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0255 0255     SUBTENSE BAR (TWO METRES)    Theodolites   Surveying Instrument Collection  Maker Wild, Heerbrugg, Switzerland Model Two-Metre Subtense Bar Serial Number 35194 Dimensions 2142 x 105 x 135 mm Image    Description Before the introduction of electronic distance measurement (EDM), the 2 m subtense bar was used for precise distance measurements on close range or for distance measurements in traversing, usually with forced centring.. The subtense bar features two large targets 2.0000 m apart, a target in the centre and a sighting telescope. The latter has no 'collimator' that allows to check from the instrument if the bar is properly aligned. The two targets carry a red disk at the bottom of the white triangular targets. Only these red disks can be illuminated through a power socket in the centre. The subtense bar is folded for transport and comes with a carrying bag. . This particular subtense bar features a very old tribrach. Forced centring is possible through the bottom plate only. The upper part of the tribrach is part of the subtense bar. The distance (D) is derived from the angle (alpha) between the two targets at 2 m distance (base b) from each other. D = (b/2) cot (alpha/2) = 1.0000 cot (alpha/2) When measuring alpha to 1 second of arc, the precision of the derived distance (sD) is about 1 mm at 20 m, 6 mm at 50 m and 25 mm at 100 m: sD = (1/400) D*D, where sD is in mm and the distance D in metres. Inside the tubes on both sides of the bar, an invar rod of 3 mm diameter is mounted. The rod is anchored at the hinge end of the bars and tensioned by springs at the target ends (with 10 kgf, 100 N). The effect of the on the spring tension of the temperature expansion of the steel tubes is compensated by the spring and target support being made out of brass. A ten degree change in temperature affects the (2m) spacing of the two targets by only 0.01 mm. History & comments According to www.wild-heerbrugg. com WILD produced 8772 subtense bards between 1921 and 1970. Conservation in canvas bag Condition Subtense bar, bag and tribrach in good condition Remarks Sticker on bar: "E. Esdaile & Sons P/L, 42 Hunter Street, Sydney". Tribrach engraved: S.G.O. 1186 (Surveyor General's Office). Felt pen marking on canvas bag: "D3" "s/n 35194" Dates Catalogued on 25.3.2013 by JMR. Photo by JMR. [ Back to Contents ]