Torricelli's Experiment
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Torricelli's experiment was invented in
Pisa Pisa ( ; ) is a city and ''comune'' (municipality) in Tuscany, Central Italy, straddling the Arno just before it empties into the Ligurian Sea. It is the capital city of the Province of Pisa. Although Pisa is known worldwide for the Leaning Tow ...
in 1643 by the Italian scientist
Evangelista Torricelli Evangelista Torricelli ( ; ; 15 October 160825 October 1647) was an Italian people, Italian physicist and mathematician, and a student of Benedetto Castelli. He is best known for his invention of the barometer, but is also known for his advances i ...
(1608-1647). The purpose of his experiment is to prove that the source of "horror of the vacuum" by nature comes from
atmospheric pressure Atmospheric pressure, also known as air pressure or barometric pressure (after the barometer), is the pressure within the atmosphere of Earth. The standard atmosphere (symbol: atm) is a unit of pressure defined as , which is equivalent to 1,013. ...
.


Context

For much of human history, the pressure of gases like air was ignored, denied, or taken for granted, but as early as the 6th century BC, Greek philosopher Anaximenes of
Miletus Miletus (Ancient Greek: Μίλητος, Mílētos) was an influential ancient Greek city on the western coast of Anatolia, near the mouth of the Maeander River in present day Turkey. Renowned in antiquity for its wealth, maritime power, and ex ...
claimed that all things are made of air that is simply changed by varying levels of pressure. He could observe water evaporating and changing to a gas and felt that this applied even to solid matter. More condensed air made colder, heavier objects, and expanded air made lighter, hotter objects. This was akin to how gases become less dense when warmer and more dense when cooler.
Aristotle Aristotle (; 384–322 BC) was an Ancient Greek philosophy, Ancient Greek philosopher and polymath. His writings cover a broad range of subjects spanning the natural sciences, philosophy, linguistics, economics, politics, psychology, a ...
stated in some writings that "nature abhors a vacuum" and also that air has no mass/weight. The popularity of that philosopher kept this the dominant view in Europe for two thousand years. Even
Galileo Galileo di Vincenzo Bonaiuti de' Galilei (15 February 1564 – 8 January 1642), commonly referred to as Galileo Galilei ( , , ) or mononymously as Galileo, was an Italian astronomer, physicist and engineer, sometimes described as a poly ...
accepted it, believing that the pull of vacuum creates a siphon and that the pull can be overcome if the siphon is high enough. In the 17th century, Evangelista Torricelli conducted experiments with mercury that allowed him to measure the presence of air. He would dip a glass tube, closed at one end, into a bowl of mercury and raise the closed end out of it, keeping the open end submerged. The weight of the mercury would pull it down, leaving a partial vacuum at the far end. This validated his belief that air/gas has mass, creating pressure on things around it. The discovery helped bring Torricelli to the following conclusion: This test was essentially the first documented
pressure gauge Pressure measurement is the measurement of an applied force by a fluid (liquid or gas) on a surface. Pressure is typically measured in units of force per unit of surface area. Many techniques have been developed for the measurement of press ...
. In 1647
Valerianus Magnus Valerianus Magnus or Valeriano Magni (October 11, 1586 – July 20, 1661) was an Italian Capuchin Order, Capuchin, missionary preacher in Central Europe, philosopher, polemicist and author. He was one of the pioneers with the Torricelli's experim ...
published his ''Demonstratio ocularis'', in which he claims to have proved the existence of the vacuum in the court of the king of Poland, Ladislaus IV, in Warsaw by means of an experiment identical to that carried out by Torricelli three years earlier. Three months after Magnus,
Blaise Pascal Blaise Pascal (19June 162319August 1662) was a French mathematician, physicist, inventor, philosopher, and Catholic Church, Catholic writer. Pascal was a child prodigy who was educated by his father, a tax collector in Rouen. His earliest ...
published his ''Expériences nouvelles touchant le vide'', giving details of his first barometric experiments. Pascal went farther than Torricelli, having his brother-in-law try the experiment at different altitudes on a mountain and finding, indeed, that the farther down in the ocean of atmosphere, the higher the pressure.


Procedure

The experiment uses a simple
barometer A barometer is a scientific instrument that is used to measure air pressure in a certain environment. Pressure tendency can forecast short term changes in the weather. Many measurements of air pressure are used within surface weather analysis ...
to measure the pressure of air, filling it with mercury up until 75% of the tube. Any
air An atmosphere () is a layer of gases that envelop an astronomical object, held in place by the gravity of the object. A planet retains an atmosphere when the gravity is great and the temperature of the atmosphere is low. A stellar atmosph ...
bubbles in the tube must be removed by inverting several times. After that, a clean mercury is filled once again until the tube is completely full. The barometer is then placed inverted on the dish full of mercury. This causes the mercury in the tube to fall down until the difference between mercury on the surface and in the tube is about 760 mm. Even when the tube is shaken or tilted, the difference between the surface and in the tube is not affected due to the influence of atmospheric pressure.


Conclusion

Torricelli concluded that the mercury
fluid In physics, a fluid is a liquid, gas, or other material that may continuously motion, move and Deformation (physics), deform (''flow'') under an applied shear stress, or external force. They have zero shear modulus, or, in simpler terms, are M ...
in the tube is aided by the atmospheric pressure that is present on the surface of mercury fluid on the dish. He also stated that the changes of liquid level from day to day are caused by the variation of atmospheric pressure. The empty space in the tube is called the Torricellian vacuum. *760
mmHg A millimetre of mercury is a manometric unit of pressure, formerly defined as the extra pressure generated by a column of mercury one millimetre high. Currently, it is defined as exactly , or approximately 1 torr =  atmosphere = &nb ...
= 1 atm *1 atm = 1 013
mbar The bar is a metric unit of pressure defined as 100,000  Pa (100 kPa), though not part of the International System of Units (SI). A pressure of 1 bar is slightly less than the current average atmospheric pressure on Earth at sea ...
or
hPa The pascal (symbol: Pa) is the unit of pressure in the International System of Units (SI). It is also used to quantify internal pressure, stress, Young's modulus, and ultimate tensile strength. The unit, named after Blaise Pascal, is an S ...
*1 mbar or hPa = 0.7502467 mmHg 1 pascal = 1 Newton per square metre (SI unit) 1 hectopascal is 100 pascals


Additional images

{{Gallery , width=160 , height=170 , align=center , File:Water bucket for Torricelli experiment.jpg, Water is coloured with
potassium permanganate Potassium permanganate is an inorganic compound with the chemical formula KMnO4. It is a purplish-black crystalline salt, which dissolves in water as K+ and ions to give an intensely pink to purple solution. Potassium permanganate is widely us ...
for easy visualization. , File:Torricelli's experiment.jpg, Tube from the bucket is pulled up to the third floor of the building and the point where the liquid ceases to rise observed. , File:Torricelli's experiment of atmospheric pressure.jpg, The photo is taken from upward angle while performing Torricelli's experiment.


References

1643 in science Science and technology in Italy Physics experiments Pressure