How to make your own barometer
Created | Updated Jan 18, 2004
What is a Barometer?
A barometer is a device for measuring atmospheric (air) pressure. It is commonly used in weather forecasting. Barometers can be used to make a simple altimeter (as air pressure goes down with height: there is less air above you), though for this, a manometer (which is very similar) is more frequently used.
There are different types of barometer. The type mentioned here is the stick or mercury barometer1, which traditionally uses mercury, which is an ideal material, but not required.
Other types of Barometer include the aneroid barometer, which is much smaller and very accurate. However, it is also much more difficult to make.
The principle of the barometer was discovered by Evangelista Torricelli in 1643.
You will need
A long tube, sealed closed at one end. The tube must be see-through, not extremely thin and preferably quite stiff. Glass is an ideal material, and a J-shaped tube is the most convenient.
Some water or mercury, or any other fluid. If you use water, the tube will have to be very long. However, mercury is a poison, so you shouldn't touch it or eat it. You should also avoid breathing in mercury fumes. Getting hold of mercury may be difficult, and you may also have an even worse time if you want to get rid of Mercury, as it is a poison, and disposal may be covered by laws. Information is available on Mercury, courtesy of DEFRA (part of the UK government). For this reason, use of water instead is highly recommended.
A dish of some sort.
A ruler or tape measure.
How a Barometer Works
A barometer works by seeing how far up a tube a liquid can get before being stopped by gravity. The force on the open end of the tube is the force of the air, which pushes the liquid up the tube. The vacuum formed2 at the top of the tube does not push down, so the only thing stopping the liquid rising forever is the weight of the liquid.
What to Do
Fill the tube and dish with the liquid.
Put one end of the tube in the dish. As the other end of the tube is closed, no air can get in to let the liquid out again.
Raise the tube vertically above the dish, keeping the end in the liquid. No air must get into the tube. At a certain point, the force of gravity will be too much, and the liquid will not go any higher.
When the air pressure is higher, there is more pressure keeping it up, so the liquid will get higher.
Reading your Barometer
When you've made your barometer, you should be able to see the water rise and fall as the air pressure changes.
If you measure the height of the liquid (vertically upwards), this can then be converted into pressure. If you use mercury, you can read the pressure directly in millimetres of mercury, called a Torr.
Pressure can be calculated for any fluid in the SI units, Kilograms per Meter per Second Squared, a unit called the Pascal, by using the following formula (you will need to use meters for the height):
Atmospheric Pressure = The Density of the Liquid * The acceleration due to gravity3 * The height the liquid gets to (above the level of the fluid in the dish).
However, Physicists like to confuse everyone by sticking the occasional Greek letter here and there, so use the formula:
patm = ρgh
which is exactly the same.
The density of water is 1 000 kg/m3.
Mercury is used because it has a very high density, 13 600 kg/m3 (this is so high that lead will float in it). It is used because it won't go so high up the tube, around 80 cm; water, however, will need over 10m of tube.
1You don't necessarily have to make it with mercury2There will not be a total vacuum formed, as there will still be some vapour in the top3This is 9.8 meters/second2
