Short Answer:
A barometer is a scientific instrument used to measure the atmospheric pressure at a particular place. It works on the principle of balancing the weight of the atmosphere against the weight of a column of mercury or any other liquid. The height of the liquid column changes with variations in atmospheric pressure.
In simple words, a barometer tells how heavy the air around us is. It is mainly used in weather forecasting and scientific studies. The most common type is the mercury barometer, while the aneroid barometer is also widely used for portable applications.
Detailed Explanation :
Barometer
A barometer is an instrument designed to measure atmospheric pressure, which is the force exerted by the weight of air in the Earth’s atmosphere. The measurement of atmospheric pressure is essential in meteorology, aviation, and engineering because it helps in predicting weather conditions, determining altitude, and studying air behavior.
The term “barometer” comes from the Greek words “baros” meaning “weight” and “metron” meaning “measure.” Hence, it literally means “measure of weight.”
Atmospheric pressure changes with weather conditions and altitude — it decreases with height and varies with temperature and humidity. A barometer helps detect these changes accurately.
Construction of Barometer
The barometer is generally made up of the following components:
- Glass Tube:
A long glass tube (about 1 meter in length), closed at one end and open at the other. - Mercury Reservoir:
The open end of the tube is immersed in a reservoir or dish containing mercury. - Scale:
A graduated scale is fixed alongside the tube to measure the height of the mercury column, usually in millimeters or inches of mercury (mmHg or inHg). - Vacuum Space:
The upper closed end of the tube contains a vacuum (known as the Torricellian vacuum), which ensures that the mercury column height depends only on atmospheric pressure. - Support Frame:
The entire setup is supported vertically on a wooden or metallic stand for stable reading.
Working Principle of Barometer
The working principle of a barometer is based on the hydrostatic balance between the atmospheric pressure and the weight of the mercury column.
When the barometer tube is inverted into a dish of mercury:
- Mercury in the tube falls slightly, creating a vacuum at the top.
- The atmospheric pressure acting on the surface of mercury in the dish supports the column of mercury inside the tube.
- The height of this mercury column represents the atmospheric pressure.
Mathematically,
Where,
- = Atmospheric pressure (N/m²),
- = Density of mercury (13,600 kg/m³),
- = Acceleration due to gravity (9.81 m/s²),
- = Height of mercury column (m).
At standard atmospheric conditions, the height of the mercury column is approximately 760 mm (or 76 cm), which is defined as 1 atmosphere (1 atm = 101325 Pa).
Types of Barometer
Barometers are of two main types:
- Mercury Barometer
- Aneroid Barometer
(1) Mercury Barometer
The mercury barometer is the classic and most accurate type of barometer.
- It consists of a long glass tube filled with mercury and inverted into a mercury reservoir.
- The atmospheric pressure pushes the mercury up into the tube, and the height of the mercury column indicates the air pressure.
Advantages:
- Very accurate and reliable for laboratory and scientific measurements.
- Provides direct pressure readings.
Disadvantages:
- Mercury is toxic and needs careful handling.
- The barometer is heavy and not portable.
Example:
At sea level, the height of mercury in a barometer is about 760 mm, which decreases with increasing altitude.
(2) Aneroid Barometer
An aneroid barometer does not use liquid. Instead, it uses a small, sealed metal box known as an aneroid cell.
- The cell expands or contracts with changes in atmospheric pressure.
- These movements are mechanically linked to a pointer that moves on a calibrated dial to indicate pressure readings.
Advantages:
- Portable and easy to use.
- No liquid, hence no risk of mercury leakage.
- Used in portable weather instruments and aircraft altimeters.
Disadvantages:
- Slightly less accurate compared to mercury barometers.
Working of Mercury Barometer
- Normal Pressure:
When the atmospheric pressure is normal, the mercury level in the tube stands at about 760 mm. - High Pressure (Good Weather):
When atmospheric pressure increases, the mercury column rises. - Low Pressure (Bad Weather):
When atmospheric pressure decreases, the mercury level falls.
This variation helps meteorologists predict weather conditions:
- High pressure → clear skies and dry weather.
- Low pressure → cloudy and rainy conditions.
Applications of Barometer
- Weather Forecasting:
- Used to predict changes in weather based on pressure variations.
- A falling mercury level indicates storms or rain, while a rising level indicates fair weather.
- Altitude Measurement:
- Atmospheric pressure decreases with altitude, so barometers can be used to measure elevation (as in altimeters).
- Laboratory Use:
- Used to determine atmospheric pressure in scientific and engineering experiments.
- Calibration:
- Used to calibrate other pressure-measuring devices like manometers and pressure gauges.
- Aviation:
- Aneroid barometers are used in aircraft to determine flying altitude and maintain safe flight operations.
Advantages of Barometer
- Provides accurate and repeatable measurements.
- Simple principle and easy operation.
- Useful for both laboratory and environmental studies.
- Helps in real-time weather prediction.
- Can be used to determine pressure variations with altitude.
Limitations of Barometer
- Mercury barometers are heavy and not portable.
- Mercury is toxic and harmful if spilled.
- The reading may be affected by temperature and altitude changes.
- Aneroid barometers, though portable, may require frequent calibration.
Example Calculation
If the height of the mercury column is 760 mm and the density of mercury is 13,600 kg/m³, the atmospheric pressure is:
Thus, the standard atmospheric pressure equals 101.3 kPa or 1 atm.
Conclusion
In conclusion, a barometer is an essential instrument for measuring atmospheric pressure. It operates on the principle of balancing the weight of the atmosphere with a column of mercury or through mechanical deformation of a sealed cell. The mercury barometer is the most precise, while the aneroid barometer is more portable and convenient. Barometers play a vital role in weather forecasting, altitude determination, and scientific research, making them indispensable tools in meteorology and engineering.