The instrument used to measure relative humidity is called a hygrometer.

Explanation:

Hygrometer is specifically designed to measure the amount of moisture or water vapor present in the air, which is known as relative humidity. It is an essential tool in meteorology, as well as in various industries such as agriculture, construction, and manufacturing.

Here are some key points about the hygrometer and its functioning:

1. Hygrometer Types:
- There are various types of hygrometers available, including psychrometers, hair hygrometers, capacitance hygrometers, and dew point hygrometers.
- Each type of hygrometer uses different principles and techniques to measure relative humidity.

2. Psychrometers:
- Psychrometers consist of two thermometers: a dry-bulb thermometer and a wet-bulb thermometer.
- The dry-bulb thermometer measures the ambient air temperature, while the wet-bulb thermometer has a wet wick that is exposed to the air.
- By comparing the readings from both thermometers, the relative humidity can be determined using psychrometric charts or equations.

3. Hair Hygrometers:
- Hair hygrometers use human or animal hair as a sensing element.
- Hair expands or contracts based on the moisture content in the air.
- This expansion or contraction is converted into a measurement of relative humidity.

4. Capacitance Hygrometers:
- Capacitance hygrometers measure the changes in electrical capacitance due to the presence of moisture.
- They consist of a hygroscopic material that adsorbs or desorbs moisture, causing a change in capacitance.

5. Dew Point Hygrometers:
- Dew point hygrometers measure the temperature at which water vapor in the air begins to condense into liquid form.
- By measuring the dew point temperature, the relative humidity can be determined.

In conclusion, a hygrometer is the instrument used to measure relative humidity. It comes in various types and utilizes different principles to accurately determine the moisture content in the air.

Hygrometer