Theory
It is mainly used to measure horizontal as well as vertical angles with greater accuracy. The diameter of the lower plate defines the size of the theodolite.
Diagram
Construction
1. Levelling head
It consist of two parallel triangular plates called tri-branch plates each have three arms three releavecing screws or foot screws are attached to these arms. These foot screws are adjustable and keeps the plates at a fixed distance apart. One end of the foot screw having screws work in the upper tribranch and the other end bring bearing ball shoe rests in the cavity provided in the lower tribranch. Also circular hole in the centre is provided in the tribranch for suspending a plumb bob to the spindle. This part of bob to the spindle. This part of the instrument supports the main working parts of the instrument and fixing allangement for screwing on a tripode.
2. The two spindles
There are two spindes or axis. The outer spindle below and ground conical in the interior. The inner spindle is solid and also conical so as to fit into the outer spindle without play. To the inner spindle at the bottom there is a provision for locking and a hook is provided for suspending an plumb bob.
3. Lower plate
The lower plate is circular and have outer edge levelled. Standard levelled edge is graduated from 00 to 3600 to degree and half degrees or degrees and third of a degree in the clockwise direction. To make fig. distinct the outer levelled edges is sileverd the lower plate is attached to the spindle. The director of this plate defines the size of the theodolites. The lower plate is provided with a clamp and tangent or slow motion. Screw with the help of which it can be fixed accurately in any position.
4. Upper plate
The upper plate also called vernier plate caries two verniers marked A and B with magnifiers placed at two exteemeties diamaetrically opposite. These verniers help in taking reading upto 20 sec. in general on the main scale. The upper plate is attached to the inner solid spindle. The plate also carries two level tubes placed or right angled to each other. Or right angled to each other. Two vertical standards fro supporting the telescope verticle angle.
5. Telescope
The telescope being an integral part of a theodolite is rigidly fixed at the centre on a horizontal axis at right angles to main longitudinal axis of the telescope. The telescope may be a) external or b) internal focusing type.
6. Vertical circle
The vertical circle is silvered at the edges and is usually divided into four quadrants. Each quadrant is graduated to degrees half degrees in opposite direction from the two zero placed at the ends of the horizontal diameter of the vertical circle so that the line joining the zero is paralled to the line of collimation of the telescope when it is horizontal. The verticle circle is rigidly attached to the trunnion axis of the telescope and moves with telescope.
7. Index or T-frame
Station is T shaped frame and is centred on the horizontal axis of the telescope in front of the verticle circle and remains fixed. A long sensitive bubble tube is attached at the top of this index frame. The index frame consists of a vertical kg known as clippingarm and a horizontal bar known as vertical or index aim, verniers marked C and D with magnifiers are attached at the extremeties of index arm diamentially opposite to take readings upto 20” clip screw are provided at the lower end of the clipping arm to adjust the altitude bubble.
8. Compass
The compass attached to a theodolite may be of circular type or through type. The circular type is fitted in the centre of the vernier plate between the standards and the line joining north and south gradiations is paralled to the line of collimation.
9. Level Tubes
Two plate level tubes are placed at right angles to each other on the upper surface of the vernier plate. One of them is kept paralled to the horizontal axis. These plate level are centered by means of foot screws.
