Definition
A thin, pliable membrane sealed inside an instrument that flexes in response to pressure changes. In the magnetic compass, the flexible diaphragm allows the compass fluid to expand and contract with temperature changes without bursting the housing or forming air bubbles.
Plain English
A bendy, sealed disc inside the compass that bulges in or out as the liquid inside warms up or cools down, keeping the compass full and pressure-balanced.
Context Anchor
Seen in magnetic compass construction, especially when describing how a wet magnetic compass handles temperature changes in its fluid-filled case.
Derivation
From the Greek 'diaphragma,' meaning a partition or wall that divides. In instruments, it's a flexible wall that separates two spaces while still moving with pressure changes.
Why Pilots Care
Prevents fluid leaks or air bubbles that would distort compass readings during climbs, descents, or temperature changes.
Analogy
It is like a soft side on a sealed plastic container. When the liquid inside expands a little, the soft side can move instead of forcing the container to crack or leak.
Intuition Check
Do not think of a diaphragm here as the body muscle used for breathing. In this compass context, it means a thin, flexible sealing part that moves as the liquid changes volume.
Example Sentence 1
The flexible diaphragm in the magnetic compass expands as the fluid warms, preventing pressure damage to the case.
Example Sentence 2
A damaged flexible diaphragm allowed air into the compass fluid, causing erratic needle movement during the flight.