The instrument used to measure relative humidity is a

Step 1: Analyze the question The question asks which instrument measures relative humidity, which is the amount of water vapour in the air compared with the maximum amount the air can hold at that temperature (usually given as a percentage).

Step 2: Explain the concept Each instrument in the options measures something different:

• Hygrometer: measures humidity (moisture in the air). Examples are the hair hygrometer and the wet-and-dry bulb hygrometer.
• Barometer: measures atmospheric pressure.
• Hydrometer: measures the density or relative density of liquids, such as acid in a car battery or milk.
• Manometer: measures the pressure of gases or liquids, usually in a U-tube.

Step 3: Solution Relative humidity is a measure of moisture in the air, so the instrument must be a hygrometer. The prefix "hygro-" comes from the Greek word for moisture or wetness.

In the wet-and-dry bulb hygrometer, two thermometers are used. The wet bulb is cooled by evaporation, so the difference between the two readings is used with a table to find the relative humidity.

Step 4: Answer The correct option is A. Hygrometer.

Step 5: Exam tip Use the prefixes to remember the instruments:

• Hygro = moisture, so hygrometer measures humidity
• Baro = pressure (weight), so barometer measures atmospheric pressure
• Hydro = liquid, so hydrometer measures density of liquids
• Mano = gas or liquid pressure, so manometer measures pressure