The history of the barometer (and how it works) - Asaf Bar-Yosef

Aristotle famously said, "Nature fears of empty space"

when he claimed that a true vacuum, a space devoid of matter, could not exist

because the surrounding matter would immediately fill it.

Fortunately, he turned out to be wrong.

A vacuum is a key component of the barometer,

an instrument for measuring air pressure.

And because air pressure correlates to temperature

and rapid shifts in it can contribute to

hurricanes, tornadoes and other extreme weather events,

a barometer is one of the most essential tools

for weather forecasters and scientists alike.

How does a barometer work, and how was it invented?

Well, it took awhile.

Because the theory of Aristotle and other ancient philosophers

regarding the impossibility of a vacuum seemed to hold true in everyday life,

few seriously thought to question it for nearly 2,000 years --

until necessity raised the issue.

In the early 17th century, Italian miners faced a serious problem

when they found that their pumps could not raise water

more than 10.3 meters high.

Some scientists at the time, including one Galileo Galilei,

proposed that sucking air out of the pipe was what made water rise to replace the void.

But that its force was limited and could lift no more than 10.3 meters of water.

However, the idea of a vacuum existing at all

was still considered controversial.

And the excitement over Galileo's unorthodox theory,

led Gasparo Berti to conduct a simple but brilliant experiment

to demonstrate that it was possible.

A long tube was filled with water

and placed standing in a shallow pool with both ends plugged.

The bottom end of the tube was then opened

and water poured out into the basin

until the level of the water remaining in the tube was 10.3 meters.

With a gap remaining at the top, and no air having entered the tube,

Berti had succeeded in directly creating a stable vacuum.

But even though the possibility of a vacuum had been demonstrated,

not everyone was satisfied with Galileo's idea

that this empty void was exerting some mysterious

yet finite force on the water.

Evangelista Torricelli, Galileo's young pupil and friend,

decided to look at the problem from a different angle.

Instead of focusing on the empty space inside the tube,

he asked himself, "What else could be influencing the water?"

Because the only thing in contact with the water was the air surrounding the pool,

he believed the pressure from this air could be the only thing preventing

the water level in the tube from dropping further.

He realized that the experiment was not only a tool to create a vacuum,

but operated as a balance

between the atmospheric pressure on the water outside the tube

and the pressure from the water column inside the tube.

The water level in the tube decreases until the two pressures are equal,

which just happens to be when the water is at 10.3 meters.

This idea was not easily accepted,

as Galileo and others had traditionally thought

that atmospheric air has no weight and exerts no pressure.

Torricelli decided to repeat Berti's experiment

with mercury instead of water.

Because mercury was denser, it fell farther than the water

and the mercury column stood only about 76 centimeters tall.

Not only did this allow Torricelli to make the instrument much more compact,

it supported his idea that weight was the deciding factor.

A variation on the experiment used two tubes with one having a large bubble at the top.

If Galileo's interpretation had been correct, the bigger vacuum in the second tube

should have exerted more suction and lifted the mercury higher.

But the level in both tubes was the same.

The ultimate support for Torricelli's theory came via Blaise Pascal

who had such a mercury tube taken up a mountain

and showed that the mercury level dropped

as the atmospheric pressure decreased with altitude.

Mercury barometers based on Torricelli's original model

remained one of the most common ways to measure atmospheric pressure until 2007

when restrictions on the use of mercury due to its toxicity

led to them no longer being produced in Europe.

Nevertheless, Torricelli's invention,

born of the willingness to question long accepted dogmas

about vacuums and the weight of air, is an outstanding example

of how thinking outside of the box -- or the tube --

can have a heavy impact.