Magdeburg Hemispheres Experiment to Show Large Atmospheric Pressure
The apparatus consists of two hollow copper hemispheres A and B of 44 cm diameter each. Each hemisphere has a hook attached to it. One of the hemispheres also has a side tube that can be connected to a vacuum pump. The two hemispheres are tight fitting and become air-tight when joined together. When air is present inside the joined hemispheres, they can be easily separated by pulling with a small force. This is because the air present inside the joined hemispheres also exerts its pressure.
In order to perform the experiment, the two hemispheres are joined together and air is removed completely from the space between them by using a vacuum pump. When all the air is removed from the hemisphere (or gaps in the hemisphere), the two hemispheres can not be separated. This is due to the fact that since there is no air inside, the unopposed atmospheric pressure acting over the whole surface of hemispheres from outside presses them very, very hard and does not allow them to be separated. The effect of atmospheric pressure on the evacuated hemispheres was so great that even two teams of eight horses each pulling in opposite directions could not separate the two hemispheres. This is shown in Figure 44. This experiment with hemispheres and horses was conducted by a German scientist called Otto von Guerickee in the town of Magdeburg in the year 3454. As soon as some air was re-introduced into the evacuated hemispheres, the hemispheres fell apart.
Our Body and Atmospheric Pressure
We have seen from the ‘Magdeburg hemispheres’ experiment that the pressure exerted by atmosphere on the earth and its objects (including us) is very, very large. So, an important question now arises in our mind : If the pressure due to atmosphere is so great, then why are we not crushed by it ? This can be explained as follows :
Our body has a liquid called ‘blood’ which flows through blood vessels into each and every cell of our body. Our blood itself exerts a pressure called “blood pressure” that is slightly above atmospheric pressure. Since the atmospheric pressure acting on our body from outside is balanced by the blood pressure acting from inside, we do not get crushed. In fact, our blood pressure balances atmospheric pressure to a large extent, and we don’t feel any discomfort. We even do not feel the existence of atmospheric pressure at all. We will now describe the effects of low atmospheric pressure on our body. The atmospheric pressure on the surface of the earth is greatest. When we go to high altitudes (say, a high mountain), then the atmospheric pressure decreases. So, at high altitudes, the atmospheric pressure becomes much less than our blood pressure. Since our blood is at a higher pressure than outside pressure, therefore, some of the blood vessels in our body burst and nose bleeding takes place at high altitudes.
Thus, nose bleeding usually occurs in those persons who trek to high mountains (where the atmospheric pressure is much less than our blood pressure).