Atmospheric pressure is the force with which air presses on the Earth, man and everything that surrounds him. The article will tell you how in the XVII century. with the help of the experiment, the force of air pressure was shown for the first time. It is very interesting! We will learn how atmospheric pressure is indicated and how it is measured.

## Experience Otto von Guericke

How great atmospheric pressure is, the world learned in 1654. This happened thanks to the burgomaster of the city of Magdeburg (Germany) Otto von Guericke. He demonstrated experience with the so-called Magdeburg hemispheres. Then there was no talk about how air pressure is indicated, because they still did not know how to measure it. How the hemispheres look can be seen in the photo from the Magdeburg Museum.

These are two bronze hemispheres, one of them is solid, and the second has a hole. An oiled leather gasket was placed between the hemispheres for tightness and connected. Air was pumped out through the hole from the hemispheres. Interestingly, Guericke himself four years earlier, in 1650invented the vacuum pump. He is also pictured. When the air was pumped out, the hemispheres were squeezed by atmospheric pressure. In order to disconnect them from each other, they used the traction force of horses.

## Experiment with Magdeburg hemispheres

Before we learn how atmospheric pressure is indicated, let's conduct an experiment. For it, we will use the Magdeburg hemispheres model. Attach a vacuum pump to the hole of the hemisphere with 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 hemispheres from the inside decreases, and the force acting from the outside increases.

During the pumping out of air, it is impossible to separate the hemispheres, as 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 separate easily.

## What letter represents 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) in 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 letter represents pressure? The letter P. Normal atmospheric pressure (Patm) 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 area of ​​the cross-section of the hemispheres S.

S=πd2/4. F=100103 Pa3, 14(0.355 m)2/4≈10 kN (kilonewtons). This is the weight of a one-ton load, so the horses were unable to break these hemispheres.

## Barometer

How is atmospheric pressure indicated, we know, but how is it measured? The barometer, which was invented by the Italian Torricelli 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 take it to different places to predict the weather.

It was necessary to come up with a device without a glass tube, that is, without liquid. Such a barometer was invented only two hundred years later and it was called the aneroid. This word translated into Russian means liquidless. Consider what an aneroid barometer is.

This is a small device. Unlike the Torricelli mercury tube, which is one meter high, it can be easily carried with you wherever you go. What's in it inside? Let's take a look at the barometer exploded.

How is the pressure in it indicated? The device has a scale similar to a watch dial. Pressure in kilopascals is indicated by an arrow. Behind the dial we see three flattened boxes. Air is pumped out of them, and there is a spring inside. If it weren't there, the atmosphere wouldcrushed boxes. Further from the spring, the lever moves away, it transmits the movements of the boxes. Why are they moving? Boxes can change their thickness. When the atmospheric pressure is greater, the air compresses the boxes, their thickness decreases. When the pressure is less, the spring straightens out and the boxes become thicker. Through the mechanism of the levers, the movement is transmitted to the arrow.

## Liquidless barometer device

We learned how pressure is indicated in a liquidless barometer, and now we will draw its diagram.

Three boxes give the device more accuracy, but in principle one is enough. It is specially made corrugated in order to have the ability to change its thickness. Remember corrugated, and therefore flexible vacuum cleaner hoses. The bottom of the box is attached to the base. A spring is attached to the top of it, which tries to straighten the box in the same way that an aluminum ruler, if bent, tries to straighten out. Atmospheric pressure, on the contrary, tries to compress the box.

When the pressure increases, the thickness of the box decreases, which means the lever turns the axle. If you attach an arrow to the axis, it will rotate to the right when the thickness decreases, and to the left when the thickness increases.