# Encyclopædia Britannica, Ninth Edition/Telemeter

See also Telemeter on Wikipedia; Telemeter in the 11th edition; and the disclaimer.
TELEMETER, or Rangefinder. This is an instrument used in modern warfare to determine the distance or range to an enemy's position, in order that correct elevations may be given to guns or rifles directed against it. Telemeters have been made on three distinct principles, and classified as acoustic, optical, and trigonometrical respectively.
Acoustic telemeters record the time which elapses between seeing the flash or smoke and hearing the report of a gun, rifle, or shell, the range being given in yards as “the time in seconds x 364 6.” The Boulengé telemeter is the best known of this class. It consists of a graduated glass tube filled with liquid and containing a small metal traveller. At the flash the instrument is brought to a vertical position, and the traveller starts from zero; at the detonation it is turned to a horizontal position and the traveller stops. The objections to the acoustic telemeter are that the rate of transmission of sound in air is affected by wind and other local conditions and that the instrument cannot be used until firing has commenced.
Optical or perspective telemeters determine the distance to any point by observing the size of some object of known dimensions, as seen in a graduated telescope. Porro's telemeter, Elliott's telescope, and Nordenfelt's macrometer illustrate the principle. The chief defect of the system is that the objects most conveniently observed—men and horses—vary considerably in size, so that the assumption of a constant dimension may be productive of error.
Trigonometrical telemeters shorten the ordinary methods of surveying by adapting them to military purposes. They are of two kinds, field rangefinders and rangefinders for coast batteries.
(1)Field rangefinders exist in great variety, and differ from one another both in the trigonometrical methods pursued and in the mechanical peculiarities exhibited. The following are the common solutions of what is technically called ”the range-finding triangle,“ —i.e., a triangle in which O (fig. 1) is the object the distance to which is required, AOB an acute angle, and AB the base, O being visible both from A and B. (i.) Fig. 1. Where the base is a fixed length and the angles are variable. A fixed base is rarely adopted except when the base forms part of the instrument, the angles being observed by powerful telescopes. The range is usually read in yards by the assistance of verniers, extreme perfection of mechanism being necessary. Many ingenious instruments of the kind have been devised, but none have as yet proved satisfactory. With a fixed base the accuracy diminishes as the range increases, (ii.) Where the base and the angles are variable. —The base angles are generally observed by instruments of the theodolite type, and the base is actually measured or found by means of a sub-base. The range is obtained by table or calculating scale. The Nolan rangefinder, which was the first telemeter used by the British artillery, was of this kind, (iii.) Where one base angle is a right angle, the other angles and base being variable. —The instrument used is generally double-reflecting of the sextant type,—the base being found as in (ii.). The most perfect example is the Watkin rangefinder, used by the British horse and field artillery.
Fig. 2. Watkin field rangefinder.
It (fig. 2) consists of an horizon glass capable of assuming two positions, and an index glass set in a steel arm, which is worked by a movable collar on a graduated bar, and this again is moved by the turning of a graduated cylinder. O (fig. 3) being the object, the observer sets up a picket at A, and with the instrument at zero (the horizon glass being inclined 45 to the index glass) finds the right angle at the point C. A sub-base AB of 6 yards is then set off, and (with glasses set parallel and the sliding collar at 6) the observer reflects B upon A by turning the cylinder, which is thus made to record the base AC in yards. This reading being set on the graduated bar by moving the sliding collar, the observer proceeds to A, and from there 6 reflects C upon O, which causes the range to be given in yards on the cylinder. Fig. 3.
In this operation the position of the sliding collar regulates the movement of the steel bar so that the number of turns of the cylinder is always a true measure of the range OC, whatever the length of the base AC. (iv.) Where the angles are fixed and the base is a measure of the range.—The base points are determined by the use of prisms or of mirrors reflecting the particular angles adopted. The base is measured or found by a subsidiary triangle, and multiplied by a constant to give the range. The Weldon rangefinder, recently issued to the British infantry, is on this principle. It consists of three prisms, and is generally used as follows. O (fig. 4) being the object and D a convenient distant point, the observer makes with the first prism the right angle OAD. He then retires in the direction DA till the second prism records the angle OBD = ​88° 51′ 15″, when the range = 50 x AB. If it is inconvenient to measure AB, the observer can retire from B in the line OB until the third prism records the angle OCA = 74° 53′ 15″, when the range = 200 x BC. The prisms must be held in the plane of the objects and looked into at the same point. This rangefinder is very simple and portable, but is frequently inapplicable on hilly or broken ground, and does not possess great accuracy. Fig. 4.—Weldon rangefinder.
The merits of different field rangefinders depend mainly upon the balance of advantages they offer with respect to accuracy, suitability to variety of ground, simplicity, portability, and durability, these conditions being of a more or less conflicting character. The following are recognized principles: (1) the naked eye cannot with certainty appreciate less than one minute difference of angle, therefore telescopic power is necessary in proportion as the base is short compared with the range; (2) telescopes of high power cannot be steadied by hand alone; (3) the longer the base the more inconvenient are any restrictions as to its length or direction; (4) it is a disadvantage to be compelled to traverse the line joining base points; (5) the longest base which it is convenient to measure by hand is that length of measuring line which can be stretched tight in a high wind.
(2) Rangefinders for Coast Batteries.—Rangefinding is less adapted to the requirements of coast defence than “position finding,”—a method which furnishes every gun with its proper training and elevation so that it can be fired without sighting the target. Rangefinders are, however, sometimes employed. The most worthy of notice is the Watkin depression rangefinder used by the British artillery in coast batteries. The instrument resembles in principle the Watkin field rangefinder, the height above the sea-level being a vertical base. The range is found by observing the angle of depression to the object. This is done by a powerful cross-wire telescope, which forms part of the instrument. The fastest steamer can be continuously followed, and even the successive grazes of shot and shell can be observed. The instrument is levelled upon a tripod stand. When necessary, it finds its exact height in feet above the water-level in any state of tide by reference to a datum distance, and it records the range in yards automatically on a graduated cylinder. An interesting contrivance combining telemeter and gun-sight, applicable to guns in permanent emplacements over non-tidal waters, has been tried in Italy. By means of a cam the hind-sight of the gun is always maintained in the position necessary to give the proper elevation in firing, so that it only remains to make the sights cover the target. (A. W. W.*)