Why Is Mercury Used in Barometers?

Air pressure falls when the weather is changing for the worst, and rises when it is getting better. This is because humid, wet air is heavier than dry air. Barometers are instruments that measure the change in air pressure. The most accurate barometers rely on a tube of liquid mercury.

History

In 1643 an Italian scientist, Evangelista Torricelli, filled a hollow glass tube with mercury. It was closed at one end, like a straw when you cover one end. Then he placed the open end of this tube into an open cup filled with more mercury. The force of the air pressing down on the mercury in the cup made the height of the mercury in the tube rise or fall. When the air pressed down on the mercury in the cup, it forced some of the metal to push up into the tube. When the air pressure lessened, the mercury in the tube fell back down and into the cup. Torricelli was able to prove that the weight of the air in the Earth's atmosphere is equal to the amount of mercury that filled up 30 inches of a tube of a certain diameter.

Significance

This information--combined with the fact that when mercury is heated or pressurized it expands and contracts evenly--provided the basis for the invention of the first barometer. Even today, mercury barometers are more accurate that aneroid barometers for measuring changes in atmospheric pressure.

Identification

Mercury, number 80 on the Periodic Table of the Elements, is symbolized by the letters Hg. This comes from the Greek name for the element, hydrargyum, which means watery silver. Since pure mercury is a liquid metal at temperatures above negative 37.89 degrees Fahrenheit and below 674.11 degrees Fahrenheit, it got its nickname, "quick silver."

Considerations

Barometric pressure differs based on altitude. It is greater at and below sea level and less at higher elevations. Therefore, the accuracy of a mercury barometer depends on its location. Each barometer must be calibrated for the difference between the place where it is located and sea level.

Expert Insight

Evangelista Torricelli, the inventor of the mercury barometer, lived from 1608 to 1647. As one of Galileo's assistants, Torricelli was trained to make and rely on very accurate measurements. It took until 1714 for a mercury thermometer to be invented by the German Gabriel Daniel Fahrenheit. Even young children, though, can create a simulation of how the mercury barometer works using a balloon stretched over a jar filled with air. A straw placed and taped on top of the balloon will move up and down ever so slightly as the atmospheric pressure changes. This is the same principle by which Torricelli's mercury barometer worked.