Homemade barometers are very easy to make, because, despite the fine appearance of the best quality instruments designed for decoration, barometers are really quite simple devices.
The complexity comes when accuracy, consistency of measurement and clear display of the air pressure measurements are needed.
For a basic barometer, all that is needed something that changes in position with changing air pressure, and a means of seeing the change. And that's very easy to do, using stuff that's easy to find in almost every home.
But before building barometers all over the house or classroom, it's worth remembering that barometers measure Air Pressure, and here's a very simple air pressure experiment that demonstrates that air has weight and that it exerts pressure.
The Air Has Weight Experiment
Take two party balloons and partially inflate them. Tie them to the ends of a stick or a long ruler and balance it over some sort of pivot - the back of a wooden chair will do. While holding the apparatus in place at its pivot point, put a small hole in the neck of one of the balloons where the rubber is thickest. It shouldn't burst, but will start to leak.
As it leaks, the end of the ruler with the inflated balloon will slowly go down.
What's going on?
Firstly, when you inflate the balloon you are forcing air into it, at higher pressure than the air surrounding it. The same thing happens when you breathe in and inflate your lungs - your chest expands because of the higher pressure inside. You have also added air to the inside of the balloon - more air is in the balloon than in the same amount of space outside it, and the extra weight of air inside the balloon is the reason for the higher pressure.
When the balloon is punctured, air leaks out, and because there is less air in the punctured balloon than the inflated one, the inflated balloon is now heavier, and weighs its end of the ruler down.
You'll find a neat experiment demonstrating Air Pressure here, involving a tin can. It's a bit dangerous at home without adult supervision, so here's another one.
The Air Pressure Experiment
You'll need a plastic milk bottle with an air tight lid - screw on lids are probably best. Partly fill it with hot water and put on the lid. It now contains water, warm air, and a little water vapor. Watch what happens as it cools.
The bottle has partly collapsed in on itself. This is because the warm air has cooled. Normally the air would take up less volume, but it can't because the bottle is sealed. So by a fundamental physical law, if the volume stays the same, which it does, the pressure of the air must also fall as the temperature drops. This law is named after Robert Boyle, who also gave the barometer its name. Nothing more will be said about Boyle's Law, but it's a very important foundation for engineering and physical science.
Back on track then. Because the pressure dropped as the temperature fell, the pressure inside the bottle is less than that outside, and the outside air pressure presses in on the bottle, which, being flexible, starts to bend and collapse. The process is slightly helped because the water vapor in the bottle starts to condense from gas to liquid as the temperature drops, also causing a pressure drop.
If you want to know more about Home Experiments Related to Weather, just follow the link to an excellent book, and I know from experience it's not just for kids. And here are some other top introductory weatherbooks.
And while most scientific attention was concentrated on the mercury barometer, water based barometers were also in use, and in big numbers. They were known as the Weather Glass or Storm Glass. Follow the link to find out more about these elegant weather instruments.
But both of these barometers had their problems, not the least of which was keeping the mercury or water in its container.As technology improved, the better, more convenient Aneroid Barometer appeared, and that is the most common form we see today.
You can also find information onSetting and Calibrating Your Barometer, elsewhere in this site, together with reviews of Modern Barometers.
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