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.

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