Prismatic compass — ideal conditions: Which statements describe a perfect (ideal) prismatic compass setup and needle behavior?

Difficulty: Easy

Correct Answer: All of the above

Explanation:


Introduction / Context:
A prismatic compass is widely used for bearing measurements in reconnaissance, preliminary traversing, and plane table work. Its accuracy depends on the mechanical and magnetic perfection of the needle–pivot system and the alignment of components with the graduated circle read through the prism.


Given Data / Assumptions:

  • Instrument is free from local attraction or it is corrected for it.
  • Needle and pivot are in good condition.
  • Graduated ring is correctly mounted.


Concept / Approach:
For ideal behavior, the magnetic axis should coincide with the geometric axis to avoid bias. The pivot must be centered on the graduated circle to prevent eccentricity errors. The needle should be sensitive, which requires minimal friction at the pivot and proper mass balance so the needle aligns promptly with the magnetic meridian. Correct vertical and horizontal relationships of the needle ends and pivot prevent tilt and rubbing, ensuring repeatable readings.


Step-by-Step Solution:

Inspect and clean the pivot and needle cup; verify free oscillation and quick dead beat.Check that the pivot lies over the graduated circle’s center (no eccentricity when turning).Ensure the needle’s magnetic and geometric axes coincide by balancing and demagnetization as needed.Confirm sensitivity by small deflection tests under weak fields.


Verification / Alternative check:
Rotate the instrument; if readings change without changing the line of sight, eccentricity or tilt is indicated. Stable repeated bearings confirm proper setup.


Why Other Options Are Wrong:
Each individual statement is correct; the best answer is the collective condition “All of the above.”


Common Pitfalls:
Ignoring pivot wear; not checking for local attraction; misreading due to parallax in the prism.


Final Answer:
All of the above

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