Plain English Meaning
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.
Full 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.
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.