Definition 1 of 2
Definition
A flexible, accordion-style sealed chamber that expands and contracts in response to pressure or temperature changes. In a magnetic compass, a small bellows is used as a temperature-compensating element that adjusts the volume of the compass fluid as it expands or contracts with temperature, preventing bubbles from forming and keeping the float assembly properly supported.
Plain English
A small, flexible bag with folded sides — like a tiny accordion — that can squeeze in or stretch out as the liquid around it warms up or cools down. Inside the compass, it quietly takes up the slack so the fluid stays full and the compass keeps working smoothly across temperature changes.
Context Anchor
Seen in discussions of magnetic compass construction, especially how a wet compass handles temperature changes without leaking.
Derivation
From Old English 'belg', meaning bag or pouch — the same root as 'belly'. The traditional blacksmith's bellows is a folded leather bag that pumps air. In instruments, the same accordion shape lets a sealed chamber change size without leaking.
Why Pilots Care
Without a working bellows, fluid expansion from heat can crack the compass case or cause leaks, leading to inaccurate headings or complete instrument failure.
Analogy
Think of the folded paper insert in a juice box that lets it squeeze flat as you drink. The bellows works the same way — it changes size to keep the contents sealed and full as conditions change.
Grounding Statement
When the fluid inside warms and tries to take up more space, the bellows simply grows larger to absorb the change without raising pressure.
Intuition Check
Do not think of bellows here as a tool for blowing air. In a magnetic compass, bellows means a flexible sealed part that moves to handle liquid expansion and contraction.
Example Sentence 1
During the preflight check, the pilot looked for a bubble in the magnetic compass, knowing the internal bellows is meant to prevent one from forming.
Example Sentence 2
Before flight the instructor pointed out the bellows inside the magnetic compass as the part that protects against temperature-related pressure changes.