Substance bar is as shown in fig below Can be used as substance base to measure comparatively the short line in a traverse.
M/s key inverted the substance bar in which base is generally 2m or 3m. Note that the distance between the two targets is equal to the length of the base. The vane are attach to the invar rod so as to prevent temperature variation or variation by other mean so that length of the base will not vary but remain as it is. In a duralumin tube the invar rod with a spirit level it supported at a number of points.
For the purpose of accurate centering and levelling , the bar is centrally support on a levelling head which is also provide with a clamp and slow motion screw so as to rotate the substance bar about it vertical axis.
At the centre of the bar a small telescope or a pair of sight is provided for the purpose of alignment of the bar perpendicular to line OC joining the theodolite station and centre of the bar.