Measures rate-of-pressure change
The air enters the aircraft through the static port
This air enters a diaphragm (aneroid) where it expands/retracts to indicate a climb/descent by measuring the ambient changes in pressure within the static system
As pressure drops, the aneroid compresses, indicating a climb
As the pressure increases, the aneroid expands, indicating a descent
As the aircraft levels off, pressure no longer changes and the pointer returns to its zero position