Definition
The physical design of the aircraft magnetic compass, consisting of a sealed bowl filled with acid-free white kerosene, inside which a float assembly carrying two small magnets is suspended on a hardened steel pivot point. A graduated card marked with headings (in 5-degree increments, with letters N, E, S, W for the cardinal directions) is attached to the float. The card is viewed through a glass window against a fixed reference line called the lubber line. The liquid damps oscillations of the float and supports its weight to reduce friction on the pivot, while a flexible diaphragm or bellows allows the liquid to expand and contract with temperature changes.
Plain English
It describes how the magnetic compass in an aircraft is built: a small sealed bowl of clear fluid containing a floating disc with magnets attached. The disc has compass headings printed on it, and you read your heading by looking at where a fixed line on the case lines up with the numbers on the floating disc.
Context Anchor
Seen when studying magnetic compass operation, compass errors, and why the compass can lag, swing, or show temporary errors during turns and speed changes.
Derivation
"Compass" comes from the Latin com- ("together") and passus ("step" or "pace"), originally meaning something that measures or steps around — fitting for an instrument that points around the horizon. "Construction" simply means how it is built. Knowing it is a mechanical, fluid-filled, free-floating instrument helps explain why it behaves the way it does in turns and accelerations.
Why Pilots Care
How the compass is built directly causes the errors pilots must understand and correct for. The floating, liquid-suspended magnets lag and lead during turns and accelerations, and the small magnets are influenced by nearby metal and electrical equipment. Knowing the construction makes the compass's quirks predictable rather than mysterious.
Grounding Statement
Picture a small direction pointer floating in liquid, free to turn toward Earth’s magnetic field while the airplane moves around it.
Intuition Check
Do not read construction here as building or installing a compass in the airplane. In this context, it means the internal parts and design that make the magnetic compass work.
Example Sentence 1
During preflight, the instructor explained the magnetic compass construction so the student would understand why the compass swings during turns.
Example Sentence 2
Pilots study magnetic compass construction to understand why the instrument shows errors during acceleration and turns.