Brief description

A barometer is a scientific instrument used to measure atmospheric pressure. It is a critical tool for predicting short-term weather changes and measuring altitude.

Use / Function

Weather Forecasting: High pressure generally indicates fair weather, while low pressure indicates storms. Rapid pressure drops are a warning sign.
Altimetry: Measuring altitude above sea level (pressure decreases with height).
Scale: Individual to Industrial.

Operating principle

The atmosphere exerts weight (pressure) on the surface of the earth. A barometer balances this weight against a column of liquid (mercury or water) or a mechanical spring.

Mercury Barometer: Atmospheric pressure pushes down on a reservoir of mercury, forcing it up a vacuum-sealed tube. The height of the column is directly proportional to the pressure.

How to create it

Torricellian Barometer

Materials: A glass tube at least 80cm long (sealed at one end), a small bowl, and mercury.
Assembly: Fill the tube completely with mercury to remove all air. Fill the bowl partly with mercury.
Inversion: Place your thumb over the open end, invert the tube, and submerge the opening in the bowl of mercury before releasing your thumb.
Result: The mercury will drop slightly, creating a vacuum at the top. The height of the column (usually around 760mm at sea level) represents the atmospheric pressure.

Water Barometer (Goethe)

Materials: A glass jar with a spout connected to the main body (like a teapot but sealed at the top).
Principle: Traps a volume of air. When external pressure rises, it pushes water down the spout and into the body. When pressure falls, the trapped air pushes water up the spout.

Technical level: Advanced (Mercury) to Basic (Water/Goethe).

Materials needed

Essential: Glass tubing, liquid (mercury is standard, water requires a 10m tube for accuracy or a Goethe design for relative changes).
Tools: Heat source for sealing glass.
Substitutes: An aneroid barometer uses a flexible metal box with a vacuum inside, which expands/contracts with pressure (requires precision machining).

Variants and improvements

Aneroid Barometer: Non-liquid, uses a metal cell. More durable and portable.
Barograph: Records pressure changes over time on a paper chart.
Storm Glass: A chemical mixture (camphor, alcohol, etc.) that crystallizes differently based on weather (though accuracy is debated).

Limits and risks

Toxicity: Mercury is extremely toxic. Handled with great care.
Fragility: Glass tubes break easily.
Temperature: Readings must be corrected for temperature, as liquids expand with heat.