Which flight instrument is specifically designed to measure altitude changes?

The Vertical Speed Indicator (VSI) is specifically designed to measure altitude changes. It provides pilots with critical information about whether the aircraft is climbing, descending, or maintaining level flight. The VSI displays the rate of altitude change in feet per minute, allowing the pilot to assess and control the aircraft's vertical movement effectively.

In contrast, the other instruments serve different purposes. The Attitude Indicator shows the aircraft's orientation relative to the horizon, indicating whether it is climbing, banking, or in level flight, but it doesn't measure vertical speed. The Tachometer measures the engine's RPM (revolutions per minute), which is important for engine performance but irrelevant to altitude changes. The Air Speed Indicator indicates the aircraft's speed through the air, also providing essential data but not specifically related to altitude changes. Thus, the Vertical Speed Indicator is the only instrument focused solely on measuring the rate of altitude change.