The instrument used to measure the atmospheric pressure is the **barometer**. This device is specifically designed to measure the pressure exerted by the Earth's atmosphere. Here is a detailed explanation of why a barometer is used to measure atmospheric pressure:

**Barometer:**
A barometer is an instrument that measures atmospheric pressure. It consists of a long glass tube filled with mercury or another liquid, with one end sealed and the other end open to the atmosphere. The open end is submerged in a container of mercury, forming a vacuum above it. The pressure exerted by the atmosphere is balanced by the weight of the mercury in the tube, resulting in the height of the mercury column.

**Principle of Operation:**
The principle behind a barometer is based on the concept that atmospheric pressure decreases with increasing altitude. When the atmospheric pressure is high, the mercury in the tube is pushed up, resulting in a higher column height. Conversely, when the atmospheric pressure is low, the mercury column height decreases. By measuring the height of the mercury column, the atmospheric pressure can be determined.

**Types of Barometers:**
There are various types of barometers, including:

1. **Mercury Barometer:** This is the traditional barometer that uses mercury as the liquid. It is based on the principle of balancing the weight of the mercury column with the atmospheric pressure.

2. **Aneroid Barometer:** An aneroid barometer is a compact and portable instrument that uses a small, flexible metal box called an aneroid cell. The cell expands or contracts with changes in atmospheric pressure, which is then converted into a mechanical movement that can be read on a dial.

3. **Electronic Barometer:** Electronic barometers use sensors to measure atmospheric pressure and convert it into an electrical signal. These barometers provide digital readings and are often used in weather stations and meteorological instruments.

**Importance of Measuring Atmospheric Pressure:**
Measuring atmospheric pressure is important for various reasons:

1. **Weather Prediction:** Changes in atmospheric pressure indicate changes in weather conditions. High pressure is associated with fair weather, while low pressure is often associated with storms and precipitation.

2. **Altitude Determination:** By measuring atmospheric pressure, it is possible to estimate the altitude of a location. As atmospheric pressure decreases with increasing altitude, barometers can be used to determine the height above sea level.

3. **Aircraft and Weather Balloon Operations:** Atmospheric pressure measurements are crucial for aircraft operations and weather balloon launches. This information helps pilots and meteorologists understand weather patterns and make accurate predictions.

In conclusion, a barometer is the instrument used to measure atmospheric pressure. It operates on the principle of balancing the weight of a liquid column with the atmospheric pressure. By measuring the height of the liquid column, atmospheric pressure can be determined, providing valuable information for weather prediction and altitude determination.