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Telemeter Range-Finder

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TELEMETER or Position-Finder (Fr. telemetre; Ger. Distanzmesser; It. Telemetro; Russ. Dalnomier; Span. Telemetro; in the United States the word telemeter is sometimes applied to the stadia used in connexion with the tacheometer), an instrument, of which many varieties have been invented, for assisting the gunner and the infantry soldier in determining the distance or "range " 1 to their objective. Nearly all range-finders may be described as instruments which automatically solve a triangle. Usually it is a rightangled triangle, the length of the base of which is known, and one of the sides is the range it is desired to find. They are, in fact, goniometers, but the angle which they measure, whether it may be at the end of the measured base, or that subtended by it, is usually expressed as a function of the angle in terms of the measured base. Thus the range is recorded directly in metres or yards without calculation. is proposed here 1 The word " range," from O.Fr. range, from ranger, to place in a row or rank ( rang being a variant of rant, whence Eng. " rank "), meant properly a row or line of objects, as still in " mountainrange "; the secondary meanings of an area or space of ground, sphere of action, compass, extent, distance, are derived from the verb " to range," to stretch out in a line, to extend, to move about over a given area.

to describe principally the range-finding instruments in the British services (r) as used in the fleet; (2) by the army in the field; (3) in harbour defence; and (4) to refer briefly to range-finders, not under these heads, of English and foreign design.

r. The necessity for a range-finder afloat caused the British Admiralty in 1891 to issue an advertisement in the press inviting inventors to produce an instrument which would, amongst other conditions, record ranges with an accuracy of within 3% at 3000 yds. The resulting competition was declared in favour of a range-finder which is the joint invention of Professor Barr of the Glasgow University and Professor Stroud of the Yorkshire College.

The naval range-finder consists of a tube 2 which contains two telescopes. It is carried on a frame by bearings, in which the tube is free to revolve about its longer axis. To the frame is attached a weight capable of movement within a tank. St d This weight balances the range-finder and frame upon knife-edges. By means of the handle on the left of the instrument and an altitude worm beneath it, the motion of the tube is governed, and the line of sight is directed on the objective. By partially filling the tank with water, the swinging of the weight in a seaway can be checked. The frame is supported on a pedestal and can rotate in azimuth upon it (fig. I).

A rubber guar", is fitted round the eye-pieces. Its functions are to guide the eye. of the observer into the correct position, and to protect them from side light and the distressing effect of wind. It also guards the forehead against the jar occasioned by firing heavy guns. The upper portion of the field presented to the left eye is used as a finder, the lower portion is occupied by the scale upon which the ranges are engraved. The finder is a low-power telescope of large field, to the centre of which the objective is brought. When the telescope is thus correctly aligned, the objective will be seen with the right eye largely magnified, but as two partial images separated by a thin black horizontal line. When coincidence of the images is effected by means of the working head, the range can be 2 The length of tube varies from 3 ft. in the smaller to 9 ft. in the larger instruments FIG. I. - Barr and Stroud.

read off against a pointer from the scale seen with the left eye. For night use, means are provided for illuminating the scale. The range to lights may be ascertained by the use of the astigmatizer, an optical device by which a point of light is drawn out into a vertical streak. A beam of light from the objective falls on each reflector (fig. 2), r and passing through the object-glasses, each is received by an arrangement of prisms about the centre of the tube, and reflected through the right eye-piece. Two partial images are thus seen. The images could be united by the rotation of one of the reflectors, but owing to the small base used the necessary movement would be so extremely small that it would be practically impossible to measure it. The difficulty has been surmounted by utilizing fixed reflectors and effecting coincidence by means of a prism of small angle. The deflecting prism is situated in the line of the beam of light from the reflector at the right-hand end of the tube. Its multiplying action is of great delicacy. The angle available for subdivision, to measure ranges between infinity and 250 yds., is only one-third of a degree. In a travel of 6 in. the prism renders accurate measurements possible within the required limits. To bring images of distant objectives into coincidence, the prism must be moved towards the eye-piece, and for near objectives in the opposite direction. The range scale is attached to the prism. A consequent advantage is that the accuracy of the instrument is not affected by back lash arising from wear, or irregularity in the actuating mechanism. When once installed, the instrument is always ready for use. Should adjustment be required it is readily and easily applied. It is not within the sphere of this article to enter into the detail of the adjusting mechanism. For further particulars the reader is referred to the Proceedings of the Institution of Mechanical Engineers, 30th January 1896. The working of the range-finder is so simple that its use is quickly learnt by any man who can read, and with little instruction and practice he can " take a range " in 8 to 12 seconds. Besides its principal purpose, in connexion with gunnery, there are minor uses in navigation and nautical surveying to which the range-finder can be applied.

With the high speeds of modern war-vessels, guns and their objective approach each other so quickly that unless ranges can be communicated from the instrument to the guns with rapidity and accuracy the range-finder is deprived of much of its value. In connexion with the naval range-finder an apparatus is provided, which though not part of the range-finder is sufficiently important to claim passing notice. The apparatus consists of a transmitting and a receiving instrument of clockwork mechanism electrically controlled. In appearance they resemble the ordinary engine-room telegraph, on the dials of which ranges take the place of orders. The transmitter can communicate with a number of receiving instruments, disposed as required in different parts of the ship.

2. Before the introduction of the Marindin range-finder described below, the British army in the field used the " mekometer." The instruments used by the cavalry and infantry are smaller and lighter than those of the artillery pattern, but the principle involved is identical.

The mekometer is practically a box sextant. Two instruments are used simultaneously at the ends of a base of fixed length. One sextant, called the right-angle instrument, is fitted with index and horizon glasses permanently inclined at 45°. It consequently measures a right angle. In the other sextant, called the reading instrument, a graduated drum takes the place of the usual index arm and scale. The drum is graduated spirally with a scale of ranges. Both reading and right-angle instruments are fitted with a vane of gun metal with a white strip down the centre to facilitate observations. Telescopes of low power can be fitted to the instruments, and two cords of 50 (or 252) yds. are provided with which to measure the base.

Two observers attach the ends of the cord of fixed length (usually 50 yds.) to their instruments and separate until it is taut. The observer with the right-angle instrument moves into such Meko- a position that coincidence of image will be given between the objective and the vane of the instrument at the other end of the base, i.e. he makes ABC a right angle (fig. 3). When the right angle is established, the observer at C turns the graduated drum of the reading instrument until the image of the vane of the right angle instrument coincides with the direction of the objective. The range AC is then read on the drum. The ranges on the drum are measures of the angle BAC when the base BC is 50 yds.

The mekometer is open to the objection which is common to all range-finders requiring more than one observer. There is always a danger that observers may cause coincidence on different objectives or on different parts of the same objective, and thus inaccuracy in the recorded range must result. The instruments are expected to give an accuracy of less than 2 °/, at 2000 yds. For .Objectiue ranges over that dis tance, i.e. for usual Rightangle instrument B Instrument closed.

Instrument ready for use. FIG. 4. - Marindin Range-Finder.

for ranging on upright objects, or vertically, for ranging on horizontal targets.

For instance, in the diagram (fig. 5) of a road running uphill, the instrument could be held in any of the three positions indicated, a b FIG. 5.

and would give good ranges, but probably the best range would be obtained if held as at c. If it is required to use the instrument r [Iy' /?

ES L FIG. 2. - Barr and Stroud.

artillery ranges, it is desirable to use a .'double base (Ioo yds.

in length), in which case the range registered on the drum must be doubled. This operation, although slight, is a distinct disadvantage, since it adds to the time of taking a range and is a possible source of error. For field artillery, however, a range-finder is only an auxiliary adjunct. The true range can be found by a process of trial and error (see Artillery) in as short a time as the mekometer observers take to report it. It must further be remembered that as shrapnel is the principal projectile of field artillery, not only the correct elevation but also the true length of time fuse has to be found. This the range-finder cannot do. Hence it is that the range-finder for field artillery, although a valuable auxiliary, is not of the same importance as in purely defensive positions, such as batteries for harbour defence, and land forts.

The Marindin range-finder was from 1908 gradually introduced in the infantry to replace the mekometer. It was the invention of Captain A. H. Marindin, of the Black Watch (Royal Highlanders).

The principle of the instrument is that of coincidence, as in the Gautier Christie, Le Cyre, Souchier, and Barr and Stroud. But it differs from the last mentioned in that the right prism is made movable, and this movement (necessarily extremely small) is a function of the recorded range.

The steel tube, forming the base of the instrument, which carries the prisms,is supported inside an aluminium outer tube in such a way tha that no direct shock is communicated to it. The Ran appearance of the outside of the instrument, together with the names of the various parts, is shown in fig. 4.

The instrument can be used in two main positions, viz. horizontally, 'G ' --r FIG. 3.

at night, the two caps of the night-glasses should be opened. On looking through the instrument, any lamp or other light will appear like a fine, bright line, and the range can be taken in the ordinary way.

This range-finder possesses the superlative advantage of the one-man instrument, and it is claimed for it that it can range on horizontal objects, such as the crest of a hill, which has no detail suitable for use with a mekometer, and that it can be adjusted on service with no greater difficulty than the setting of a watch.

3. For harbour defence, owing to the long range of naval guns, and the fast targets which war-vessels present, an accurate range-finder is of first importance. This is largely the case because " ranging " cannot be resorted to in the same manner as in the field, where the targets are comparatively motionless and the effective ranges are less. Successful artillery practice therefore depends, in a great measure, upon the range-finder.

The instrument used in harbour forts is known as the depression range-finder. As its name suggests, it solves a triangle in the vertical plane, of which the base is the height of the instrument above sea-level. Its appearance resembles some forms of theodolite (fig. 6). A framework, capable of rotating in azimuth on a vertical FIG. 6. - Depression Range-Finder.

pivot, is supported on a plate carried by levelling screws, L, L, L. To the framework are pivoted two arms DC and FE, at C and E respectively. The arm EF is supported at F by a vertical screw H ending in a drum, upon which, in a spiral scale, the ranges are graduated. Motion in altitude is thus given to the telescope. The arm CD is supported by a slider G. This slider is set by a rack and pinion to the height above sea-level (represented on a scale of feet on EF) at which the instrument may be used. A telescope AB is suitably fitted in jaws at the top of the frame. There are spiritlevels at M and Q for adjusting purposes. The telescope is provided with cross wires which can be illuminated for night use. An azimuth circle X and pointer Y enable the direction of any vessel to be indicated, the range of which it is desired to know. The instrument rests on a base plate R, to which it is locked by the top-plate O. The observer directs the cross wires of the telescope upon the water-line of the objective, by means of the drum I and the azimuth handle P, the top of which just appears in the diagram. The reader watches the arrow on the drum and calls out the ranges as the figures arrive beneath it. The ranges are communicated to the officers at the guns by various devices, which differ according to local requirements.

Position-Finder

The range-finding instrument known in the British service as the Position-Finder (invented by Colonel Watkin, C.B., R.A.) is practically a large depression rangefinder. It posesses, however, certain additional appliances which render it capable of automatically recording, upon an oriented chart, the position or course of a vessel. And further, by electrical means it automatically records to a distant battery the range and bearing of the desired objective. The position-finder can therefore, from a concealed and safe position, automatically control the fire of a group of guns, whose detachments need not necessarily see the target engaged. As the observer follows the objective with the telescope of the instrument the range and bearing is simultaneously shown in the battery on convenient dials. The distance and direction thus communicated are the range and bearing from the guns, not as measured from the range-finder. The correction due to the displacement between gun and instrument is automatic. In localities where the height does not admit of using the depression system, an alternative arrangement is provided, known as the Horizontal Position-Finder. It is open to the objections common to two-man range-finders, and is only employed where necessity compels its use. Briefly, there are two observing stations at either end of a measured and electrically connected base. One is known as the transmitting and the other the receiving station; the latter contains the principal instrument, which usually is capable of independent use for medium and short ranges as a depression instrument.

It will be seen that the difference between the two systems is, that the first described solves the range triangle in the vertical, and the latter in the horizontal plane. There have been various methods proposed for using the position-finder. The best results are obtained by placing range and bearing dials on the gun-mounting in a position where they can be easily seen by the men elevating and training the gun. The gun is kept directed upon the objective and fired as quickly as it can be loaded. A position-finder can be used for firing mines in a mine field, and instruments are issued to the Royal Navy for this purpose.

In the United States of America the term " positionfinder " is applied to a range-finder which gives direction as well as distance. This is substantially correct, but custom, in the British service, confines the use of the expression as defined above.

4. Various appliances, not strictly range-finders, are sometimes used to assist in estimating distance. The following examples are not without interest: Acoustic telemeters, depending upon the velocity of sound, are obviously unsuited to the requirements of modern warfare. The names of Thouvenin, Redier and Le Boulenge are connected with such instruments - that of the last-named is perhaps the most convenient. It consists of a graduated glass tube filled with liquid, of suitable density, and containing a small metal traveller. At the flash of discharge of a gun or rifle 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 at the point on the scale indicating the range.

On this principle is the rough method of ascertaining the distance, in yards, of a thunderstorm, viz. multiply the number of seconds elapsing between the perception of the lightning and that of the thunder by the number of days in the year.

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 prin- Toe_ ciple. The chief defect of the system is that the objects most conveniently observed - men and horses - vary con siderably in size, so that the assumption of a constant dimension may be productive of error.

On the continent of Europe the perspective telemeter for military purposes has attracted more attention than in England. The French in their precise terminology call such an instrument " Stadia militaire," a term which at once distinguishes it from a " telemetre," and describes its nature. In rapid military sketching, in locating positions upon maps, &c., perspective telemeters find a use. The telescopes issued to field batteries and to coast forts in France are provided with a scale in the field of view. By comparing this scale with known heights, such as the average height of a man on foot, or the known height of funnels, masts, turrets, &c., of a war-vessel,. distance can be estimated with fair accuracy.

The " jumelle Souchier," which can be used as an ordinary fieldglass, is constructed on the stadia principle. By its means ranges can be _estimated within an accuracy of to %. A stand or rest, however, is necessary for good results.

General Percin of the French army has shown, in an interesting pamphlet, that a piece of wood or card cut to a known fraction of the distance between the eye and the end of the thumb, when the arm is fully extended, can be used to estimate distances. Thus it is. easy to find a penny in good condition of which the thickness is, T hth part of the arm-length in a man of average height. Provided' with such a coin an observer finds its rim to exactly cover a distant man 6 ft. (or 2 yds. high). The range therefore is 400 X 2 = g oo yds. Similarly, if the man's height appeared to be but half the thicknessof the coin the range would be 4 X 400 = 1600 yds. With a little practice the eye estimates the proportion between the object of known height and the stadia used. General Percin gives many useful applications of this simple device.

Various range-finders have been produced in countries outside the British Isles which, as they are the outcome of similar necessity and required for identical purposes, naturally resemble, more or less, the instruments already described.

Field artillery officers of all countries usually claim their gun to be their best range-finder. This may be another way of saying that a durable, one-man range-finder, capable of instantaneously finding modern artillery ranges with accuracy, has yet to be invented. In France the " telemetre Goutier " for field artillery, a two-man instrument, corresponds with the Watkin mekometer.

The " ` Gautier, used by the Italian field artillery, is a one-man instrument, but requires a measured base-line. The " Aubry " telemeter, used by some of the Russian batteries in Manchuria, is very portable, but requires a measured base-line, and a slide rule to find the range. In the French and Russian infantry the " prismetelemetre," the invention of Colonel Souchier, is used. It is small, very light, and can be carried in the same manner as field-glasses. French machine guns are ranged by the " telemetre instantane," an instrument of the Barr and Stroud type, with an aluminium base 1 metre in length.

For work in the field the modern tendency abroad is to follow Barr and Stroud. In Germany, Hahn, Goerz and Zeiss have produced handy and fairly light short base range-finders, in outward appearance more or less similar to Marindin's instrument.

The Zeiss range-finder, however, depends on the stereoscopic principle. It is open to the objection that best results can only be obtained with it by persons who are capable of-seeing stereoscopically, and also, in individuals possessing this particular gift (a comparatively small proportion of the human race), stereoscopic vision may vary in power from day to day. Nevertheless the Zeiss rangefinder has found favour in many countries, notably as the infantry range-finder in Italy. For naval and harbour defence purposes the Barr and Stroud range-finder is very largely used throughout the world. In Italy a Barr and Stroud instrument, with the large base of 5 metres, was in 1908 under trial for coast artillery.

Of the depression range-finder type in France, " le telemetre Deve " is used at all heights of about 70 ft. and upwards.

Brazil possesses, in the invention of Captain Mario Netto, an excellent range-finder. It is supplied to the harbour defences of that country. It is accurate, handy, easily transported and reerected where required, and is not affected by the concussion of heavy gun-fire. The German coast range-finder of Hahn closely resembles the earlier Watkin instruments. In Italy the Amici instrument is being replaced by the Braccialine. The latter inventor has also supplied his country with a horizontal base instrument.

After extended competitive trials in the U.S.A. the Lewis depression range-finder has been found superior to others presented to the Range-Finding Committee, and is recommended for adoption. It is a neat, workmanlike instrument, and gave an average mean error of 24 yds. in the ranges recorded during the trials. The maximum range was 12,000 yds. and the height of base 1352 ft.

The details of position-finders abroad, as in the British service, are confidential, and but little is published of the " telemetre par recoupement " of the French coast batteries, or the " telegoniometro Sollier " of Italy. In the United States, B. A. Fiske has ingeniously adapted the principle of the Wheatstone bridge in the construction of the position-finder which bears his name.

See de Marre, Instruments pour la mesure de distances (Paris, 1880); Abridgments of Specifications, Class 97, Patent Office, London; Handbooks and Instructions for Range-Finder, published by the British War Office; Barr and Stroud, Proc. Inst. Mech. Eng., 30th Jan. 1896; Zeiss pamphlet by Carl Zeiss of Jena, which gives a candid statement of the difficulty attending the stereoscopic principle, &c. (F. M. L.*)