Difference Between Aneroid and Mercury Barometer

Main Difference – Aneroid vs Mercury Barometer

A barometer is a device that is used to measure atmospheric pressure. Atmospheric pressure, sometimes also called barometric pressure, is the pressure due to the weight of air within the atmosphere of Earth. This atmospheric pressure changes from one point to another depending on the distance (height) from the sea level. There are different types of barometers. Aneroid barometer and mercury barometer are two such barometers. The main difference between aneroid and mercury barometer is that aneroid barometer measures the atmospheric pressure using the expansion of a metal whereas mercury barometer measures the atmospheric pressure by adjusting the height of mercury inside a tube.

Key Areas Covered

1. What is Aneroid Barometer
     – Definition, How It Works, Uses
2. What is Mercury Barometer
    – Definition, Structure, How It Works
3. What is the Difference Between Aneroid and Mercury Barometer
     – Comparison of Key Differences

Key Terms: Aneroid Barometer, Atmospheric Pressure, Barometer, Barometric Pressure, Mercury Barometer, Metal

What is an Aneroid Barometer

An aneroid barometer is a type of barometer that uses a small flexible metal box to measure the atmospheric pressure. It is widely used as an alternative barometer for liquid barometers. This is because aneroid barometer is a solid device which is easy to handle and transport. The value of the atmospheric pressure is easy to read. It is also a small device that is easy to use.

Figure 1: Aneroid Barometer

Inside the aneroid barometer, there is a flexible metal box. This metal box is called the aneroid cell. It is often made of a metal alloy containing beryllium and copper. The box is evacuated, and any change of the external pressure can cause the metal box to either expand or contract accordingly.

There are levers inside the barometer which are moved with the expansion or the contraction of the metal box. These levers cause a needle to move. The needle points out the correct atmospheric pressure. Therefore, the pressure is easily displayed.

The aneroid barometer is calibrated using a mercury barometer. Unlike other simple barometers, machinery is needed for the construction of the barometer. Aneroid barometers are commonly used in homes, recreational boats, aircrafts, etc.

What is Mercury Barometer

Mercury barometer is a simple barometer that uses mercury to measure the atmospheric pressure. Mercury is a metal and is in the liquid phase at room temperature. A mercury barometer is composed of two main parts: a vertical glass tube and an open, mercury-filled basin.

The vertical glass tube has one end open and one sealed. At first, it is filled with mercury. Then this glass tube is placed in a mercury-filled basin upside down. Then the open end is immersed in mercury, and the sealed end is at the top. The mercury inside the glass tube falls down creating a vacuum at the top of the glass tube. The mercury in the glass tube falls until the weight of the mercury in the glass tube equals the weight of air above the mercury level of the basin.

Figure 2: A Diagram of Mercury Barometer

Once the mercury level is balanced, and no more change of height occurs, the reading is taken as the height of mercury in the vertical glass tube. The atmospheric pressure at sea level is 760 mmHg. This means the height of the mercury in the vertical glass tube is 760 mm when the barometer is taken to sea level.

Difference Between Aneroid and Mercury Barometer

Definition

Aneroid Barometer: Aneroid barometer is a type of barometer that uses a small flexible metal box to measure the atmospheric pressure.

Mercury Barometer: Mercury barometer is a simple barometer that uses mercury to measure the atmospheric pressure.

Components

Aneroid Barometer: Aneroid barometer uses a metal box to measure the atmospheric pressure.

Mercury Barometer: Mercury barometer uses mercury to measure the atmospheric pressure.

Technique

Aneroid Barometer: Aneroid barometer measures the external pressure using the expansion or contraction of a flexible metal box which leads to moving a needle on a pressure scale to get the reading.

Mercury Barometer: Mercury barometer measures the external pressure by measuring the rise or the height of mercury inside the vertical glass tube.

Handling

Aneroid Barometer: Since an aneroid barometer is a solid and compact device, it is easy to handle and transport.

Mercury Barometer: The mercury barometer is large (about 3 feet high) and fragile it is hard to handle and transport.

Taking the Measurement

Aneroid Barometer: It is easy to take a measurement from the aneroid barometer since it directly gives a value.

Mercury Barometer: It is hard to take a measurement from the mercury barometer since the height should be measured accurately after it gets balanced.

Construction

Aneroid Barometer: The aneroid barometer need machinery for its construction.

Mercury Barometer: Mercury barometer needs no machinery for the construction.

Stable

Aneroid Barometer: Aneroid barometers are stable devices.

Mercury Barometer: Mercury barometers are unstable.

Uses

Aneroid Barometer: Aneroid barometers are used in homes, recreational boats, aircrafts, etc.

Mercury Barometer: Mercury barometers are used in laboratories, for weather forecasting, to determine the altitude of a place, etc.

Conclusion

A barometer is a system that is used to measure atmospheric pressure. There are many types of barometers. Aneroid barometer and mercury barometer are such two types. The main difference between aneroid and mercury barometer is that aneroid barometer measures the atmospheric pressure using the expansion of a metal whereas mercury barometer measures the atmospheric pressure by adjusting the height of mercury inside a tube.

References:

 1. Worboys, Jenny. “How a Barometer Works and Helps Forecast Weather.” ThoughtCo, Jul. 9, 2017, Available here.
2. “Barometer.” Encyclopædia Britannica, Encyclopædia Britannica, inc., 3 Feb. 2017, Available here.
3. “Barometer.” Wikipedia, Wikimedia Foundation, 15 Dec. 2017, Available here.

Image Courtesy:

1. “Aneroid barometer J2” By Jamain – Own work (CC BY-SA 3.0) via Commons Wikimedia
2. “Quecksilber-Barometer Prinzip” (CC BY-SA 3.0) via Commons Wikimedia