Verious Part of a Transit Theodolite

The following are the essential parts of a theodolite:

1. Trivet: It is a circular plate having a central, threaded hole for fixing the theodolite on the tripod stand by a wing nut. It is also called the base plate. Three foot screws are secured to the present plate by means of a ball-and-socket arrangement.
2. Foot Screws: These are meant for levelling the instrument. The lower a part of the foot screws are secured within the trivet by means of a ball-and-socket arrangement, and therefore the upper threaded part passes through the threaded hole in the tribrach plate.

3. Tribrach: It is a triangular plate carrying three foot screws at its ends.

4. Levelling Head: The trivet, foot screws and the tribrach constitute a body which is known as the levelling head.
5. Spindles: The theodolite consists of two spindles or axes one inner and the other outer. The inner axis is solid and conical, and therefore the outer is hollow. The two spindles are coaxial.
6. Lower Plate: The lower plate is attached to the outer axis, and is also known as the scale plate. It is bevelled and the scale is graduated from 0 to 360 in a clockwise direction. Each degree is again subdivided into two, three or four divisions: thus, the worth of 1 small division could also be 30', 20 or 15' respectively. The lower plate is given a clamp screw and a tangent screw which control its movements. When the clamp screw is tightened. This plate is fixed with the outer axis. For fine adjustment of the lower plate, the tangent screw is rotated to the extent required. The size of the theodolite is designated consistent with the diameter of the lower plate.
7. Upper Plate: The upper plate contains the venier scales 4 and B. It is attached to the inner axis. Its notion is controlled by the upper clamp screw and the upper tangent screw. When the clamp screw is tightened, the vernier scales are fixed with the inner axis, and for fine adjustment of the scales the tangent screw is rotated.
8. Plate Bubble: Two plate bubbles are mounted at right angles to each other on the upper surface of the vernier plate. One bubble is kept parallel to the horizontal axis of the theodolite. Sometimes one plate bubble is provided on the vernier plate. The bubbles are meant for levelling the instrument at the time of measuring the horizontal angles.
9. Standard or A-frame: Two frames (shaped like the letter 4) are provided on the upper plate to support the telescope, the vertical circle and the vernier scales. These frames are known as standards or A-frames.
10. Telescope: The telescope is pivoted between the standards at right angles to the horizontal axis. It are often rotated about its horizontal axis during a vertical plane. The telescope Is given a focussing screw, clamping screw and tangent screw.
11. Vertical Circle: The vertical circle is rigidly fixed with the telescope and moves with it. It is divided into four quadrants. Each quadrant is graduated from 0 to 90° in opposite directions, with the 'zero' mark at the ends of the horizontal diameter of the great circle .
The line joining the "zero' marks, corresponds to the road of collimation. The subdivisions of the vertical circle are similar to these of the horizontal circle. The great circle are often clamped or finely adjusted with the assistance of the clamping screw and therefore the tangent screw provided along side the telescope.
12. Index Bar or T-frame: The index bar is provided on the standard in front of the vertical circle. It carries two verniers (C and D) at the 2 ends of the horizontal arm. The vertical leg of the index bar is given a clip screw at the lower end by means of which the altitude bubble are often delivered to the centre.
13. Altitude Bubble: A long sensitive bubble tube is provided on the top of index bar. The bubble it contains is understood because the altitude bubble. This bubble is brought to the centre by The clip screw at the time of measuring the vertical angle. A mirror is provided on the top of the bubble to help observe it when the instrument is set up above normal height.
14. Compass: Sometimes a circular box compass is mounted on the vernier scale between the standards. In modern theodolites, an adjustable trough compass or tubular compass are often fitted with a screw to the quality . The compass is provided for taking the magnetic bearing of a line.