Aneroid

The sensitive component in an altimeter or barometer that measures the absolute pressure of the air. An aneroid is a sealed, flat capsule made of thin disks of corrugated metal soldered together and evacuated by pumping all of the air out of it. Evacuating the aneroid allows it to expand or collapse as the air pressure on the outside changes. The pressure of the air can be found by measuring the amount the aneroid expands or collapses.