Question

For minor adjustment of horizontal angles measured using a theodolite, the tangential screw is adjusted after
1. both the plates are unclamped
2. the lower plate is clamped and the upper plate is unclamped
3. the topper plate is clamped and the lower plate is unclamped
4. both the plates are clamped

Answer 1

Correct Answer - Option 4 : both the plates are clamped

The manipulation of the upper plate and lower plate clamp screws provide the following three conditions:

1. When both the upper plate clamp screw and the lower plate clamp screw are tightened, the tangential screw is adjusted thereafter and the instrument gets fully fixed.

2. When the upper plate clamp screw is tightened and the lower plate clamp screw is opened, the instrument rotates on its outer axis, there is no relative motion between the two plate and the readings in the horizontal vernier scales do not change.

3. When the lower plate clamp screw is tightened, and the upper plate is opened, the instrument rotates on the inner axis with outer axis fixed. The readings in the horizontal vernier scales change.