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Three types of barometers are used: the mercury barometer, the aneroid barometer, and the barograph.
The mercury barometer is so simple in its design, that it has been used since it was invented by Evangelista Torricelli in 1642. Today, it is still the most commonly used barometer.
A three foot tall glass tube is filled with mercury. One end of the tube is stopped up. The other end of the tube is put down into a container filled with more mercury. The mercury barometer acts like a balance. On one side is the weight or pressure of the column of air. On the other side is the column of mercury in the glass tube. As the outside air pressure increases, the mercury inside the barometer's column rises. As it decreases, the mercury falls. The changes in the height of the mercury column are measured in "inches of mercury" or millibars (metric.)
However, mercury is very expensive and mercury barometers can be bulky, so sometimes a different kind of barometer is used. The aneroid barometer has no liquid inside of it. Instead, it has a spring balance that balances the weight of the atmosphere and a known force on the spring. The spring is inside of a contain from which most of the air has been taken. This is called a partial vacuum. Because of the partial vacuum, the pressure of the air outside the can presses in. The spring inside the can keeps it from collapsing. As the air pressure outside of the can increases and decreases, the spring measures how much or how little the air is pressing in on the can.
The aneroid barometer doesn't record the air pressure as accurately as the mercury barometer, but it is smaller and cheaper. It can also be changed to become a barograph. A barograph is a barometer that is attached to a pen. It keeps a written record of the changes in the barometer. The chart it creates is called a barogram.
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