Atmospheric pressure refers to the force with which air presses on the Earth, man and everything that surrounds him. The article tells how in the XVII century. using the experiment, the force of air pressure was first shown. It is very interesting! We learn how atmospheric pressure is indicated and how it is measured.
Otto von Guericke's Experience
How great is atmospheric pressure, the world recognized in 1654. This happened thanks to the mayor of the city of Magdeburg (Germany) Otto von Guericke. He demonstrated experience with the so-called Magdeburg hemispheres. Then there was no question of how air pressure is indicated, because they were not yet able to measure it. What the hemispheres look like can be seen in the photograph from the Magdeburg Museum.
These are two bronze hemispheres, one of them is solid, and the second has a hole. An oiled leather pad was placed between the hemispheres for tightness and connected. Air was pumped out of the hemispheres through an opening. Interestingly, Guericke himself invented the vacuum pump four years before, in 1650. It is also captured in the photo. When the air was pumped out, hemispheres were compressed by atmospheric pressure. In order to disconnect them from each other, used the traction of horses.
Experiment with the Magdeburg Hemispheres
Before we learn how atmospheric pressure is indicated, we will conduct an experiment. For him we will use the model of the Magdeburg hemispheres. Connect the vacuum pump to the hemisphere hole using a rubber hose. Turn it on, open the tap on one of the hemispheres. The pressure in the space between them will decrease. Consequently, the force acting on the hemisphere from the inside decreases, and the force acting outside - increases.
During the pumping out of the air, it is impossible to separate the hemispheres, since they fit snugly together. Turn off the pump, disconnect the rubber hose. Air will begin to enter the space between the hemispheres. Then they will be easily divided.
What letter denotes air pressure
Let's try to calculate the force that squeezed the hemispheres. When we pump out air, only the force of atmospheric pressure acts on the hemispheres. It compresses the hemispheres and is directed from the inner walls of the hollow spheres to the center of the space between them. The diameter of the hemispheres (d) at Guericke was 35.5 cm.
Based on the fact that we could not separate the hemispheres, it becomes clear that the pressure force is very large. Even eight horses on each side could not break these hemispheres. Here is an engraving that illustrates the experience of Otto von Guericke.
What is the letter for pressure? The letter R. Normal atmospheric pressure (P atm ) is 100 kilopascals (kPa). Such a force acts on each part of the hemisphere. The pressure force F is equal to the product of atmospheric pressure and the cross-sectional area of the hemispheres S.
S = π * d 2/4. F = 100 * 10 3 Pa * 3.14 * (0.355 m) 2 / 4≈10 kN (kilonewtons). This is the weight of a cargo weighing one ton, so the horses could not break these hemispheres.
Barometer
How atmospheric pressure is indicated, we know, and how is it measured? The barometer that the Italian Torricelli invented in the first half of the 17th century had flaws. It could easily be broken, it was filled with poisonous mercury, and you really wanted to carry it to different places in order to predict the weather.
It was necessary to come up with a device without a glass tube, i.e., non-liquid. Such a barometer was invented only two hundred years later and was called its aneroid. This word in translation into Russian means liquid-free. Consider what is an aneroid barometer.
This is a small device. Unlike a one meter tall Torricelli mercury tube, it can easily be carried with you wherever you go. What is inside of it? Let's take a look at the unassembled barometer.
What is the pressure in it? The device has a scale similar to the dial of a watch. The pressure in kilopascals is indicated by an arrow. Behind the dial we see three flattened boxes. Air is pumped out of them, and inside is a spring. If she were not there, the atmosphere would crush the boxes. Further from the spring, the lever moves away, it transfers the movements of the boxes. Why are they moving? Boxes can change their thickness. When atmospheric pressure is greater, air compresses the boxes, their thickness decreases. When the pressure is less, the spring straightens and the boxes become thicker. Through the mechanism of levers, the movement is transmitted to the arrow.
Liquidless barometer device
We learned how the pressure in a liquid-free barometer is indicated, and now we will draw its diagram.
Three boxes give the device greater accuracy, but in principle one is enough. It is specially made corrugated in order to have the ability to change its thickness. Remember the corrugated, and therefore flexible hoses of vacuum cleaners. The bottom of the box is attached to the base. A spring is attached to its upper part, which tries to straighten the box in the same way as an aluminum ruler, if it is bent, it tries to straighten. Atmospheric pressure, on the contrary, is trying to squeeze the box.
When the pressure increases, the thickness of the box decreases, which means that the lever rotates the axis. If you attach an arrow to the axis, it will turn to the right when the thickness decreases, and to the left when the thickness increases.