The Torricelli's experiment It was carried out by the Italian physicist and mathematician Evangelista Torricelli in 1644 and resulted in the first measurement of atmospheric pressure.
This experiment arose from the need to improve the water supply in cities. Evangelista Torricelli (1608-1647), who was a court mathematician to the Grand Duke of Tuscany Ferdinand II, had studied hydraulic phenomena together with Galileo.
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In 1644, Torricelli did the following experiment:
- He introduced mercury inside a 1 m long tube, open at one end and closed at the other.
- When the tube was completely full, he inverted it and dumped it into a container that also contained mercury..
- Torricelli observed that the column descended and stopped at about 76 cm high.
- He also noticed that a vacuum had been created in the space that was free, although not perfect..
Torricelli repeated the experiment using different tubes. He even made a small variation: he added water to the bucket, which, being lighter, floated on the mercury. Then he slowly raised the tube containing mercury to the surface of the water..
Then the mercury went down and the water went up. The vacuum obtained, as we have already said, was not perfect, because there were always remnants of mercury vapor or water.
The atmosphere is a mixture of gases in which nitrogen and oxygen predominate, with traces of other gases such as argon, carbon dioxide, hydrogen, methane, carbon monoxide, water vapor and ozone..
The gravitational attraction exerted by the Earth is responsible for keeping the whole surrounding the planet.
Of course, the composition is not uniform, and neither is the density, since it depends on the temperature. Near the surface there is a good amount of dust, sand and pollutants from natural events and also from human activity. The heaviest molecules are closer to the ground.
Since there is so much variability, it is necessary to choose a reference altitude for the atmospheric pressure, which for convenience has been taken as sea level.
Here it is not just any sea level, because that also presents fluctuations. The level o datum a geodetic reference system is chosen with the help of common agreement between the experts.
How much is the atmospheric pressure near the ground worth? Torricelli found its value when he measured the height of the column: 760 mm of mercury.
At the top of the tube the pressure is 0, since a vacuum has been established there. Meanwhile, on the surface of the mercury vat the pressure P1 is atmospheric pressure.
Let's choose the origin of the reference frame on the free surface of the mercury, at the top of the tube. From there until the surface of the mercury in the container is measured H, the height of the column.
The pressure at the point marked in red, at the depth and1 it is:
P1 = Por + ρHg . g.y1
Where ρHg is the density of mercury. Since Y1 = H Y Po = 0:
P1 = ρHg . g.H
H = P1/ ρHg.g
Since the density of mercury is constant and gravity is constant, it turns out that the height of the mercury column is proportional to P1, which is atmospheric pressure. Substituting known values:
H = 760 mm = 760 x 10 -3 m
g = 9.8 m / stwo
ρHg = 13.6 g / cc = 13.6 x 10 3 kg / m3
P1 = 13.6 x 10 3 kg / m3 x 9.8 m / stwo x 760 x 10 -3 m = 101.293 N / mtwo= 101.3 kN / mtwo
The unit for pressure in the International System is the pascal, abbreviated Pa. According to Torricelli's experiment, atmospheric pressure is 101.3 kPa.
Torricelli observed that the level of mercury in the tube underwent slight variations every day, so he deduced that the atmospheric pressure must also change..
Atmospheric pressure is responsible for much of the climate, however its daily variations go unnoticed. It is because they are not as noticeable as storms or cold, for example.
However, these variations in atmospheric pressure are responsible for the winds, which in turn influence rainfall, temperature and relative humidity. When the ground heats up, the air expands and tends to rise, causing the pressure to drop.
Whenever the barometer indicates high pressures, good weather can be expected, while with low pressures there is the possibility of storms. However, to make accurate predictions of the weather, you need more information about other factors..
Although it sounds strange, since pressure is defined as force per unit area, in meteorology it is valid to express atmospheric pressure in millimeters of mercury, as established by Torricelli.
It is because the mercury barometer continues to be used today with little variation since that time, so that in honor of Torricelli, 760 mm of Hg is equal to 1 torr. In other words:
1 torr = 760 mm Hg = 30 inches of Hg = 1 atmosphere pressure = 101.3 kPa
If Torricelli had used water instead of mercury, the height of the column would be 10.3 m. The mercury barometer is more practical as it is more compact.
Other widely used units are bars and millibars. One millibar equals one hectopascal or 10two pascals.
An altimeter is an instrument that indicates the height of a place, comparing the atmospheric pressure at that height with that on the ground or another place of reference.
If the height is not very great, in principle we can assume that the density of the air remains constant. But this is an approximation, since we know that the density of the atmosphere does decrease with height.
Using the equation used above, the density of air is used instead of that of mercury:
P1 = Por + ρair . g.H
In this expression Por is taken as the atmospheric pressure at ground level and P1 is that of the place whose altitude is to be determined:
H = (P1 - Por) / ρair . g
The altimetric equation shows that pressure decreases exponentially with height: for H = 0, P1= Por what if H → ∞, then P1=0.
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