument for orienting cannon or mortar fire. The multipurpose instrument is used to obtain angle of site and azimuth readings. This photo from U.S. Army field manual FM 21-26 Map Reading illustrates the essential features and components of the Artillery Compass M2:
Artillery Compass M2, from U.S. Army field manual FM 21-26 Map Reading (March 1956).
Components of the M-2 Artillery Compass
The M-2 Artillery Compass is designed to be a superior and highly accurate tool for artillery operations. The components of the M2 compass are:
M2 Artillery Compass.
Azimuth Scale. The azimuth scale is numbered every 200 mils from 0 to 6400. The scale is graduated every 20 mils and can be read to an accuracy of 10 mils.
Sights. The compass has front and rear leaf sights and a mirror in the cover for sighting and reading angles.
Levels. The compass is equipped with a circular level for leveling the instrument before the azimuth values read. A tubular level is used with the elevation scale to measure angles of site.
Angle-of-Site Mechanism. Rotation of the level lever causes the elevation level and the elevation scale index to rotate as a unit. The index clamps against the bottom piece to keep the mechanism from moving unless it is actuated by the level lever.
Magnetic Needle and Lifting Mechanism. The magnetic needle (the white end of the needle) shows a magnetic north direction for orienting purposes. The needle is delicately balanced and jewel-mounted, on a pivot, to rotate freely. The magnetic needle reading is taken when the bubble is centered in the circular level. The lifting mechanism includes a needle-lifting (locking) pin and a needle lifting lever. The lower end of the pin engages the lever. The upper end projects slightly above the body of the compass to engage the cover when it is closed. Thus, it automatically lifts the needle from its pivot and holds it firmly against the glass window.
Azimuth Scale Adjuster Assembly. The azimuth scale adjuster assembly rotates the azimuth scale to introduce the declination constant. Two teeth at the adjuster engage teeth on the underside of the azimuth scale. Thus, turning the adjuster with a screwdriver rotates the azimuth scale approximately 1,800 mils. The scale is read against a fixed index under the rear sight hinge.
The M2 Compass is the military version of the Brunton Pocket Transit. It was issued originally with a good quality leather case but more recently comes with a plastic case, as discussed on the Olive-Drab.com page on Compass Cases.
During WW II its technical manual was TM 9-1596 Ordnance Maintenance, Compass M2 (14 August 1942).
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