Newtons Experiments - air pressure and bell jar

bell Jar

While we are on the earth we are surronded by our atmosphere, perhaps 50 miles of air above our heads (there is no well defined cut-off point for the atmosphere). Hydrogen and Helium are light enough to escapes the Earths gravity and so are lost into space but the other gases: e.g. O2, N2, CO2 etc. remain. All this gas above us adds up to a lot of mass (weight) and so everything below continually experiences the pressure of this gas - this is what we call atmospheric pressure (which at sea level is called 1atmos or 1 bar and amounts to about 14.7 pounds / square inch, 65 N / square inch).

The weight (mass) of the air above is greater the lower we go on earth. In general the pressure will increase below sea level and decrease higher up. Barometers can be used by mountineers to determine height up a mountain and also by pilots to determine the height flying in aircrafts.

The insides of our body are at about the same pressure as the outside so we are generaly unaware of atmospheric pressure but if we quickly go up a hill or down into a valley our bodies take a while to equalise with the corresponding pressure change and so we feel this when our ears 'pop' etc.

The bell jar apparatus is used to create our very own atmosphere contained within the glass jar which for example we can use to explore and understand our larger Earths atmosphere. We have a glass jar that rests on a rubber base which is air tight (seals well). The top of the jar has a bung attached to it through which a metal tube passes. This has a fexible tube attached to it that goes to a hand vacuum pump. I tried various pumps but found that the best hand powered pump was made using a wine preserver.

bladder colapser bladder colapser,

Experiment 1 - bladder collapser
If we blow up a balloon and then let the air out we get a saggy balloon which we can use in these experiments. Attach the balloon to a bung which has a hole drilled through it. Place the bung hole over the tube within the jar and fit the jar onto the base, see picture above. Initially the air pressure inside the jar is the same as outside so nothing happens. If we now pump on the hand pump we will remove much of the air in the tube and start to evacuate the balloon. As the air around the balloon is at atmospheric pressure when we reduce the pressure in the balloon all the forces will tend to collapse the balloon. Once we have removed most of the air the balloon looks completly flat - vacuum packed!

You might think that putting a 'super dooper' high tech motor pump will flatten the balloon even more but in fact once you have removed the air from the balloon there is simply no more air to remove. It collapses because of the air pressure outside rather than due to a great 'suck' from the pump. A larger more powerful pump might collapse the balloon quicker but it wont make it any flatter !

As you can only ever take out the air that was in the jar the maximum difference in pressure between the bell-jar and outside can only be about 1 atmophere. As a result explosion (or rather implosion) is possible but unlikely. If you are worried wrapping the jar in cling film and use a perspex (plexyglass) screen between the apparatus and spectators can safe guard aganist accidents.

They didnt have balloons in Newtons day so they used to use animal bladders for these type of experiments - hence the strange name given to this experiment!

balloon inflator

Experiment 2 balloon inflater
Now if the equipment is dismantled carefully and a partly inflated and knotted ballon is put inside the bell Jar we can see what effect will take place if we remove the air in the jar. So instead of sucking on the air inside of the balloon we are going to remove the air from around the balloon (i.e. remove the air from the bell jar). What will happen ?

As the pump removes the air from the bell jar the pressure obviously reduces around the balloon. Normally the balloon has this air pressure around it limiting its size, but as this air is removed and the pressure around it reduces the gas in the balloon can expand and so the balloon appears to inflate ! If you let the air back in the bell jar it will shrink back to its original size once more. This is a little bit like what is happening in your body when you go over a hill or down into a valley and your ears 'pop'.

barometer, barometer

Experiment 3 - the barometer
Barometers are devices that measure air pressure and can be used to try and predict changes in the weather. As masses of cold and hot air move the world they cause corresponding changes in the local air pressure and of course bring weather changes with them. So a change in the barometer reading often means that the weather will soon change.

Here we have a simple little water barometer. A glass jar contains coloured water (e.g. cranberry juice!) and is topped with a bung having a tube fitted that dips into the water. As you push on the bung the air is slightly compressed and some water goes up the tube. You can reset the water level inside the tube to zero by momonterily taking the little bung out of the small second tube.

Changes in pressure outside will cause a pressure difference between the tube (open to the air) and the air trapped inside the device to change and this leads to in water level. As the pump removes air from the jar you can see the water level shoot up on the little barometer.

The change in water level 'effect' can help explain why food jars and drinking bottles can leak when travelling around from place to place. The pressure change associated with moving about can cause movement of the contents in and out of badly sealed containers.

Experiment 4 - alarm clock
We hear sounds when they pass across the room and into our ears. Sound travels in waves in the air. Without air we can have no sound (although we can hear a bit by vibrations being transmitted to our ears in solid things such as through our head / bones etc.). So as the movies say "in space no one can hear you scream" because there is no air in the vacuum of space to transfer the sound.

In this experiment we set up a mechanical alarm clock to go off while inside the bell-jar. If we remove a lot of the air in the jar using the pump the alarm ringing will seem very much quieter. The effect is greatest when the very last traces of air are removed but this is often not possible using simple hand pumps so this experiment works best with a motorised pump that can remove the air more effectively.

Experiment 5 - animals need to breath
Hundreds of years ago people were interested the importance of air to life. Now of course we know that you have to breath air (oxygen and nitrogen). They used to take a little bird and place him into the jar and pump out the air until it fell over / fainted. Then they would then let the air back in and the bird would come back to life. Often all this was too much for the poor little bird and it would die in the process : - (

It is debatable how much 'science' was learnt in this process.

back to Newton page


Dr Jonathan Hare, The University of Sussex
Brighton, East Sussex. BN1 9QJ

home | diary | whats on | CSC summary | latest news