Derivation device

The derivation device was employed by the Bulgaria fortress-siege artillery to determine the lateral deviation of the projectiles. It consisted of an oblong board and an alidade.

At one end of the board was an axis (o) around which the alidade rotated, and at the other end was an iron arc (a) described with a radius equal to the distance between the axis and the arc. Radial divisions were engraved on the arc, their value was 4’. The degrees ranged on both sides from 0 to 5°; to measure deviations greater than 5°, extensions of the axis (б в) were used which were attached to it on both sides.

The alidade was a wooden line, one end of which rotated around the axis of the table, and the other ran on the arc; a thin iron plate was attached to the ends, the dioptres of the eyepiece (О) and the objective (П), which served to direct the alidade towards the target. The end of the alidade, which ran on the arc, had an iron chain, on which was engraved a line, which served as a pointer. The pointer and both dioptres lay in line with the axis around which the alidade rotates.

Use of the derivation device

For use, the device was placed on a four-legged chair or on the parapet and directed towards the point from which the deviation of the projectiles would be evaluated, so that the pointer passed through O, which is located at the centre of the arc. At the moment of the shot, the observer, while looking through the dioptres, moved the trailing end of the alidade to direct the line of sight to the point where the projectile had exploded. The reading of the alidade thus directed gave the angle of deviation of the projectile in minutes, from which the deviation could also be determined in meters by proceeding as with the aiming triangle.

SOURCE :

Ръководство за занятията въкреспостата артилерия. Част IV. Подготовка и служба на наблюдателите, Sofia 1902, pp. 39-41.