Atmospheric Science

It was a beautiful day in early May 1654. Before the emperor, great dignitaries from all over Germany and other important people, a curious representation unfolds, a mixture of science and spectacle. As master of ceremonies, the burgomaster of Magdeburg, a guy who knew very well how to entertain the staff and, by the way, mark a date in red on the calendar of the history of science.

That daring character was none other than Otto Von Guericke, a lawyer and judge of the city of Magdeburg for many years. Not only had he devoted himself to studying laws because, since machines and the functioning of the world were food for his insatiable curiosity, he also studied mathematics, at the University of Leiden and was busy experimenting in various areas of physics.

One of those fields was pneumatics. Based on previous studies of great experimenters like Torricelli, Von Guericke decided to unravel the mysteries of that invisible ocean in which we live, the air. In 1650 he successfully built a pneumatic machine capable of partially extracting air from closed containers.

With that simple and primitive method of approaching the vacuum, Von Guericke managed to discover some very interesting things. Without air… the flame of the candles was impossible to maintain! The little animals died without remedy!

The sound stops being transmitted! Naturally, he was also attracted to other new areas of research, such as electrostatics, which led him to develop machines capable of generating electric sparks, which led him to maintain that lightning possessed the same nature as those harmless and tiny sparks that arose after electrifying various bodies.

That of electrostatics was spectacular, but the air had been installed in von Guericke’s mind as the central axis of his research, so… why not develop an impressive experiment that demonstrates the “power” of our atmosphere?

We live “swimming” in a sea that is a more or less homogeneous mixture of gases. Like the water-formed ocean, in which pressure increases with depth, everything on the emerged land surface is also subject to considerable pressure, namely that exerted by the “sea” of air above us. We are accustomed to living under that pressure, after all our body is optimally adapted to that condition.

But, of course, it is one thing to think about it today, knowing what air is, and it is quite another to demonstrate that this pressure exists and that it is of considerable importance at a time when it is being discussed whether emptiness really can exist – the horror vacui campaba at its leisure – or even whether the air was more than an illusion.

Von Guericke thought that the best way to demonstrate that atmospheric pressure was something real and powerful was to carry out an experiment that would be witnessed by the highest personalities of his land and his time. Well, nothing, said and done, he decided to take advantage of the Regensburg Imperial Diet as a luxury spectator.

On that day in May 1654, the burgamaster was probably nervous, his prestige was at stake, and perhaps something else, even his position. Von Guericke carefully designed the demonstration. It had to be rigorous but, at the same time, so spectacular that the potentates present were left with their mouths open with astonishment.

The emperor himself must have been impressed. In the midst of an almost circus environment was developed what, in the history of science, has come to be known as the experiment of the Magdeburg hemispheres. From that day on nothing was ever the same for physics, the demand for pneumatic machines grew without measure and the passion to study the atmosphere and its “powers” gained a momentum never seen before. All of this was due to the fact that the demonstration developed perfectly, following the script of Von Guericke.

In front of the dignitaries, surely surprised at the curious assembly that unfolded before them, appeared two half spheres -hemispheres- carefully elaborated in copper. One half faced the other, forming a large sphere. To this sphere was connected Von Guericke’s pneumatic machine, which extracted as much air as it could from the interior. Just as when we suck the air from an empty plastic bottle with our mouths and it deforms, so the “vacuum” was created inside the sphere. Now, the atmospheric pressure kept the two hemispheres “glued” together. How much force would it take to separate them?

It was time to demonstrate the power of atmospheric pressure. The hemispheres had several rings which were attached to thick ropes associated with two groups of eight horses, one group on each side. The sphere is on the ground, the horses agitate nervously, Von Guericke’s helpers wait for his order. In an instant, before a vigorous order issued by the burgomaster, the helpers encourage the two groups of horses that, with all their strength pull in opposite directions in each of the hemispheres.

The audience rises, the murmurs grow, no one believes what they are seeing. Logic” told them that the two hemispheres -which were neither welded nor glued together nor joined to each other by nothing more than the force of the external atmospheric pressure manifested by making the “void” inside them- would separate almost instantly when they felt the extraordinary force of the horses on the rings. But no, those stubborn copper hemispheres, united only by contact, endured the effort of the horses for a considerable time.

Von Guericke’s vacuum pump had achieved a “miracle,” the partial vacuum inside the copper sphere revealed the reality of atmospheric pressure, and with the cheers of those present Von Guericke put the icing on the cake, for he showed that simply by slightly opening the valve of the sphere, again allowing air to enter, the hemispheres could separate by themselves. As the internal and external pressure were equal, no horses or other aids were needed for the two pieces to separate. The bet had been risky, the assembly spectacular and the result.

With the help of a bottle, a little water and a thumbtack (it is also advisable to use a small basin so as not to get dirty at home), we will explain the principles of surface tension, atmospheric pressure and gravity.

In our experiment we have air outside the bottle, but inside as well. The water will stay in or out depending on which factors predominate.

The first thing is to make a small hole in a plastic bottle, we cover it provisionally with the help of our finger and fill the bottle with water, then capping it by the stopper. When you remove your finger from the hole you can see that no water comes out. But, if we remove the cap from the bottle, we see that it does.