Automatic/engineer’s level: at what location is the tangent to the liquid surface in the bubble tube parallel to the axis of the tube? Select the correct condition used for precise levelling.

Introduction / Context:
In precision levelling, the sensitivity and geometry of the spirit bubble are critical. The relationship between the bubble surface and the instrument axis is used to set line-of-sight horizontal and to quantify small tilts from the bubble’s motion.

Given Data / Assumptions:

• The bubble resides in a slightly curved (circular-arc) tube partially filled with liquid.
• The axis of the level is tangent to the bubble surface at a specific location.
• We consider the tube correctly filled and in normal operation.

Concept / Approach:

For a circular-arc tube, the liquid surface forms a meniscus where the bubble touches it. The tangent to this surface is parallel to the tube axis at the bubble’s center. That is why the bubble is centered at the mid-point when the line of sight is horizontal. Away from the midpoint, geometric symmetry is lost, and the tangent does not remain parallel to the axis in the same way.

Step-by-Step Solution:

Verification / Alternative check:

Instrument manuals instruct to bring the bubble to the center (mid-point) using foot screws to ensure the line of sight is horizontal before taking staff readings.

Why Other Options Are Wrong:

“Every point” or “either end” do not satisfy the geometric condition; “nowhere” contradicts practice; over-filling is not a correct operational state.

Common Pitfalls:

Reading the bubble off-center; misunderstanding that the bubble’s mid position—not merely being visible—defines horizontality.

Final Answer:

the mid-point of the bubble