What instrument is designed to measure changes in resistivity due to vapor pressure variations?

The instrument specifically designed to measure changes in resistivity due to vapor pressure variations is the hair hydrometer. This device employs human or synthetic hair, which responds to humidity by expanding or contracting. As the moisture content in the air changes, the hair's length varies, leading to corresponding changes in its resistivity. This property allows for precise measurements of humidity levels by interpreting the changes in resistivity that occur with shifts in vapor pressure.

In contrast, other instruments such as hygrometers typically measure humidity directly using different principles rather than resistivity changes. Psychrometers, which include wet-bulb and dry-bulb thermometers, calculate humidity based on temperature differences, while barometers measure atmospheric pressure and do not focus on changes in vapor pressure. Each of these other instruments serves important but distinct functions in meteorology.