Which device measures humidity by detecting moisture-related changes in materials?

• A. Barometer
• B. Thermometer
• C. Anemometer
• D. Hygrometer

Answer: (D)

Humidity measurement relies on sensors that respond to water content in the air. A hygrometer uses a material whose properties change as it absorbs moisture from the surroundings. Classic examples include a hair-based hygrometer, where the length of human hair increases with higher humidity, and more modern versions that detect humidity through changes in capacitance or electrical resistance caused by moisture affecting the material. Because these changes reliably reflect how much moisture is present, the hygrometer is the device designed to measure humidity.

Barometers relate to air pressure, thermometers to temperature, and anemometers to wind speed, so they don’t track moisture-induced changes in materials.