Theodolite Adjustment: Plate Bubble

Proper adjustment of a theodolite ensures accurate measurements. The plate bubble is a key component used for leveling the instrument.

Plate Bubble Condition Explained

• The plate bubble is used to ensure the instrument's vertical axis is aligned perfectly vertical.
• When the theodolite is correctly leveled, the bubble in the plate bubble will be centered.
• The fundamental condition for this adjustment is that the tangent to the plate bubble must be perpendicular to the vertical axis.

Condition Verification

Evaluating the given statements:

• Option 1: "Horizontal line passes through the centre of the horizontal circle." While related to leveling, this is not the direct adjustment criterion for the plate bubble itself.
• Option 2: "Tangent to plate bubble must be perpendicular to horizontal axis." This is incorrect. The plate bubble's leveling function directly relates to the vertical axis.
• Option 3: "Tangent to plate bubble must be perpendicular to vertical axis." This statement accurately describes the condition for a properly adjusted plate bubble.
• Option 4: "Vertical line passes through the centre of the vertical circle." This pertains to the perpendicularity of the horizontal axis to the vertical axis, not the plate bubble adjustment.

Therefore, the correct statement defining the plate bubble's adjustment is its perpendicularity to the vertical axis.