Kern 1B invar substense bar.
A subtense bar is a bar of known length, with targets at either end. It is usually made of a stable material such as invar. When used with a tacheometer/theodolite, it serves as a quick and convenient method of measuring distances indirectly.
The following is abstracted from the accompanying leaflet:
The Kern Invar Subtense Bar is used for the measurement of distance by the by the method of parallactic angle measurement. The angle subtended by the two targets of the two-metre bar is measured with a one second or other precision theodolite. This angle is used as the argument to obtain the horizontal distance directly from a distance table. With a root mean square (RMS) angle of +/- 3cc/degree, the RMS error in distance is +/- 2.4 cm at 100 m. The distance error is proportional to the square of the distance.
The advantages of the Invar Subtense Bar lie in its high measuring precision for short distances, its adaptability to autocentering, and its elimination of the need for reducing distances to the horizontal. It is especially well suited to the measurement of distance in auxiliary triangulation and in traversing. With the aid of a simple expansion net, it can be used economically to measure distances up to about 2 km (1,25 miles).
It consists of two tubes of an anodised lightweight alloy which are connected by a bayonet joint. The exact distance between the two targets is maintained by an invar wire under tension. A compensating device makes the subtense bar virtually insensitive to temperature change. The Kern 1B set consists of the rod, a rod holder with a sighting device for orientation, a rod base and a distance table.
The complete set is packed in a heavy canvas bag case with carrying strap.