Wild subtense bar BL16-21.
A subtense bar is a rigid fixed length rod, usually of a material insensitive to changes in temperature (eg invar). It is mounted on a tripod over the station to which the distance is desired. It is brought to level and a small telescope on the bar allows it to be orientated perpendicular to the line of sight to an angle measuring station. At the angle measuring station, a theodolite is used to measure the angle between indicators on the two ends of the subtense bar. The distance is determined by simple geometry to be the altitude of a triangle with the theodolite at the upper vertex and the subtense bar length at its base.
This item was used for teaching purposes by the donor, Falkirk College of Further and Higher Education.