Definition
A sealed, flexible metal capsule (or stack of capsules) with most of the air evacuated from inside, used as the pressure-sensing element in instruments such as the altimeter, vertical speed indicator, and some airspeed indicators. As the surrounding atmospheric pressure changes, the capsule expands or contracts, and that movement is transferred through mechanical linkages to drive the instrument's pointer.
Plain English
A small sealed metal bellows with the air sucked out. When outside air pressure changes, the bellows flexes — and that tiny movement is what makes pressure instruments work, without needing any liquid inside.
Context Anchor
Seen in discussions of altimeters and other aircraft instruments that use outside air pressure to give a cockpit reading.
Derivation
From Greek 'a-' (without) and 'neros' (water) — meaning 'without liquid.' Earlier barometers used a column of mercury or water to measure pressure. The aneroid cell measures pressure without any liquid at all, using a flexing metal capsule instead. Knowing this helps explain why pilots rely on it: it's compact, rugged, and works in any aircraft attitude.
Why Pilots Care
It forms the core sensing element inside an altimeter, converting pressure changes into altitude readings.
Analogy
Think of a small sealed accordion with the air pumped out. Squeeze the outside air pressure and it shrinks; reduce the pressure and it expands. That gentle in-and-out motion is what the instrument reads.
Grounding Statement
As an aircraft climbs and outside air pressure drops, an aneroid cell expands slightly; as the aircraft descends and pressure rises, it contracts.
Intuition Check
Do not read “cell” here as a battery or a living cell. In this context, it means a small sealed pressure-sensing chamber.
Example Sentence 1
When the altimeter pointer froze during the climb, the mechanic suspected a stuck aneroid cell.
Example Sentence 2
During preflight the pilot checks that the aneroid cell in the altimeter responds correctly to known field elevation.