Parts of a Military Compass
- Instead of a compass needle, military lensatic compasses use a balanced marked dial with a magnetic strip acting as the needle. The chamber containing the dial allows the disc to dip horizontally as well as rotate on the central pin. Free movement guarantees accuracy worldwide without adjustment. Electromagnetic induction dampens the dial movement, eliminating the need for a liquid filled chamber.
- Obvious markings include the 360 degrees of the dial and luminous markings make the compass readable even in the dark. Index lines on the bezel and transparent face aid readings taken through the lens of the compass. Luminous sighting dots above and below the sighting slot assist in taking night time readings, and a sighting wire in the slot lines up with the dial's index line to increase daytime accuracy.
- Unfolded, the halves of the compass case form a straight edged ruler used for plotting lines on maps and measuring scaled distances. A magnifying lens in the folding eyepiece increases the accuracy of compass readings. The compass can be used without the lens but with less precision. When folded, the case presses against a spring wire that holds down the compass dial and prevents wear and tear to the mechanism. The steel wire thumb loop provides a steady grip when taking readings.
- Real accuracy depends on declination adjustments to the heading the compass reads. Magnetic north wanders in a predictable pattern. Readings will be off by several degrees from true north without this correction. Taking accurate physical sightings is equally important to plotting an accurate course. The lensatic compass design emphasizes correct procedures. Held properly at eye level both target and dial can be seen at almost the same time, resulting in a compass heading accurate to within one degree of true.
- Air filled lensatic compasses have not proven reliable in all weather conditions. Accuracy suffers in extreme cold and the mechanism itself can wear out from constant use. Some parts are breakable. If damaged, water could enter the dial chamber and alter readings. Corrosion and the weight of water droplets could unbalance the dial. Some models used dangerous radioactive elements in the luminous markings and were eventually withdrawn because of safety concerns.