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The history of the barometer (and how it works) - Asaf Bar-Yosef - YouTube
Channel: TED-Ed
[6]
Aristotle famously said,
"Nature fears of empty space"
[11]
when he claimed that a true vacuum,
a space devoid of matter, could not exist
[16]
because the surrounding matter
would immediately fill it.
[19]
Fortunately, he turned out to be wrong.
[21]
A vacuum is a key component
of the barometer,
[25]
an instrument for measuring air pressure.
[27]
And because air pressure
correlates to temperature
[30]
and rapid shifts in it can contribute to
[32]
hurricanes, tornadoes
and other extreme weather events,
[35]
a barometer is one of the
most essential tools
[38]
for weather forecasters and scientists alike.
[41]
How does a barometer work,
and how was it invented?
[44]
Well, it took awhile.
[46]
Because the theory of Aristotle
and other ancient philosophers
[49]
regarding the impossibility of a vacuum
seemed to hold true in everyday life,
[54]
few seriously thought to question it
for nearly 2,000 years --
[59]
until necessity raised the issue.
[61]
In the early 17th century,
Italian miners faced a serious problem
[65]
when they found that their pumps
could not raise water
[68]
more than 10.3 meters high.
[70]
Some scientists at the time,
including one Galileo Galilei,
[74]
proposed that sucking air out of the pipe
was what made water rise to replace the void.
[80]
But that its force was limited and could lift
no more than 10.3 meters of water.
[85]
However, the idea of a
vacuum existing at all
[88]
was still considered controversial.
[90]
And the excitement over
Galileo's unorthodox theory,
[93]
led Gasparo Berti to conduct a simple
but brilliant experiment
[98]
to demonstrate that it was possible.
[100]
A long tube was filled with water
[102]
and placed standing in a shallow pool
with both ends plugged.
[106]
The bottom end of the tube
was then opened
[108]
and water poured out into the basin
[111]
until the level of the water remaining
in the tube was 10.3 meters.
[116]
With a gap remaining at the top,
and no air having entered the tube,
[120]
Berti had succeeded in
directly creating a stable vacuum.
[124]
But even though the possibility
of a vacuum had been demonstrated,
[128]
not everyone was satisfied
with Galileo's idea
[131]
that this empty void
was exerting some mysterious
[134]
yet finite force on the water.
[136]
Evangelista Torricelli,
Galileo's young pupil and friend,
[140]
decided to look at the problem
from a different angle.
[143]
Instead of focusing on the
empty space inside the tube,
[147]
he asked himself,
"What else could be influencing the water?"
[150]
Because the only thing in contact
with the water was the air surrounding the pool,
[154]
he believed the pressure from this air
could be the only thing preventing
[158]
the water level in the tube
from dropping further.
[161]
He realized that the experiment
was not only a tool to create a vacuum,
[165]
but operated as a balance
[167]
between the atmospheric pressure
on the water outside the tube
[170]
and the pressure from the
water column inside the tube.
[173]
The water level in the tube decreases
until the two pressures are equal,
[178]
which just happens to be
when the water is at 10.3 meters.
[182]
This idea was not easily accepted,
[184]
as Galileo and others
had traditionally thought
[187]
that atmospheric air has no weight
and exerts no pressure.
[192]
Torricelli decided to
repeat Berti's experiment
[194]
with mercury instead of water.
[196]
Because mercury was denser,
it fell farther than the water
[200]
and the mercury column stood
only about 76 centimeters tall.
[203]
Not only did this allow Torricelli to make
the instrument much more compact,
[208]
it supported his idea that weight
was the deciding factor.
[212]
A variation on the experiment used two tubes
with one having a large bubble at the top.
[217]
If Galileo's interpretation had been correct,
the bigger vacuum in the second tube
[222]
should have exerted more suction
and lifted the mercury higher.
[225]
But the level in both tubes was the same.
[228]
The ultimate support for Torricelli's theory
came via Blaise Pascal
[233]
who had such a mercury tube
taken up a mountain
[236]
and showed that the mercury level dropped
[238]
as the atmospheric pressure
decreased with altitude.
[242]
Mercury barometers based on
Torricelli's original model
[245]
remained one of the most common ways
to measure atmospheric pressure until 2007
[250]
when restrictions on the use of
mercury due to its toxicity
[253]
led to them no longer
being produced in Europe.
[256]
Nevertheless, Torricelli's invention,
[258]
born of the willingness to question
long accepted dogmas
[262]
about vacuums and the weight of air,
is an outstanding example
[265]
of how thinking outside of the box
-- or the tube --
[269]
can have a heavy impact.
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