Definition
An instrument that measures atmospheric pressure using one or more sealed metal capsules (aneroid wafers) that have had most of the air removed from them. As outside air pressure changes, the capsules expand or contract, and that mechanical movement is linked to a pointer or other indicator to show the current pressure.
Plain English
A pressure-measuring device that uses small sealed metal cans to sense changes in air pressure. The cans squeeze or swell as the surrounding pressure changes, and that tiny movement is shown on a dial.
Context Anchor
Seen in altimeter discussions, especially when explaining how an aircraft altimeter turns air pressure changes into an altitude indication.
Derivation
From the Greek 'a-' meaning 'without' and 'neros' meaning 'liquid' — literally 'without liquid.' The name distinguishes it from older mercury barometers, which used a column of liquid mercury. An aneroid barometer does the same job using sealed metal capsules instead of liquid.
Why Pilots Care
The sensitive altimeter relies on stacked aneroid capsules to detect small pressure changes and display accurate altitude, directly affecting safe altitude management in instrument flight.
Analogy
Think of a small sealed metal can with most of the air pumped out. When outside pressure rises, the can gets squeezed a little; when pressure drops, it puffs out a little. That tiny flexing is what drives the needle.
Grounding Statement
Imagine a thin sealed metal can that gently bulges outward when outside air pressure drops and flattens when pressure increases, moving the altitude needle.
Intuition Check
Do not think of a barometer here as just a weather-prediction tool. In an altimeter, an aneroid barometer is the pressure-sensing mechanism that helps show altitude.
Example Sentence 1
The altimeter uses an aneroid barometer to sense changes in atmospheric pressure as the aircraft climbs or descends.
Example Sentence 2
Inside the sensitive altimeter, the aneroid capsules respond to pressure drops by expanding and driving the hands to show higher altitude.