---
title: "Mercury Barometer | College Physics I"
description: "Mercury barometer measures atmospheric pressure by the height of a mercury column, linking pressure measurement to absolute pressure in College Physics I."
canonical: "https://fiveable.me/intro-college-physics/key-terms/mercury-barometer"
type: "key-term"
subject: "College Physics I – Introduction"
unit: "Unit 11"
---

# Mercury Barometer | College Physics I

## Definition

A mercury barometer is a device that measures atmospheric pressure with a column of mercury in a sealed glass tube. In College Physics I, it shows how absolute pressure is found from hydrostatic balance.

## What It Is

A mercury barometer is a pressure-measuring device in College Physics I that uses a column of mercury to show the absolute pressure of the atmosphere. You read it by measuring how high the mercury rises in the glass tube above the reservoir surface.

The setup is simple but very physics-heavy. A glass tube is filled with mercury, inverted, and placed into a dish of mercury. Some mercury falls out of the tube, but not all of it, because the outside air presses on the reservoir and supports the column inside the tube.

What stops the column from falling farther is hydrostatic equilibrium. The pressure from the atmosphere pushing on the mercury in the reservoir balances the pressure created by the weight of the mercury column. That balance gives the relationship P = ρgh, where ρ is mercury’s density, g is gravitational acceleration, and h is the column height.

That means the barometer is not measuring an arbitrary liquid level, it is converting pressure into height. If atmospheric pressure increases, it pushes harder on the reservoir and forces the mercury column higher. If pressure drops, the column falls. Near sea level, the column is about 760 mm high under standard atmospheric pressure.

This is one reason mercury works so well in this device. It is very dense, so a manageable column height can represent a large pressure. If the fluid were much less dense, the column would need to be much taller to balance the same atmosphere.

A common point of confusion is that the barometer does not measure gauge pressure. It measures atmospheric pressure itself, which is an absolute pressure relative to a vacuum at the top of the sealed tube. That is why mercury barometers show the pressure of the air around you, not the difference between two nearby pressures.

## Why It Matters

Mercury barometers connect the idea of pressure to a visible physical height, which makes them a classic bridge between formulas and real measurements in College Physics I. When you see P = ρgh in this setting, you are not just memorizing a math relationship, you are watching pressure support a liquid column against gravity.

This term also helps you separate absolute pressure from gauge pressure. That distinction shows up all over pressure problems, especially when you compare atmospheric pressure, tire pressure, or fluid columns in manometers. If you can explain why the barometer reads atmospheric pressure directly, you are in good shape for related problems.

It also shows up in lab work and data interpretation. A question may ask you to read a barometer, compare pressure values at different locations, or explain why a higher column means higher atmospheric pressure. The concept gives you a physical picture for why pressure changes with weather and altitude.

## Connections

### [Atmospheric Pressure](/intro-college-physics/key-terms/atmospheric-pressure)

A mercury barometer is built to measure atmospheric pressure directly. The air outside the tube pushes on the mercury reservoir, and that pressure supports the column inside the tube. If atmospheric pressure changes with weather or altitude, the column height changes too, which is why barometers can track pressure trends.

### Absolute Pressure

Mercury barometers measure absolute pressure, not pressure relative to the air around you. The reference point is effectively a vacuum at the top of the sealed tube, so the reading reflects the full pressure of the atmosphere. That makes it different from common gauges, which usually report pressure above atmospheric pressure.

### [Hydrostatic Equilibrium](/intro-college-physics/key-terms/hydrostatic-equilibrium)

The mercury column stays at a steady height when the pressure from the atmosphere balances the weight of the liquid column. That balance is hydrostatic equilibrium. If the pressure on the reservoir changes, the column shifts until a new balance is reached, which is the whole operating idea behind the barometer.

### [Aneroid Barometer](/intro-college-physics/key-terms/aneroid-barometer)

An aneroid barometer measures pressure without liquid mercury, usually with a flexible metal chamber that changes shape as pressure changes. It is often more portable and less fragile than a mercury barometer. The two devices do the same measurement job, but they use different physical mechanisms.

## On the AP Exam

A quiz or problem-set question might give you a mercury column height and ask for the atmospheric pressure using P = ρgh. You may need to convert units, especially if the height is given in millimeters of mercury or centimeters. Another common task is identifying whether the reading is absolute or gauge pressure. If a diagram shows an inverted tube and reservoir, you should recognize that the air pressure on the reservoir supports the mercury column and that higher pressure means a taller column. In a lab question, you might explain why the barometer reading changes when weather systems move through.

## Mercury Barometer vs Aneroid Barometer

A mercury barometer uses a mercury column in a glass tube, while an aneroid barometer uses a sealed flexible metal chamber. Both measure atmospheric pressure, but the aneroid version is more portable and less fragile. If you see liquid mercury and a height reading, it is the mercury barometer.

## Key Takeaways

- A mercury barometer measures atmospheric pressure by the height of a mercury column in a sealed tube.
- The column height is set by hydrostatic balance, where atmospheric pressure supports the weight of the mercury.
- The basic relationship is P = ρgh, so taller columns mean higher atmospheric pressure.
- This device measures absolute pressure, not gauge pressure.
- Mercury barometers are accurate but fragile, which is why many modern setups use other pressure sensors instead.

## FAQs

### What is a mercury barometer in College Physics I?

It is a device that measures atmospheric pressure using a column of mercury in an inverted glass tube. The atmosphere pushes on the mercury reservoir, and the column height shows the pressure. In physics, it is a classic example of pressure balanced by liquid weight.

### How does a mercury barometer measure pressure?

Air pressure on the mercury reservoir supports the mercury in the tube until the weight of the column balances that pressure. The taller the column, the higher the atmospheric pressure. You can model the reading with P = ρgh.

### Is a mercury barometer gauge pressure or absolute pressure?

It measures absolute pressure. The sealed top of the tube acts like a near-vacuum reference, so the reading reflects the full atmospheric pressure. Gauge pressure is different because it is measured relative to atmospheric pressure.

### Why use mercury instead of another liquid?

Mercury is very dense, so a reasonable column height can balance atmospheric pressure. A less dense liquid would need a much taller column to represent the same pressure. That is why mercury barometers are compact compared with many other liquid-column setups.

## Related Study Guides

- [11.6 Gauge Pressure, Absolute Pressure, and Pressure Measurement](/intro-college-physics/unit-11/6-gauge-pressure-absolute-pressure-pressure-measurement/study-guide/90rXMcQ75CCjAWkW)

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