Evangelista Torricelli: Inventing the Barometer and Measuring Atmospheric Pressure

Before Torricelli, scientists debated: does air have weight?

Some argued: air is weightless, nature abhors vacuum.

In 1643, Torricelli designed an elegant experiment: fill a tube with mercury, invert it over mercury pool.

Mercury column didn't fall completely—it hung suspended at ~760 millimeters height.

Atmospheric pressure pressed on mercury below, supporting the column above.

Torricelli proved air has weight and invented the barometer.

The Background

Galileo had observed something puzzling: pumps couldn't siphon water above 10 meters (32 feet).

Galileo attributed it to "nature's horror of vacuum"—nature forbids empty space.

But Torricelli, Galileo's student, suspected the real cause: air's weight.

If air weighs, it exerts pressure. At some height, water's weight exceeds air pressure—water can't rise higher.

The Experiment

Torricelli filled a glass tube (closed at top, open at bottom) completely with mercury.

He inverted the tube into a shallow mercury pool, keeping it vertical.

Mercury poured out temporarily, but then stopped, hanging suspended.

The mercury column was consistently 760 millimeters high.

Why didn't it fall? Mercury weighed less than atmospheric pressure could support.

The Interpretation

Torricelli realized:

The weight of mercury column (760 mm) exactly equals atmospheric pressure.

Atmospheric pressure pressing on mercury surface below supported the column above.

A vacuum (or near-vacuum) formed at top of tube—no air there to push down.

Mercury rose to height where atmospheric pressure = mercury weight.

The Barometer

Torricelli's apparatus became the barometer—measuring atmospheric pressure.

Principle: Mercury height indicates pressure.

• Higher mercury → higher pressure
• Lower mercury → lower pressure

Simple, elegant, immediate readout.

Units

Mercury's height became pressure standard:

• 1 atmosphere = 760 mm Hg (millimeters of mercury)
• 1 bar (SI unit) ≈ 100,000 Pascals

When weather reports say "29.92 inches of mercury," they're using Torricelli's barometer principle.

(29.92 inches ≈ 760 mm)

Scientific Implications

Torricelli's experiment proved:

1. Air has weight - it exerts pressure
2. Vacuum can exist - top of tube is empty (or near-empty)
3. Pressure is measurable - using mercury column height

These three conclusions revolutionized physics.

The "Horror Vacui" Debate

Torricelli settled an old philosophical debate:

Aristotle and medieval philosophers: "Nature abhors vacuum" (empty space can't exist)

Torricelli: The space at barometer's top is nearly vacuum

This didn't settle all vacuum debates (Newton's colleagues still argued), but provided experimental proof that vacuum is possible.

Atmospheric Pressure Variations

Torricelli observed mercury height varied slightly day-to-day.

This variation correlated with weather changes.

This observation launched modern meteorology—pressure changes predict weather.

Barometers became essential to weather forecasting.

Boyle and Boyle's Law

Torricelli's demonstration of pressure inspired Robert Boyle.

Boyle conducted experiments with closed tubes and pressure—discovering Boyle's Law (PV = constant).

Boyle's Law explains compressibility of gases—a consequence of molecular behavior at pressure.

The Calculation

Mercury column height = 760 mm

Mercury density ≈ 13,600 kg/m³

Atmospheric pressure = (density) × (height) × (gravity)

= 13,600 × 0.76 × 9.8 ≈ 101,293 Pascals ≈ 1 atmosphere

The numbers work out exactly, confirming Torricelli's explanation.

Modern Barometers

Modern barometers no longer use mercury (toxic):

Aneroid barometer: Metal bellows expand/contract with pressure, mechanized indicator

Digital barometer: Electronic pressure sensor

Phone barometer: Many smartphones have pressure sensors

All measure atmospheric pressure—principle unchanged from Torricelli's mercury column.

Altitude and Pressure

Torricelli's principle explains altitude:

As altitude increases, atmospheric pressure decreases.

Barometer reading drops as you climb mountain.

This relationship (barometric formula) lets surveyors determine elevation using barometer.

Torricelli's Life

Evangelista Torricelli (1608-1647):

• Italian physicist and mathematician
• Student of Galileo (corresponded, never met in person)
• Became mathematics professor in Florence
• Died at 39 from typhoid fever

Brief life, but profound contribution.

The Vacuum Question

Torricelli's empty space at barometer's top troubled philosophers.

Is it truly empty (vacuum)? Or filled with ethereal substance?

Only 17 years later, Otto von Guericke created vacuum pump, removing air from enclosed space.

This definitively proved vacuum possible—vacuum exists in Torricelli's barometer.

Application to Height

Torricelli proved mercury height measures pressure.

This became basis of barometric altimetry:

Every airplane carries barometric altimeter. Every hiker with altimeter uses Torricelli's principle.

Elevation = determined from atmospheric pressure using barometric formula.

The Vacuum Controversy

Torricelli's empty space at barometer's top troubled philosophers for decades:

Aristotle and medievals: "Nature abhors vacuum"—empty space cannot exist.

Descartes: Space must be filled with ethereal fluid (invisible medium).

Torricelli: The space is essentially vacuum—air has been removed, creating near-perfect void.

Only 17 years later, Otto von Guericke created the vacuum pump, removing air from enclosed space experimentally. His famous Magdeburg Hemispheres (1654) demonstrated vacuum's power: two copper hemispheres with air removed between them were so tightly held together that 16 horses (8 pulling each way) couldn't separate them. This definitively proved vacuum is possible and can exert enormous force (atmospheric pressure pressing on hemispheres).

Torricelli and von Guericke together settled the vacuum debate: empty space exists and plays fundamental role in atmospheric physics.

Barometric Measurement in Practice

Modern barometers measure pressure using Torricelli's principle:

Mercury barometer: Column height directly indicates pressure. 760 mm = 1 atmosphere = 101.3 kPa.

Aneroid barometer: Metal bellows expand/contract with pressure change. Mechanical linkage moves pointer on dial. No mercury (eliminates toxicity).

Digital/electronic: Pressure sensor converts atmospheric pressure to electrical signal. Display shows numerical value (hPa, inches Hg, or kPa).

All operate on Torricelli's principle: pressure causes displacement proportional to force.

Altitude Determination

Torricelli's principle enables barometric altimetry:

As altitude increases, atmospheric density decreases exponentially. Barometer reading drops correspondingly. The barometric formula relates altitude to pressure:

h = (RT/g) × ln(P₀/P)

Where:

• h = altitude
• R = gas constant
• T = temperature
• P₀ = pressure at sea level
• P = pressure at altitude
• g = gravitational acceleration

This allows:

• Surveyors to determine elevation using barometer
• Pilots to measure altitude (all aircraft carry barometric altimeter)
• Hikers with altimeter watches to track elevation gain

The formula assumes constant temperature (reasonable for small altitude changes; inaccurate over large ranges).

Weather Prediction from Barometric Pressure

Torricelli observed mercury height varies with weather:

• Rising pressure: Usually indicates improving weather (high pressure systems bring clear skies)
• Falling pressure: Usually indicates worsening weather (low pressure systems bring storms)
• Rapid fall: Indicates severe weather development (strong storms, possible hurricanes)

Modern meteorology quantifies this:

High pressure systems: 1020+ hPa, clear weather, calm conditions.

Low pressure systems: Below 1000 hPa, stormy weather, strong winds.

This relationship, discovered by Torricelli, remains foundation of weather forecasting. Meteorologists track pressure changes to predict weather 1-7 days ahead. Pressure alone isn't sufficient (temperature, humidity, and upper-atmosphere conditions matter), but it's a crucial indicator.

Torricelli's Life and Context

Evangelista Torricelli (1608-1647):

• Italian physicist and mathematician
• Student of Galileo (corresponded, never met in person—worked at distance)
• Became mathematics professor in Florence
• Died at 39 from typhoid fever

Brief life, profound contribution. Worked in post-Galileo era when experimental physics was emerging. His barometer exemplified new experimental approach: ask nature questions through carefully designed experiments.

Pressure Units Legacy

Torricelli's measurement became standard pressure unit:

1 atmosphere = 760 mm Hg (millimeters of mercury)

The term "inches of mercury" (29.92 inches) still appears on weather reports—directly from Torricelli's barometer. Modern SI unit is bar (≈ 1 atmosphere) or pascal (Pa).

The persistence of "mm Hg" and "inches of mercury" in everyday use demonstrates Torricelli's lasting impact. Most non-scientists don't realize they're using a unit based on a 17th-century experiment with mercury tube.

Source: Torricelli, Evangelista (1644) — "Letter describing barometer experiment to Vincenzo Viviani"

Source: Heilbron, John L. (1979) — "Electricity in the 17th and 18th Centuries" (historical context)

Source: Middleton, W.E.K. (1964) — "The History of the Barometer" (comprehensive historical treatment)

Frequently Asked Questions

Q: Why does mercury height measure pressure specifically?
A: Mercury is heavy (density 13,600 kg/m³—13 times denser than water). Its weight creates the "pressure balance": mercury column's weight equals atmospheric pressure pressing from below. Heavier liquid = shorter column for same pressure. Water barometer would need 10-meter column (760 mm mercury × 13 = ~10 meters water). Mercury's density made barometer practical.

Q: How accurate is the barometer for weather prediction?
A: Moderately accurate. Pressure trends (rising, falling, steady) correlate well with weather changes. However, pressure alone isn't sufficient. Temperature, humidity, and upper-atmosphere wind patterns matter. Modern forecasts use barometric data plus satellite imagery, radar, and computer models. Pressure is necessary but not sufficient information.

Q: Why do hikers' altimeters sometimes show wrong elevation?
A: Barometric altimeters assume standard temperature profile with altitude. If actual temperature differs (warm day in mountains, cold air mass), altitude calculation errors occur. Solution: recalibrate at known elevation when possible. Modern GPS eliminates this problem (measures absolute position via satellites), but GPS is less precise vertically than barometric methods.

Q: Could you use water instead of mercury in barometer?
A: Yes, but impractically. Water is ~13 times less dense than mercury, so water column would need to be ~10 meters tall—huge, impractical. Mercury's density made compact barometer possible. Disadvantage: mercury is toxic. Modern electronic barometers eliminate toxicity by replacing mercury with electronic pressure sensor.

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