The graduated ruler (мерна линейка, reglette de direction) is a pewter plate on which divisions are drawn, the interval between which corresponds to 5 millièmes, when viewed from a distance of 5 cm.
The ruler is used to roughly measure the angular distances between two points. At its two ends, there is a hole for tying the ends of a string. The length of the string is such that when the observer puts it around his neck, the wide side of the ruler is 50 cm from his eye.
Both sides of the ruler have divisions, which allow moving to the goniometer divisions without calculations. The distance between the divisions is 2.5 mm, which corresponds to 5 millièmes. Their numbering is the same as the numbering of the divisions of the goniometer, the lateral adjustments mechanism and the battery telescope. The initial division is 1000 and the values of the numbers decrease when the auxiliary point is to the left of the target and increase when it is to the right of it. The word target (цел) is written next to number 1000, because that number must always be placed in the direction of the target. In order not to direct the observer inadvertently towards the target, the edges of the ruler on either side of the number 1000 are bevelled.
Since progressive fire is used against targets whose width is greater than 100 m, to determine it at various distances without calculation, the ruler has parallel lines with dashes bearing the number 1, 1.5, 2, 2.5 and 3 km.
In the middle of the ruler there is a slot with two round windows; the width of the slot corresponds to height of the normal shrapnel burst, which is equal to 3/1000. By means of this slot, it is possible to compare the actual burst height with the normal one. The ruler has a holster on one end for carrying it.
The measuring scale ruler (мерно-мащабна линейка) can be used as a simple ruler for calculating distances on the meters and versts maps then used by the Bulgarian Army and contains the following measures:
- first side up : a centimetre scale;
- first side down : a scale made for the three versts Russian map (1: 126000), with a unit of 0.8 cm;
- second side up : a scale made for the one verst map (1: 42000), with a unit of 0.95 cm;
- second side down : the scales of the stakes for measuring the slopes along the horizontal plane.
It can also be used as a graduated ruler, tying the ends with a string which, when placed around the neck, will be 50cm away from the eye. In this case, 1 mm measures 2 millièmes of the distance.
Use of the graduated ruler
To define the deviation of a gun from some auxiliary aiming point, the ruler is located in the place where the gun will be, or if the gun is already in place – instead of the sight.
a) When the aiming point is to the right of the target, place the ruler with the flat side facing the eyes so that the string passed through the neck is taut. Hold it in the right hand horizontally with the ends equidistant from the eyes and with the word “target” at its left end. In this position, the remaining divisions drawn on the upper edge increase from left to right. Bring the division 1000 to the vertical line passing through the target, close the left eye and, looking at the target and the auxiliary point simultaneously, move the thumb until the nail rests on the vertical line passing through the auxiliary point. The division is read on the ruler, which at the same time determines the direction of the gun.
b) When the aiming point is to the left of the target, turn the ruler, which is held in the left hand, to present towards you the face which bears the word “target” with the remaining divisions decrease from target to the aiming point. Bringing the division 1000 to the vertical line passing through the target, close the left eye and then, looking at the target and the aiming point simultaneously, and moving the thumb, bring it to the vertical line passing through the aiming point. The division is read on the ruler, which at the same time determines the direction of the gun.
To determine the width of the front of a target, or to determine the angular distance between two points, place the ruler with the division 1000 against the left point, so that the divisions increase from left to right, then close the left eye and moving the nail of the right thumb, count the division against which it is as it comes to the vertical line which passes through the right point. In this calculation 1000 is ignored, for example reading 1080, the angular distance between the two points will be only 80 millièmes.
The image above shows the réglette de direction in use in the French army, which is the same as the Bulgarian one, with the only difference that the lettering is in French and not in Bulgarian (but instead of цел etc.).